CONTROL CIRCUIT FOR DC/DC CONVERTER

Abstract

A main comparator compares a feedback voltage VFB corresponding to an output voltage VOUT of a DC/DC converter with a reference voltage VREF and asserts a turn-on signal when the feedback voltage VFB falls below the reference voltage VREF. A timer circuit generates a turn-off signal S2 that transitions in level after an ON time TON proportional to (VOUT−VIN/VOUT has elapsed from assertion of the turn-on signal.

Description

BACKGROUND

1. Technical Field

The present disclosure relates to a DC/DC converter.

2. Description of the Related Art

A DC/DC converter is used when a DC (direct current) voltage of a certain voltage value is converted into a DC voltage of another voltage value. As a control method of the DC/DC converter, a ripple control method is known. The ripple control method is a method in which an output voltage of the DC/DC converter is compared with a threshold voltage, and when the output voltage exceeds (or falls below) the threshold voltage, the switching transistor is switched on and off with the output voltage as a trigger. The ripple control method has an advantage that a response speed is high and power consumption can be reduced as compared with a voltage mode control method or an electric current mode control method using an error amplifier.

The present inventors have studied application of ripple control of bottom detection and constant on time (COT) to a boost converter.

This method is easy to design because phase compensation is unnecessary and has a feature of excellent high-speed transient response. On the other hand, when the ON time is fixed, there is a problem that switching frequency greatly fluctuates.

SUMMARY OF THE INVENTION

One embodiment of the present disclosure has been made in view of the related problems, and an exemplary general purpose of one embodiment is to provide a control circuit of a boost DC/DC converter in which the switching frequency can be stabilized.

Another exemplary general purpose of one embodiment of the present disclosure is to provide a DC/DC converter and a control circuit thereof capable of reducing an output voltage in an overcurrent state.

1. One embodiment of the present disclosure relates to a control circuit of a DC/DC converter that boosts an input voltage V_IN and generates an output voltage V_OUT. The control circuit includes a main comparator structured to compare a feedback voltage corresponding to an output voltage of the DC/DC converter with a reference voltage and to asserts a turn-on signal when the feedback voltage falls below the reference voltage, and a timer circuit structured to generate a turn-off signal transitioning in level after an on time proportional to (V_OUT−V_IN)/V_OUT has elapsed from the assertion of the turn-on signal.

2. One embodiment of the present disclosure relates to a control circuit of a DC/DC converter including a switching transistor. The control circuit includes a main comparator structured to compare a feedback voltage corresponding to an output voltage of a DC/DC converter with a reference voltage, and to assert a turn-on signal when the feedback voltage falls below the reference voltage; an ON time generation circuit structured to assert a turn-off signal after a lapse of an ON time from turn-on of a switching transistor; an overcurrent detection circuit structured to assert an overcurrent detection signal when a current flowing through the switching transistor exceeds an overcurrent threshold in an ON period of the switching transistor; a turn-on inhibition circuit structured to generate a turn-on inhibition signal to be asserted in a period from turn-on of the switching transistor until a lapse of a predetermined time; a logic circuit structured to generate a pulse signal that transitions to an ON level when the turn-on signal is asserted in a period in which the turn-on inhibition signal is negated, and transitions to an OFF level when the turn-off signal or the overcurrent detection signal is asserted; and a driver structured to drive the switching transistor according to the pulse signal.

It is to be noted that any arbitrary combination or rearrangement of the above-described structural components and so forth is effective as and encompassed by the present embodiments. Moreover, this summary does not necessarily describe all necessary features so that the disclosure may also be a sub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:

FIG. 1 is a circuit diagram of a DC/DC converter according to the first embodiment.

FIG. 2 is an equivalent circuit diagram of an output circuit of a DC/DC converter.

FIG. 3 is a circuit diagram illustrating a basic configuration of a timer circuit.

FIG. 4 is an operation waveform diagram of the timer circuit of FIG. 3.

FIG. 5 is a circuit diagram of a timer circuit according to Example 1.1.

FIG. 6 is an operation waveform diagram of the timer circuit of FIG. 5.

FIG. 7 is a circuit diagram of a timer circuit according to Example 1.2.

FIG. 8 is an operation waveform diagram of the timer circuit of FIG. 7.

FIG. 9 is a circuit diagram of a timer circuit according to Example 1.3.

FIG. 10 is a circuit diagram of a timer circuit according to Example 1.4.

FIG. 11 is a circuit diagram of a DC/DC converter according to the second embodiment.

FIG. 12 is an operation waveform diagram of the DC/DC converter of FIG. 11 in a normal state (a non-overcurrent state).

FIG. 13 is an operation waveform diagram in the overcurrent state of the DC/DC converter of FIG. 11.

FIG. 14 is an equivalent circuit diagram of an output circuit of the DC/DC converter.

FIG. 15 is a circuit diagram illustrating a basic configuration of an ON time generation circuit;

FIG. 16 is an operation waveform diagram of the ON time generation circuit of FIG. 15.

FIG. 17 is a circuit diagram of an ON time generation circuit according to Example 2.1.

FIG. 18 is an operation waveform diagram of the ON time generation circuit of FIG. 17.

FIG. 19 is a circuit diagram of an ON time generation circuit according to Example 2.2.

FIG. 20 is an operation waveform diagram of the ON time generation circuit of FIG. 19.

FIG. 21 is a circuit diagram of an ON time generation circuit according to Example 2.3.

FIG. 22 is a circuit diagram of an ON time generation circuit according to Example 2.4.

DETAILED DESCRIPTION

Outline of Embodiments

A summary of several example embodiments of the disclosure follows. This summary is provided for the convenience of the reader to provide a basic understanding of such embodiments and does not wholly define the breadth of the disclosure. This summary is not an extensive overview of all contemplated embodiments and is intended to neither identify key or critical elements of all embodiments nor to delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later. For convenience, the term “one embodiment” may be used herein to refer to a single embodiment or multiple embodiments of the disclosure.

1. One embodiment disclosed in the present description relates to a control circuit for a DC/DC converter that boosts an input voltage V_IN and generates an output voltage V_OUT. The control circuit includes a main comparator that compares a feedback voltage corresponding to an output voltage of the DC/DC converter with a reference voltage and asserts a turn-on signal when the feedback voltage falls below the reference voltage, and a timer circuit that generates a turn-off signal transitioning in level after an on time proportional to (V_OUT−V_IN)/V_OUT has elapsed from the assertion of the turn-on signal.

According to this embodiment, a switching frequency can be stabilized by adaptively changing the ON time according to the input voltage and the output voltage.

The timer circuit may include a first capacitor, a current source that is connected to the first capacitor and generates a current proportional to V_OUT, and a comparator that detects that a voltage change proportional to (V_OUT−V_IN) occurs in the first capacitor.

The current generated by the current source is I=α×V_OUT. Time T_ON required for the voltage change ΔV=β×(V_OUT−V_IN) proportional to (V_OUT−V_IN) to occur in the first capacitor is expressed as

T _ON =ΔV/I=β×(V_OUT−V_IN)/(α×V_OUT)=(β/α)×(V_OUT−V_IN)/V_OUT,

and the ON time proportional to (V_OUT−V_IN)/V_OUT can be generated.

One end of the first capacitor may be grounded. The timer circuit may further include a threshold voltage generation circuit that generates a threshold voltage according to (V_OUT−V_IN). The comparator may compare a voltage at the other end of the first capacitor with the threshold voltage.

The threshold voltage generation circuit may include a second capacitor. The threshold voltage generation circuit may charge the second capacitor with (V_OUT−V_IN) in the OFF state of the switching transistor, apply the switching voltage of a connection node between the inductor and the switching transistor to one end of the second capacitor in an ON period of the switching transistor, and set the voltage at the other end of the second capacitor as the threshold voltage. The switching voltage during the ON period is I×R_ON1. I is a current flowing through the switching transistor, and R_ON1 is an ON resistance of the switching transistor. Therefore, according to this configuration, it is possible to generate the ON time in consideration of the ON resistance of the switching transistor.

The threshold voltage generation circuit may include a second capacitor, a first selector that applies an input voltage V_IN to one end of the second capacitor in the OFF state of the switching transistor and connects the one end of the second capacitor with the inductor of the DC/DC converter and the connection node of the switching transistor during an ON period of the switching transistor, and a second selector that applies an output voltage V_OUT to the other end of the second capacitor in the OFF state of the switching transistor and connects the other end of the second capacitor with the comparator during the ON period of the switching transistor. According to this configuration, it is possible to generate the ON time in consideration of the ON resistance of the switching transistor.

The threshold voltage generation circuit may include an inverter that inverts a switching voltage generated at an inductor of the DC/DC converter and a connection node of the switching transistor, and a filter that smooths an output of the inverter and generates a threshold voltage. According to this configuration, the ON time can be generated in consideration of the influence of the ON resistances of the switching transistor and the synchronous rectification transistor as well as the equivalent series resistance of the inductor.

The filter may be an RC filter. The threshold voltage generation circuit may charge the capacitor of the RC filter with V_OUT−V_IN while operating in the discontinuous current mode. According to this configuration, when returning from the discontinuous current mode to the continuous current mode, the operation can be resumed from an appropriate ON time.

The filter may be an RC filter including a resistor and a capacitor. The threshold voltage generation circuit may further include a third selector that applies an output voltage of the inverter to one end of the resistor during the continuous current mode and applies the output voltage V_OUT to one end of the resistor during the discontinuous current mode, and a fourth selector that applies a ground voltage to the other end of the capacitor during the continuous current mode and applies the input voltage V_IN to the other end of the capacitor during the discontinuous current mode. According to this configuration, when returning from the discontinuous current mode to the continuous current mode, the operation can be resumed from an appropriate ON time.

The input voltage V_IN may be applied to one end of the first capacitor. The comparator may compare the voltage at the other end of the first capacitor with the output voltage V_OUT. Since the ON resistance of the transistor and the equivalent series resistance of the inductor are ignored, the ON time can be generated with a simple configuration although the frequency at a heavy load becomes faster.

The control circuit may be integrally integrated on one semiconductor substrate. The term “integrally integrated” includes a case where all components of a circuit are formed on a semiconductor substrate and a case where main components of the circuit are integrally integrated, and some resistors, capacitors, and the like may be provided outside the semiconductor substrate for adjusting a circuit constant. By integrating the circuits on one chip, a circuit area can be reduced, and the characteristics of circuit elements can be kept uniform.

2. One embodiment disclosed in the present description relates to a control circuit for a DC/DC converter including a switching transistor. A control circuit includes a main comparator that compares a feedback voltage corresponding to an output voltage of a DC/DC converter with a reference voltage, and asserts a turn-on signal when the feedback voltage falls below the reference voltage; an ON time generation circuit that asserts a turn-off signal after a lapse of an ON time from the assertion of the turn-on signal; an overcurrent detection circuit that asserts an overcurrent detection signal when a current flowing through a switching transistor exceeds an overcurrent threshold in an ON period of the switching transistor; a turn-on inhibition circuit that generates a turn-on inhibition signal to be asserted in a period from the turn-on of the switching transistor to the lapse of a predetermined time; a logic circuit that generates a pulse signal that transitions to an on-level when the turn-on signal is asserted in a period in which the turn-on inhibition signal is negated, and transitions to an OFF level when the turn-off signal or the overcurrent detection signal is asserted; and a driver that drives the switching transistor according to the pulse signal.

According to this embodiment, the output voltage can be lowered in the overcurrent state.

The DC/DC converter may be a boost type that boosts the input voltage V_IN and generates an output voltage V_OUT.

The ON time may be proportional to (V_OUT−V_IN)/V_OUT. A switching frequency can be stabilized by adaptively changing the ON time according to the input voltage and the output voltage.

The ON time generation circuit may include a first capacitor, a current source that is connected to the first capacitor and generates a current proportional to V_OUT, and a comparator that detects that a voltage change proportional to (V_OUT−V_IN) occurs in the first capacitor.

The current generated by the current source is I=α×V_OUT. The time T_ON required for the voltage change ΔV=β×(V_OUT−V_IN) proportional to (V_OUT−V_IN) to occur in the first capacitor is expressed as

$\begin{array}{c}{T}_{\mathrm{ON}}=\Delta V/I=\beta \times \left(V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right)/\left(\alpha \times V_{\mathrm{OUT}}\right)\\ =\left(\beta /\alpha \right)\times \left(V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right)/V_{\mathrm{OUT}},\end{array}$

and the ON time proportional to (V_OUT−V_IN)/V_OUT can be generated.

One end of the first capacitor may be grounded. The ON time generation circuit may further include a threshold voltage generation circuit that generates a threshold voltage according to (V_OUT−V_IN). The comparator may compare a voltage at the other end of the first capacitor with the threshold voltage.

The threshold voltage generation circuit may include a second capacitor. The threshold voltage generation circuit may charge the second capacitor with (V_OUT−V_IN) in the OFF state of the switching transistor, apply the switching voltage of a connection node between the inductor and the switching transistor to one end of the second capacitor in an ON period of the switching transistor, and set the voltage at the other end of the second capacitor as the threshold voltage. The switching voltage during the ON period is I×R_ON1. I is a current flowing through the switching transistor, and R_ON1 is an ON resistance of the switching transistor. Therefore, according to this configuration, it is possible to generate the ON time in consideration of the ON resistance of the switching transistor.

The threshold voltage generation circuit may include a second capacitor, a first selector that applies an input voltage V_IN to one end of the second capacitor in the OFF state of the switching transistor and connects the one end of the second capacitor with the inductor of the DC/DC converter and the connection node of the switching transistor during an ON period of the switching transistor, and a second selector that applies an output voltage V_OUT to the other end of the second capacitor in the OFF state of the switching transistor and connects the other end of the second capacitor with the comparator during the ON period of the switching transistor. According to this configuration, it is possible to generate the ON time in consideration of the ON resistance of the switching transistor.

The threshold voltage generation circuit may include an inverter that inverts a switching voltage generated at an inductor of the DC/DC converter and a connection node of the switching transistor, and a filter that smooths an output of the inverter and generates a threshold voltage. According to this configuration, the ON time can be generated in consideration of the influence of the ON resistances of the switching transistor and the synchronous rectification transistor as well as the equivalent series resistance of the inductor.

The filter may be an RC filter. The threshold voltage generation circuit may charge the capacitor of the RC filter with V_OUT−V_IN while operating in the discontinuous current mode. According to this configuration, when returning from the discontinuous current mode to the continuous current mode, the operation can be resumed from an appropriate ON time.

The filter may be an RC filter including a resistor and a capacitor. The threshold voltage generation circuit may further include a third selector that applies an output voltage of the inverter to one end of the resistor during the continuous current mode and applies the output voltage V_OUT to one end of the resistor during the discontinuous current mode, and a fourth selector that applies a ground voltage to the other end of the capacitor during the continuous current mode and applies the input voltage V_IN to the other end of the capacitor during the discontinuous current mode. According to this configuration, when returning from the discontinuous current mode to the continuous current mode, the operation can be resumed from an appropriate ON time.

The input voltage V_IN may be applied to one end of the first capacitor. The comparator may compare the voltage at the other end of the first capacitor with the output voltage V_OUT. Since the ON resistance of the transistor and the equivalent series resistance of the inductor are ignored, the ON time can be generated with a simple configuration although the frequency at a heavy load becomes faster.

The control circuit may be integrally integrated on one semiconductor substrate. The term “integrally integrated” includes a case where all components of a circuit are formed on a semiconductor substrate and a case where main components of the circuit are integrally integrated, and some resistors, capacitors, and the like may be provided outside the semiconductor substrate for adjusting a circuit constant. By integrating the circuits on one chip, a circuit area can be reduced, and the characteristics of circuit elements can be kept uniform.

EMBODIMENTS

Hereinafter, the present disclosure will be described based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes illustrated in the drawings are denoted by the same reference numerals, and redundant description will be omitted as appropriate. Furthermore, the embodiments are not intended to limit the invention but are examples, and all features described in the embodiments and combinations thereof are not necessarily essential to the invention.

In the present description, “a state in which a member A is connected to a member B” includes not only a case where the member A and the member B are physically and directly connected to each other, but also a case where the member A and the member B are indirectly connected to each other via another member that does not substantially affect an electrical connection state between the member A and the member B or that does not impair a function or an effect exhibited by coupling between the two members.

Similarly, “a member C is provided between the member A and the member B” includes not only a case where the member A and the member C, or the member B and the member C are directly connected to each other, but also a case where the members are indirectly connected to each other via another member that does not substantially affect an electrical connection state between the members or that does not impair a function or an effect exhibited by the connection between the members.

In addition, “a signal A (voltage, current) is corresponding to a signal B (voltage, current)” means that the signal A has a correlation with the signal B. Specifically, it means (i) when the signal A is the signal B, (ii) when the signal A is proportional to the signal B, (iii) when the signal A is obtained by level-shifting the signal B, (iv) when the signal A is obtained by amplifying the signal B, (v) when the signal A is obtained by inverting the signal B, (vi) or any combination thereof, or the like. It is understood by a person skilled in the art that a range of “corresponding to” is determined according to types and applications of the signals A and B.

A vertical axis and a horizontal axis of a waveform diagram and a time chart referred to in the present description are appropriately enlarged and reduced for easy understanding, and each waveform shown is simplified or exaggerated or emphasized for easy understanding.

First Embodiment

FIG. 1 is a circuit diagram of a DC/DC converter 100 according to the first embodiment. The DC/DC converter 100 is a boost converter, boosts an input voltage V_IN of an input line (an input terminal) 102, stabilizes the input voltage to a predetermined voltage level, and supplies the voltage to a load 4 connected to an output line (output terminal) 104.

The DC/DC converter 100 includes an output circuit 110 and a control circuit 300. The output circuit 110 includes an inductor L1, a switching transistor (a low-side transistor) M1, a synchronous rectification transistor (a high-side transistor) M2, and an output capacitor C1.

The control circuit 300 is a controller of a ripple control method, more specifically, a bottom detection method, and includes a switching pin SW and an output pin OUT. An external inductor L1 is connected to the switching pin SW, and an external output capacitor C1 and the output line 104 are connected to the output pin OUT.

The control circuit 300 includes a voltage dividing circuit 302, a main comparator 308, a logic circuit 312, a first driver 314, a second driver 316, a timer circuit 320, a switching transistor M1, and a synchronous rectification transistor M2, and is an integrated circuit (IC) integrated on one semiconductor substrate.

The voltage dividing circuit 302 includes the resistors R11 and R12 and divides the output voltage V_OUT to generate the feedback voltage V_FB.

The main comparator 308 compares the feedback voltage V_FB corresponding to the output voltage V_OUT of the DC/DC converter 100 with a reference voltage V_REF and asserts a turn-on signal S1 when the feedback voltage V_FB falls below the reference voltage V_REF. The turn-on signal S1 is a pulse signal indicating a magnitude relationship between V_FB and V_REF, and one of a positive edge and a negative edge can be associated with assertion.

The logic circuit 312 generates pulse signals Sp1 and Sp2 instructing on and off of the switching transistor M1 and the synchronous rectification transistor M2 based on the turn-on signal S1.

The logic circuit 312 changes a start signal STARTX with assertion of the turn-on signal S1 as a trigger to operate the timer circuit 320. The timer circuit 320 generates a turn-off signal S2 that transitions in level after a lapse of an ON time T_ON proportional to (V_OUT−V_IN)/V_OUT from assertion of the turn-on signal S1. The turn-off signal S2 indicates a turn-off timing of the switching transistor M1.

The first pulse signal Sp1 is at the ON level (for example, high) during the on time T_ON from assertion of the turn-on signal S1 to assertion of the turn-off signal S2, and is at an OFF level (for example, low) until assertion of the next turn-on signal S1.

In the continuous current mode (CCM), the logic circuit 312 complementarily changes the second pulse signal Sp2 with the first pulse signal Sp1. In the discontinuous current mode (DCM), the zero crossing of the current flowing through the synchronous rectification transistor M2 is detected, and the OFF levels of both the first pulse signal Sp1 and the second pulse signal Sp2 are maintained from the current zero crossing to the assertion of the next turn-on signal S1.

FIG. 2 is the equivalent circuit diagram of the output circuit 110 of the DC/DC converter 100. R_DC is an equivalent series resistance such as the inductor L1 and wiring. R_ON1 represents the ON resistance of the switching transistor M1, and R_ON2 represents the ON resistance of the synchronous rectification transistor M2.

A switching cycle is defined as T. In an ON state φ_ON of the switching transistor M1, I_L=I_M1, and the voltage across the inductor L1 is {V_IN−(R_ON1+R_DC)×I_L}. Accordingly, an increase in width ΔI_ON of an inductor current I_L in the ON state φ_ON is expressed by Equation (1). T_ON is a length of the ON state and is referred to as the ON time.

ΔI_ON=T_ON/L×{V_IN−(R_ON1+R_DC)×I_L}  (1)

In the OFF state (PUFF of the switching transistor M1, I_L=I_M2, and the voltage across the inductor L1 is {V_OUT+(R_ON1+R_DC)×I_L−V_IN}. Accordingly, a decrease in width ΔI_OFF of the inductor current I_L in the OFF state (PUFF is expressed by Equation (2).

ΔI_OFF=(T−T_ON)/L×{V_OUT−(R_ON2+R_DC)×I_L−V_IN}  (2)

When the output voltage V_OUT is stabilized in the continuous current mode, ΔI_ON=ΔI_OFF holds. Accordingly, a duty cycle d is expressed by Equation (3).

$\begin{array}{cc}\begin{array}{c}d=T_{\mathrm{ON}}/T\\ =\left\{V_{\mathrm{OUT}}-V_{\mathrm{IN}}+\left(R_{\mathrm{ON}2}+R_{\mathrm{DC}}\right)\times I_L\right\}/\left\{V_{\mathrm{OUT}}-\left(R_{\mathrm{ON}1}-R_{\mathrm{ON}2}\right)\times I_L\right\}\end{array}& \left(3\right)\end{array}$

Assuming R_ON1=R_ON2=R_DC=0, Equation (4) is obtained.

$\begin{array}{cc}\begin{array}{c}d=T_{\mathrm{ON}}/T\\ =\left\{V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right\}/V_{\mathrm{OUT}}\end{array}& \left(4\right)\end{array}$

According to the control circuit 300 of FIG. 1, in the boost converter, the switching frequency can be kept constant by adaptively changing the ON time T_ON according to the input voltage V_IN and the output voltage V_OUT in such a way to satisfy Equation (4).

The present disclosure extends to various apparatuses and methods understood as a block diagram or a circuit diagram of FIG. 1 or derived from the description above and is not limited to a specific configuration. Hereinafter, more specific configuration examples and embodiments will be described not in order to narrow the scope of the present invention but to be of help in understanding the essence and operation of the invention and to clarify them.

Next, a configuration of the timer circuit 320 will be described based on some embodiments.

FIG. 3 is a circuit diagram illustrating a basic configuration of the timer circuit 320. The timer circuit 320 includes a first capacitor C11, a current source CS1, a comparator 322, and a threshold voltage generation circuit 330.

The current source CS1 is connected to the first capacitor C11 and generates a current I (∝V_OUT) proportional to V_OUT. For example, the current source CS1 may be a V/I conversion circuit. The comparator 322 monitors the voltage V_C11 between both ends of the first capacitor C11 and detects that a voltage change proportional to (V_OUT−V_IN) has occurred.

In FIG. 3, one end of the first capacitor C11 is grounded. The threshold voltage generation circuit 330 generates a threshold voltage V_TH∝(V_OUT−V_IN) proportional to (V_OUT−V_IN). The comparator 322 compares the voltage V_C11 at the other end of the first capacitor C11 with the threshold voltage V_TH. A switch SW1 is connected in parallel with the first capacitor C11 and is controlled according to the start signal STARTX.

FIG. 4 is an operation waveform diagram of the timer circuit 320 of FIG. 3. Before time to, the start signal STARTX is high, and the voltage V_C11 of the first capacitor C11 is 0 V. When the start signal STARTX transitions from high to low at the time to, the first capacitor C11 is charged by the current I generated by the current source CS1, and the voltage V_C11 of the first capacitor C11 rises with a slope proportional to the current I.

I=αV _OUT

The voltage V_C11 of the capacitor after a lapse of time t from the time to is expressed by Equation (5).

V _C11 =αV _OUT ×t/C11  (5)

It is assumed that the threshold voltage V_TH is V_TH=β×(V_OUT−V_IN). When the time until a capacitor voltage V_C11 reaches the threshold voltage V_TH is represented by τ, Equation (6) holds.

αV_OUT×τ/C11=β×(V_OUT−V_IN)  (6)

When this is solved for τ, Equation (7) is obtained.

τ=α/β×C11×(V_OUT−V_IN)/V_OUT  (7)

Therefore, according to the timer circuit 320 of FIG. 2, it is possible to generate the turn-off signal S2 that changes after a time T proportional to (V_OUT−V_IN)/V_OUT elapses after the start signal STARTX changes. By driving the DC/DC converter 100 with the time T as the ON time T_ON, the switching frequency can be stabilized.

Example 1.1

FIG. 5 is a circuit diagram of a timer circuit 320A according to Example 1.1. The threshold voltage generation circuit 330A includes a second capacitor C12.

The threshold voltage generation circuit 330A charges the second capacitor C12 with (V_OUT−V_IN) in the OFF state φ_OFF of the switching transistor M1. In addition, in the ON state φ_ON of the switching transistor M1, the threshold voltage generation circuit 330A applies the voltage (switching voltage) V_SW of the switching pin SW, which is the connection node between the inductor L1 and the switching transistor M1, to one end of the second capacitor C12, and supplies the voltage at the other end of the second capacitor C12 to the comparator 322 as the threshold voltage V_TH.

For example, the threshold voltage generation circuit 330 incudes a first selector 332 and a second selector 334 in addition to the second capacitor C12. The first selector 332 applies the input voltage V_IN to one end of the second capacitor C12 in the OFF state φ_OFF of the switching transistor M1 and connects one end of the second capacitor C12 to the switching pin SW of the DC/DC converter 100 in the ON state φ_ON of the switching transistor M1.

The second selector 334 applies the output voltage V_OUT to the other end of the second capacitor C12 in the OFF state of the switching transistor M1 and connects the other end of the second capacitor C12 to the comparator 322 in the ON state φ_ON of the switching transistor M1.

The above is the configuration of the timer circuit 320A. FIG. 6 is an operation waveform diagram of the timer circuit 320A of FIG. 5. Before the time to, the OFF state φ_OFF is set, and the second capacitor C12 is charged with (V_OUT−V_IN).

At the time to, the state changes to the ON state φ_ON. When the switch SW1 is turned off in response to the start signal STARTX, charging of the first capacitor C11 is started, and the capacitor voltage V_C11 rises with a slope proportional to the output voltage V_OUT.

Since a potential difference of the second capacitor C12 is maintained during the ON state (porn, the threshold voltage V_TH is calculated as

$\begin{array}{cc}\begin{array}{c}{V}_{\mathrm{TH}}=\left(V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right)+V_{\mathrm{SW}}\\ =\left(V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right)+R_{\mathrm{ON}1}·I_L\end{array}.& \left(8\right)\end{array}$

Therefore, the ON time T_ON generated by the timer circuit 320A is expressed as

T _ON =C11/α×{(V_OUT−V_IN)+R_ON1·I_L}/V_OUT  (9).

As described above, according to the timer circuit 320A of FIG. 5, the ON time T_ON in consideration of a coil current I_L (that is, a load current) and an ON resistance R_ON1 of the switching transistor M1 can be generated.

Furthermore, since a low-pass filter is unnecessary as in Example 1.2 and Example 1.3 to be described later, mounting can be performed in a small circuit area.

Example 1.2

FIG. 7 is a circuit diagram of a timer circuit 320B according to Example 1.2. The timer circuit 320B is different from the threshold voltage generation circuit 330A of FIG. 5 in the configuration of a threshold voltage generation circuit 330B.

The threshold voltage generation circuit 330B includes an inverter 336 and a low-pass filter 338. The inverter 336 inverts the switching voltage V_SW generated in the switching pin SW. The output voltage V_OUT is supplied to the power supply terminal of the inverter 336, and thus an amplitude of the output signal of the inverter 336 is equal to the output voltage V_OUT.

The low-pass filter 338 smooths the output of the inverter 336 and generates the threshold voltage V_TH. For example, the low-pass filter 338 can be configured by an RC filter.

FIG. 8 is an operation waveform diagram of the timer circuit 320B of FIG. 7. The output of the low-pass filter 338 is expressed by Equation (10).

V _TH =V _OUT ×d  (10)

d is the duty cycle of the first pulse signal Sp1. Since Equation (4) is established in the steady state of the continuous current mode, Equation (11) is obtained from Equations (4) and (10).

V _TH =V _OUT ×{V _OUT −V _IN }/V _OUT =V _OUT −V _IN

In other words, the threshold voltage V_TH proportional to V_OUT−V_IN can be generated.

Example 1.3

In Example 1.2, during the continuous current mode, Equation (4) holds, but in the discontinuous current mode in which Equation (4) does not hold, the threshold voltage V_TH deviates from an appropriate voltage level. Accordingly, a frequency fluctuation rises immediately after the transition from the discontinuous current mode to the continuous current mode. In Embodiment 1.3, a configuration for solving this problem will be described.

FIG. 9 is a circuit diagram of a timer circuit 320C according to Example 1.3. A threshold voltage generation circuit 330C is configured to charge a capacitor C of an RC filter 338 with V_OUT−V_IN while operating in the discontinuous current mode. Specifically, during the discontinuous current mode, V_OUT is applied to one end of the capacitor C, and the input voltage V_IN is applied to the other end thereof. For example, the threshold voltage generation circuit 330C includes a third selector 340 and a fourth selector 342 in addition to the inverter 336 and the low-pass filter 338.

The third selector 340 applies the output voltage of the inverter 336 to one end of the resistor R during a continuous current mode φ_CCM and applies the output voltage V_OUT to one end of the resistor R during a discontinuous current mode φ_DCM. Also, the fourth selector 342 applies a ground voltage of 0 V to the other end of the capacitor C during the continuous current mode φ_CCM and applies an input voltage V_IN to the other end of the capacitor C during the discontinuous current mode φ_DCM.

Accordingly, the voltage between both ends of the capacitor C is maintained at V_OUT V_IN during the discontinuous current mode φ_DCM, so that the operation can be resumed from the appropriate threshold voltage V_TH when the mode is transitioned to the continuous current mode φ_CCM next.

Example 1.4

FIG. 10 is a circuit diagram of a timer circuit 320D according to Example 1.4. The timer circuit 320D includes the comparator 322, the current source CS1, the capacitor C11, and the switch SW1. An input voltage V_IN is applied to one end of the capacitor C11.

When the switch SW1 is in the ON state, the capacitor voltage V_C11 is equal to the input voltage V_IN. When the switch SW1 is turned off, the capacitor voltage V_C11 rises with a slope proportional to the output voltage V_OUT with the input voltage V_IN as an initial value. The comparator 322 compares the capacitor voltage V_C11 with the output voltage V_OUT. An output S2 of the comparator 322 transitions in level when the capacitor voltage V_C11 changes by V_OUT V_IN.

In this configuration, since R_ON and R_DC are ignored, the switching frequency becomes faster in a heavy load state where I_L is large, but the switching frequency can be stabilized with a simple configuration.

Modifications related to the first embodiment will be described.

Modification 1.1

In the first embodiment, the switching transistor M1 and the synchronous rectification transistor M2 are integrated in the control circuit 300, but the present invention is not limited thereto, and the switching transistor M1 and the synchronous rectification transistor M2 may be external discrete elements. In addition, the synchronous rectification transistor M2 may be an N-channel MOSFET, and in that case, a bootstrap circuit may be added to the second driver 316.

Second Embodiment

As a result of studying overcurrent protection in a converter of the ripple control of bottom detection and constant on time (COT), the present inventors have recognized the following problems.

The overcurrent protection monitors a current flowing through the switching transistor (or the inductor) during an ON period of the switching transistor and turns off the switching transistor when a threshold value of the overcurrent is exceeded.

In general, in the overcurrent state, a drooping characteristic in which an output voltage decreases is required. However, in the COT method of bottom detection, since the bottom of the output voltage is maintained at the reference voltage even in the overcurrent state, the drooping characteristic cannot be obtained.

In the second embodiment, a DC/DC converter capable of lowering the output voltage in the overcurrent state and a control circuit thereof will be described.

FIG. 11 is a circuit diagram of the DC/DC converter 100 according to the second embodiment. The DC/DC converter 100 is a boost converter, boosts an input voltage V_IN of an input line (an input terminal) 102, stabilizes the input voltage to a predetermined voltage level, and supplies the voltage to a load 4 connected to an output line (output terminal) 104.

The DC/DC converter 100 includes the output circuit 110 and a control circuit 400. The output circuit 110 includes an inductor L1, a switching transistor (a low-side transistor) M1, a synchronous rectification transistor (a high-side transistor) M2, and an output capacitor C1.

The control circuit 400 is a controller of a ripple control method, more specifically, a bottom detection method, and includes the switching pin SW and the output pin OUT. An external inductor L1 is connected to the switching pin SW, and an external output capacitor C1 and the output line 104 are connected to the output pin OUT.

The control circuit 400 includes a voltage dividing circuit 402, a main comparator 408, a logic circuit 412, a first driver 414, a second driver 416, an ON time generation circuit 420, an overcurrent detection circuit 450, a turn-on inhibition circuit 460, the switching transistor M1, and the synchronous rectification transistor M2, and is an integrated circuit (IC) integrated on one semiconductor substrate.

The voltage dividing circuit 402 includes the resistors R11 and R12 and divides the output voltage V_OUT to generate the feedback voltage V_FB.

The main comparator 408 compares the feedback voltage V_FB corresponding to the output voltage V_OUT of the DC/DC converter 100 with the reference voltage V_REF and asserts the turn-on signal TURN_ON when the feedback voltage V_FB falls below the reference voltage V_REF. The turn-on signal TURN_ON is a pulse signal indicating a magnitude relationship between V_FB and V_REF, and one of the positive edge and the negative edge can be associated with the assertion.

The ON time generation circuit 420 generates a turn-off signal TURN_OFF to be asserted after the elapse of the ON time T_ON from the turn-on of the switching transistor M1. The ON time T_ON may be a predetermined constant time or may be adaptively controlled according to the state of the DC/DC converter 100. The turn-off signal TURN_OFF is a trigger for turning off the switching transistor M1.

The turn-on inhibition circuit 460 asserts a turn-on inhibition signal TURNON_DIS_B for a period until a predetermined time (referred to as the minimum period) Tp_(MIN) elapses from the assertion of a start signal START indicating the turn-on of the switching transistor M1. In the present description, _B represents a negative logic, where assertion is assigned low, and negation is assigned high. For example, the start signal START is a signal indicating that the first pulse signal Sp1 instructing on/off of the switching transistor M1 has transitioned to the ON level.

The ON time generation circuit 420 and the turn-on inhibition circuit 460 can be constituted by a timer circuit. The logic circuit 412 supplies the start signal START that triggers operation start to the ON time generation circuit 420 and the turn-on inhibition circuit 460. The start signal START is a signal indicating a turn-on of the switching transistor M1. The start signal START may be the first pulse signal Sp1.

When the current flowing through the switching transistor M1 exceeds an overcurrent threshold value I_OCP in the ON period of the switching transistor M1, the overcurrent detection circuit 450 asserts (for example, high) an overcurrent detection signal OCP.

The logic circuit 412 generates the pulse signals Sp1 and Sp2 instructing on and off of the switching transistor M1 and the synchronous rectification transistor M2 based on the turn-on signal TURN_ON, the turn-off signal TURN_OFF, the turn-on inhibition signal TURNON_DIS_B, and the overcurrent detection signal OCP.

When the turn-on signal TURN_ON is asserted in a period in which the turn-on inhibition signal TURNON_DIS_B is negated (high), the logic circuit 412 causes the first pulse signal Sp1 to transition to the ON level (high). During a period in which the turn-on inhibition signal TURNON_DIS_B is asserted (low), even when the turn-on signal TURN_ON is asserted, the first pulse signal Sp1 maintains the OFF level (low).

When the turn-off signal TURN_OFF or the overcurrent detection signal OCP is asserted, the logic circuit 412 causes the first pulse signal Sp1 to transition to the OFF level (low).

In the continuous current mode (CCM), the logic circuit 412 complementarily changes the second pulse signal Sp2 with the first pulse signal Sp1. In the discontinuous current mode (DCM), the zero crossing of the current flowing through the synchronous rectification transistor M2 is detected, and the OFF levels of both the first pulse signal Sp1 and the second pulse signal Sp2 are maintained from the current zero crossing to the assertion of the next turn-on signal TURN_ON.

The above is the configuration of the control circuit 400. Next, the operation will be described.

FIG. 12 is an operation waveform diagram of the DC/DC converter 100 of FIG. 11 in the normal state (the non-overcurrent state). Here, the operation is performed in the continuous mode. The ON time T_ON generated by the ON time generation circuit 420 is determined such that the switching cycle in the normal state is longer than the minimum cycle Tp_(MIN) generated by the turn-on inhibition circuit 460. FIG. 12 illustrates a waveform in which a circuit delay is ignored.

When the feedback voltage V_FB decreases to the reference voltage V_REF at the time to, the turn-on signal TURN_ON is asserted. At this timing, since the turn-on inhibition signal TURNON_DIS_B is negated (high), the first pulse signal Sp1 transitions to the ON level in response to the assertion of the turn-on signal TURN_ON. The second pulse signal Sp2 transitions complementarily with the first pulse signal Sp1.

When the switching transistor M1 is turned on at the time to, the ON time generation circuit 420 starts clocking, and the turn-off signal TURN_OFF is asserted at time t₁ after the ON time T_ON has elapsed. In this way, the first pulse signal Sp1 transitions to the OFF level, and the switching transistor M1 is turned off. Thereafter, when the feedback voltage V_FB decreases to the reference voltage V_REF at time t₂, the turn-on signal TURN_ON is asserted. This operation is repeated in the normal state.

FIG. 13 is an operation waveform diagram of the DC/DC converter 100 of FIG. 11 in the overcurrent state. When the feedback voltage V_FB falls below the reference voltage V_REF at the time to, the turn-on signal TURN_ON is asserted (low). At this timing, since the turn-on inhibition signal TURNON_DIS_B is negated (high), the first pulse signal Sp1 transitions to the ON level in response to the assertion of the turn-on signal TURN_ON. The second pulse signal Sp2 transitions complementarily with the first pulse signal Sp1.

When the switching transistor M1 is turned on at the time to, the ON time generation circuit 420 and the turn-on inhibition circuit 460 start clocking, and the turn-on inhibition signal TURNON_DIS_B is asserted (low). In the overcurrent state, the overcurrent detection signal OCP is asserted at the time t₁ before the ON time T_ON elapses, the first pulse signal Sp1 transitions to the OFF level in response to the overcurrent detection signal OCP, and the switching transistor M1 is turned off. This provides pulse-by-pulse overcurrent protection.

The feedback voltage V_FB increases immediately after the switching transistor M1 is turned off, and then decreases with time. When the voltage falls below the reference voltage V_REF at the time t₂, the turn-on signal TURN_ON is asserted. However, at the time t₂, since the minimum period Tp_(MIN) has not elapsed from the time to at which the immediately preceding switching transistor M1 is turned on, the turn-on inhibition signal TURNON_DIS_B is asserted. Therefore, the first pulse signal Sp1 maintains the OFF level.

Thereafter, when the turn-on inhibition signal TURNON_DIS_B is negated at time t₃ after the minimum period Tp_(MIN) has elapsed from the time t₁, the first pulse signal Sp1 transitions to the ON level in response to the turn-on signal TURN_ON already asserted (low) at that time, and the switching transistor M1 is turned on.

When the first pulse signal Sp1 transitions to the ON level at the time t₃, the turn-on inhibition signal TURNON_DIS_B is asserted (low), and the turn-on inhibition circuit 460 starts clocking. When the overcurrent detection signal OCP is asserted at time t₄, the first pulse signal Sp1 becomes the OFF level, and the switching transistor M1 is turned off.

In the overcurrent state, the operations from t₁ to t₄ are repeated. The above is the operation of the DC/DC converter 100.

According to the DC/DC converter 100, in the overcurrent state, the switching frequency can be maintained at 1/Tp_(MIN) while the pulse-by-pulse overcurrent protection is performed. In addition, in the overcurrent state, the feedback voltage V_FB, that is, the output voltage V_OUT can be decreased with time, and the drooping characteristic can be realized.

The present invention extends to various apparatuses and methods understood as the block diagram or cross-sectional diagram of FIG. 11 or derived from the description above and is not limited to a specific configuration. Hereinafter, more specific configuration examples and embodiments will be described not in order to narrow the scope of the present invention but to be of help in understanding the essence and operation of the invention and to clarify them.

A specific configuration example of the control circuit 400 will be described.

FIG. 14 is an equivalent circuit diagram of the output circuit 110 of the DC/DC converter 100. R_DC is an equivalent series resistance such as the inductor L1 and wiring. R_ON1 represents the ON resistance of the switching transistor M1, and R_ON2 represents the ON resistance of the synchronous rectification transistor M2.

A switching cycle is defined as T. In an ON state φ_ON of the switching transistor M1, I_L=I_M1, and the voltage across the inductor L1 is {V_IN−(R_ON1+R_DC)×I_L}. Accordingly, an increase in width ΔI_ON of an inductor current I_L in the ON state φ_ON is expressed by Equation (1). T_ON is a length of the ON state and is referred to as the ON time.

ΔI_ON=T_ON/L×{V_IN−(R_ON1+R_DC)×I_L}  (1)

In the OFF state φ_OFF of the switching transistor M1, I_L=I_M2, and the voltage across the inductor L1 is {V_OUT+(R_ON1+R_DC)×V_IN}. Accordingly, a decrease in width ΔI_OFF of the inductor current I_L in the OFF state φ_OFF is expressed by Equation (2).

ΔI_OFF=(T−T_ON)/L×{V_OUT+(R_ON2+R_DC)×I_L−V_IN}  (2)

When the output voltage V_OUT is stabilized in the continuous current mode, ΔI_ON=ΔI_OFF holds. Accordingly, a duty cycle d is expressed by Equation (3).

$\begin{array}{cc}\begin{array}{c}d=T_{\mathrm{ON}}/T\\ =\left\{V_{\mathrm{OUT}}-V_{\mathrm{IN}}+\left(R_{\mathrm{ON}2}+R_{\mathrm{DC}}\right)\times I_L\right\}/\left\{V_{\mathrm{OUT}}-\left(R_{\mathrm{ON}1}-R_{\mathrm{ON}2}\right)\times I_L\right\}\end{array}& \left(3\right)\end{array}$

Assuming R_ON1=R_ON2=R_DC=0, Equation (4) is obtained.

$\begin{array}{cc}\begin{array}{c}d=T_{\mathrm{ON}}/T\\ =\left\{V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right\}/V_{\mathrm{OUT}}\end{array}& \left(4\right)\end{array}$

Therefore, when a target period in a non-overcurrent state is Tp_(REF), the ON time generation circuit 420 of the control circuit 400 calculates to generate the ON time T_ON satisfying the equation below

T _ON ={V _OUT −V _IN }/V _OUT ×T _REF.

This makes it possible to keep the switching frequency of the DC/DC converter 100 constant.

Next, a configuration of the ON time generation circuit 420 will be described based on some embodiments.

FIG. 15 is a circuit diagram illustrating a basic configuration of the ON time generation circuit 420. The ON time generation circuit 420 includes the first capacitor C11, the current source CS1, a comparator 422, and a threshold voltage generation circuit 430.

The current source CS1 is connected to the first capacitor C11 and generates a current I (∝V_OUT) proportional to V_OUT. For example, the current source CS1 may be a V/I conversion circuit. The comparator 422 monitors the voltage V_C11 between both ends of the first capacitor C11 and detects that the voltage change proportional to (V_OUT−V_IN) has occurred.

In FIG. 15, one end of the first capacitor C11 is grounded. The threshold voltage generation circuit 430 generates the threshold voltage V_TH∝(V_OUT−V_IN) proportional to (V_OUT−V_IN). The comparator 422 compares the voltage V_C11 at the other end of the first capacitor C11 with the threshold voltage V_TH. The switch SW1 is connected in parallel with the first capacitor C11 and is controlled according to a start signal START_B.

FIG. 16 is an operation waveform diagram of the ON time generation circuit 420 of FIG. 15. Before time to, the start signal START_B is high, and the voltage V_C11 of the first capacitor C11 is 0 V. When the start signal START_B transitions from high to low at the time to, the first capacitor C11 is charged by the current I generated by the current source CS1, and the voltage Von of the first capacitor C11 rises with a slope proportional to the current I.

I=αV _OUT

The voltage V_C11 of the capacitor after a lapse of time t from the time to is expressed by Equation (5).

V _C11 =αV _OUT ×t/C11  (5)

It is assumed that the threshold voltage V_TH is V_TH=β×(V_OUT−V_IN). When the time until a capacitor voltage V_C11 reaches the threshold voltage V_TH is represented by τ, Equation (6) holds.

αV_OUT×τ/C11=β×(V_OUT−V_IN)  (6)

When this is solved for τ, Equation (7) is obtained.

τ=α/β×C₁₁×(V_OUT−V_IN)/V_OUT  (7)

Therefore, according to the ON time generation circuit 420 in FIG. 15, it is possible to generate the turn-off signal TURN_OFF that changes after the lapse of the time T proportional to (V_OUT−V_IN)/V_OUT after the start signal START_B changes. By driving the DC/DC converter 100 with the time T as the ON time T_ON, the switching frequency can be stabilized.

Example 2.1

FIG. 17 is a circuit diagram of an ON time generation circuit 420A according to Example 2.1. The threshold voltage generation circuit 430A includes the second capacitor C12.

The threshold voltage generation circuit 430A charges the second capacitor C12 with (V_OUT−V_IN) in the OFF state φ_OFF of the switching transistor M1. In addition, in the ON state φ_ON of the switching transistor M1, the threshold voltage generation circuit 430A applies the voltage (switching voltage) V_SW of the switching pin SW, which is the connection node between the inductor L1 and the switching transistor M1, to one end of the second capacitor C12, and supplies the voltage at the other end of the second capacitor C12 to the comparator 422 as the threshold voltage V_TH.

For example, the threshold voltage generation circuit 430 includes a first selector 432 and a second selector 434 in addition to the second capacitor C12. The first selector 432 applies the input voltage V_IN to one end of the second capacitor C12 in the OFF state φ_OFF of the switching transistor M1 and connects one end of the second capacitor C12 to the switching pin SW of the DC/DC converter 100 in the ON state φ_ON of the switching transistor M1.

The second selector 434 applies the output voltage V_OUT to the other end of the second capacitor C12 in the OFF state of the switching transistor M1 and connects the other end of the second capacitor C12 to the comparator 422 in the ON state φ_ON of the switching transistor M1.

The above is the configuration of the ON time generation circuit 420A. FIG. 18 is an operation waveform diagram of the ON time generation circuit 420A in FIG. 17. Before the time to, the OFF state φ_OFF is set, and the second capacitor C12 is charged with (V_OUT−V_IN).

At the time to, the state changes to the ON state φ_ON. When the switch SW1 is turned off in response to the start signal START_B, charging of the first capacitor C11 is started, and the capacitor voltage V_C11 rises with a slope proportional to the output voltage V_OUT.

Since a potential difference of the second capacitor C12 is maintained during the ON state φ_ON, the threshold voltage V_TH is calculated as

$\begin{array}{cc}\begin{array}{c}{V}_{\mathrm{TH}}=\left(V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right)+V_{\mathrm{SW}}\\ =\left(V_{\mathrm{OUT}}-V_{\mathrm{IN}}\right)+R_{\mathrm{ON}1}·I_L\end{array}.& \left(8\right)\end{array}$

Therefore, the ON time T_ON generated by the ON time generation circuit 420A is expressed as follows.

T _ON =C11/α×{(V_OUT−V_IN)+R_ON1·I_L}/V_OUT  (9).

As described above, according to the ON time generation circuit 420A of FIG. 17, the ON time T_ON in consideration of the coil current I_L (that is, the load current) and an ON resistance R_ON1 of the switching transistor M1 can be generated.

In addition, since a low-pass filter is unnecessary as in Example 2.2 and Example 2.3 to be described later, mounting can be performed in a small circuit area.

Example 2.2

FIG. 19 is a circuit diagram of an ON time generation circuit 420B according to Example 2.2. The ON time generation circuit 420B is different from the threshold voltage generation circuit 430A of FIG. 17 in the configuration of the threshold voltage generation circuit 430B.

The threshold voltage generation circuit 430B includes an inverter 436 and a low-pass filter 438. The inverter 436 inverts the switching voltage V_SW generated in the switching pin SW. The output voltage V_OUT is supplied to the power supply terminal of the inverter 436, and thus an amplitude of the output signal of the inverter 436 is equal to the output voltage V_OUT.

The low-pass filter 438 smooths the output of the inverter 436 and generates the threshold voltage V_TH. For example, the low-pass filter 438 can be configured by the RC filter.

FIG. 20 is an operation waveform diagram of the ON time generation circuit 420B in FIG. 19. The output of the low-pass filter 438 is expressed by Equation (10).

V _TH =V _OUT ×d  (10)

d is the duty cycle of the first pulse signal Sp1. Since Equation (4) is established in the steady state of the continuous current mode, Equation (11) is obtained from Equations (4) and (10).

V _TH =V _OUT ×{V _OUT −V _IN }/V _OUT =V _OUT −V _IN

In other words, the threshold voltage V_TH proportional to V_OUT−V_IN can be generated.

Example 2.3

In Example 2.2, Equation (4) holds during the continuous current mode, but in the discontinuous current mode in which Equation (4) does not hold, the threshold voltage V_TH deviates from an appropriate voltage level. Accordingly, a frequency fluctuation rises immediately after the transition from the discontinuous current mode to the continuous current mode. In Example 2.3, a configuration for solving this problem will be described.

FIG. 21 is a circuit diagram of an ON time generation circuit 420C according to Example 2.3. A threshold voltage generation circuit 430C is configured to charge the capacitor C of an RC filter 438 with V_OUT−V_IN while operating in the discontinuous current mode. Specifically, during the discontinuous current mode, V_OUT is applied to one end of the capacitor C, and the input voltage V_IN is applied to the other end thereof. For example, the threshold voltage generation circuit 430C includes a third selector 440 and a fourth selector 442 in addition to the inverter 436 and the low-pass filter 438.

The third selector 440 applies the output voltage of the inverter 436 to one end of the resistor R during a continuous current mode φ_CCM and applies the output voltage V_OUT to one end of the resistor R during a discontinuous current mode φ_DCM. Also, the fourth selector 442 applies a ground voltage of 0 V to the other end of the capacitor C during the continuous current mode φ_CCM and applies an input voltage V_IN to the other end of the capacitor C during the discontinuous current mode φ_DCM.

Accordingly, the voltage between both ends of the capacitor C is maintained at V_OUT V_IN during the discontinuous current mode φ_DCM, so that the operation can be resumed from the appropriate threshold voltage V_TH when the mode is transitioned to the continuous current mode φ_CCM next.

Example 2.4

FIG. 22 is a circuit diagram of an ON time generation circuit 420D according to Example 2.4. The ON time generation circuit 420D includes the comparator 422, the current source CS1, the capacitor C11, and the switch SW1. An input voltage V_IN is applied to one end of the capacitor C11.

When the switch SW1 is in the ON state, the capacitor voltage V_C11 is equal to the input voltage V_IN. When the switch SW1 is turned off, the capacitor voltage V_C11 rises with a slope proportional to the output voltage V_OUT with the input voltage V_IN as an initial value. The comparator 422 compares the capacitor voltage V_C11 with the output voltage V_OUT. The output TURN_OFF of the comparator 422 transitions in level when the capacitor voltage V_C11 changes by V_OUT−V_IN.

In this configuration, since R_ON and R_DC are ignored, the switching frequency becomes faster in a heavy load state where I_L is large, but the switching frequency can be stabilized with a simple configuration.

Hereinafter, modifications related to the second embodiment will be described.

Modification 2.1

In the second embodiment, the switching transistor M1 and the synchronous rectification transistor M2 are integrated in the control circuit 400, but the present invention is not limited thereto, and the switching transistor M1 and the synchronous rectification transistor M2 may be external discrete elements. In addition, the synchronous rectification transistor M2 may be an N-channel MOSFET, and in that case, a bootstrap circuit may be added to the second driver 416.

Modification 2.2

Although the boost converter has been described in the second embodiment, the present invention is also applicable to a buck converter and a boost-buck converter.

The embodiments merely illustrate the principle and application of the present invention, and many modifications and changes in configuration can be made to the embodiments without departing from the gist of the present invention defined in the claims.

Claims

1. A control circuit of a DC/DC converter that boosts an input voltage VIN and generates an output voltage VOUT, the control circuit comprising: a main comparator structured to compare a feedback voltage corresponding to an output voltage of the DC/DC converter with a reference voltage, and to assert a turn-on signal when the feedback voltage falls below the reference voltage; anda timer circuit structured to generate a turn-off signal that transitions in level after an ON time that is proportional to (VOUT−VIN)/VOUT has elapsed from assertion of the turn-on signal.

2. The control circuit according to claim 1, wherein the timer circuit includesa first capacitor;a current source structured to be connected to the first capacitor and to generate a current proportional to VOUT; anda comparator structured to detect that a voltage change proportional to (VOUT−VIN) has occurred in the first capacitor.

3. The control circuit according to claim 2, wherein one end of the first capacitor is grounded,the timer circuit further includes a threshold voltage generation circuit structured to generate a threshold voltage according to (VOUT−VIN), andthe comparator is structured to compare a voltage of the other end of the first capacitor with the threshold voltage.

4. The control circuit according to claim 3, wherein the threshold voltage generation circuit includes a second capacitor, charges the second capacitor with (VOUT−VIN) during an OFF period of a switching transistor, applies a switching voltage of a connection node between an inductor and the switching transistor to one end of the second capacitor during an ON period of the switching transistor, and sets a voltage of the other end of the second capacitor as the threshold voltage.

5. The control circuit according to claim 3, wherein the threshold voltage generation circuit includes:a second capacitor;a first selector structured to apply the input voltage VIN to one end of a second capacitor during an OFF period of a switching transistor and to connect the one end of the second capacitor with an inductor of the DC/DC converter and a connection node of the switching transistor during an ON period of the switching transistor; anda second selector structured to apply the output voltage VOUT to the other end of the second capacitor during the OFF period of the switching transistor, and to connect the other end of the second capacitor to the comparator during the ON period of the switching transistor.

6. The control circuit according to claim 3, wherein the threshold voltage generation circuit includes:an inverter structured to invert a switching voltage generated at a connection node between an inductor of the DC/DC converter and a switching transistor; anda filter structured to smooth an output of the inverter and to generate the threshold voltage.

7. The control circuit according to claim 6, wherein the filter is an RC filter, and the threshold voltage generation circuit charges a capacitor of the RC filter with VOUT−VIN while operating in a discontinuous current mode.

8. The control circuit according to claim 6, wherein the filter is an RC filter including a resistor and a capacitor, andthe threshold voltage generation circuit further includes:a third selector structured to apply the output voltage of the inverter to one end of the resistor during a continuous current mode, and to apply the output voltage VOUT to the one end of the resistor during a discontinuous current mode; anda fourth selector structured to apply a ground voltage to the other end of the capacitor during the continuous current mode, and to apply the input voltage VIN to the other end of the capacitor during the discontinuous current mode.

9. The control circuit according to claim 2, wherein the input voltage VIN is applied to one end of the first capacitor, and the comparator compares a voltage of the other end of the first capacitor with the output voltage VOUT.

10. The control circuit according to claim 1, wherein the control circuit is integrally integrated on one semiconductor substrate.

11. A power supply circuit comprising: an output circuit of a boost DC/DC converter, andthe control circuit according to claim 1.

12. A control circuit of a DC/DC converter including a switching transistor, the control circuit comprising: a main comparator structured to compare a feedback voltage corresponding to an output voltage of the DC/DC converter with a reference voltage, and to assert a turn-on signal when the feedback voltage falls below the reference voltage;an ON time generation circuit structured to assert a turn-off signal after an ON time elapses from turn-on of the switching transistor;an overcurrent detection circuit structured to assert an overcurrent detection signal when a current flowing through the switching transistor exceeds an overcurrent threshold value in an ON period of the switching transistor;a turn-on inhibition circuit structured to generate a turn-on inhibition signal to be asserted during a period from turn-on of the switching transistor to elapse of a predetermined time;a logic circuit structured to generate a pulse signal that transitions to an ON level when the turn-on signal is asserted and transitions to an OFF level when the turn-off signal or the overcurrent detection signal is asserted, during a period in which the turn-on inhibition signal is negated; anda driver structured to drive the switching transistor according to the pulse signal.

13. The control circuit according to claim 12, wherein the DC/DC converter may be a boost type that boosts an input voltage VIN and generates an output voltage VOUT.

14. The control circuit according to claim 13, wherein the ON time is proportional to (VOUT−VIN)/VOUT.

15. The control circuit according to claim 12, wherein the control circuit is integrally integrated on one semiconductor substrate.

16. A power supply circuit comprising: an output circuit of a DC/DC converter; andthe control circuit according to claim 12.

17. A control method for a DC/DC converter including a switching transistor, the method comprising: comparing a feedback voltage corresponding to an output voltage of the DC/DC converter with a reference voltage, and asserting a turn-on signal when the feedback voltage falls below the reference voltage;asserting the turn-off signal after an ON time elapses from turn-on of the switching transistor;asserting an overcurrent detection signal when a voltage flowing through the switching transistor exceeds an overcurrent threshold value in an ON period of the switching transistor;asserting a turn-on inhibition signal in a period from the turn-on of the switching transistor until elapse of a predetermined time;transitioning a pulse signal to an ON level when a turn-on signal is asserted in a period in which the turn-on inhibition signal is negated;transitioning the pulse signal to an OFF level when a turn-off signal or the overcurrent detection signal is asserted; anddriving the switching transistor corresponding to the pulse signals.