Patent Application
20130200873
The present disclosure relates generally to electronic circuits. More particularly, the disclosure relates to methods and apparatuses for a soft-start function with auto-disable.
In electronic applications, improper start-up conditions for voltages and/or currents may damage circuit components. For instance, in electronic applications that make use of DC-DC voltage converters, improper start-up conditions can damage circuits powered by such DC-DC voltage converters and/or damage the DC-DC voltage converters themselves. Many circuits have a maximum limit on rate of supply voltage ramp-up, which dictates a maximum rate at which a supplied voltage powering the circuit can rise to a target level during a start-up phase. Similarly, many circuits have a maximum limit on voltage that can be applied. If the rate were to be higher than the maximum limit of a particular circuit, the voltage may overshoot and a particular circuit under consideration can be damaged.
According to a first aspect, a circuital arrangement is provided, wherein the circuital arrangement is adapted to be coupled to a reference voltage generator, the reference voltage generator adapted to generate a reference voltage when not coupled to the circuital arrangement, the circuital arrangement comprising: a ramp generator, wherein, during operation, the ramp generator is configured to generate a ramp voltage that increases in magnitude over time; a control loop comprising a comparator coupled with a first switching element on one end and the ramp generator on another end, wherein, during operation: the comparator is configured to receive the ramp voltage and a soft-start voltage, the comparator is configured to apply a comparator output voltage to the first switching element, and the comparator output voltage gradually turns off the first switching element as the ramp voltage increases toward the reference voltage, wherein the soft-start voltage is a function of the ramp voltage, the comparator output voltage, and the reference voltage.
According to a second aspect, a method for increasing a soft-start voltage from a first voltage to a second voltage is provided, the second voltage being substantially equal in value to a reference voltage, wherein the reference voltage is adapted to be generated via a reference voltage generator, the method comprising: generating a ramp voltage via a ramp generator, wherein, during operation of the ramp generator, the ramp voltage increases in magnitude over time; providing a control loop, wherein the control loop comprises a comparator coupled with a first switching element on one end and the ramp generator on another end; generating a comparator output voltage via the comparator based on the ramp voltage and the soft-start voltage; providing the first switching element, wherein: the first switching element is adapted to receive the comparator output voltage, and the comparator output voltage gradually turns off the first switching element as the ramp voltage increases toward the reference voltage; providing the first voltage as the soft-start voltage; increasing the soft-start voltage toward the second voltage when the ramp voltage is less than the reference voltage; and retaining the soft-start voltage at the second voltage when the ramp voltage is greater than the reference voltage, thus increasing the soft-start voltage from the first voltage to the second voltage.
Further embodiments are provided in the specification, drawings, and claims of the present application.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description of example embodiments, serve to explain the principles and implementations of the disclosure.
A “soft-start” circuit can be used to limit rate at which a voltage and/or current is output from the soft-start circuit. A soft-start circuit can be implemented with or as part of a DC-DC converter, where the soft-start circuit can be used to limit rate at which voltage of the DC-DC converter rises and/or rate of in-rush of current at start-up to circuits powered by the DC-DC converter. The rate at which voltage rises can be referred to as voltage ramp-up and similarly rate at which current rises can be referred to as current ramp-up.
According to many embodiments of the present disclosure, one or both of the voltage and current ramp-up can be controlled to allow a smooth transition from start-up to when the voltage or current output from the soft-start circuit reaches regulation. “Reaching regulation” refers to a state where a soft-start circuit has reached a target voltage or current (also referred to as a steady-state voltage or current).
Aside from DC-DC converters, soft-start circuits can be utilized in, for instance, a linear regulator such as a low-dropout regulator in order to control rate of voltage increase and prevent voltage overshoot. Other devices identifiable by a person skilled in the art can utilize the limitations on voltage ramp-up and/or current ramp-up that can be realized with soft-start circuits.
As used herein, the terms “switching device” and “switch” are used interchangeably and may refer to a device that can turn on or off in response to one or more control signals. The present disclosure can be built using various switching devices comprising one or more of microelectromechanical system (MEMS) switches, diodes, bipolar junction transistors, field effect transistors (e.g., metal-oxide-semiconductor field effect transistor (MOSFET)), and other switching devices identifiable by a person skilled in the art. As is well known, a switch can be configured to serve as digital switches, which are essentially fully on or fully off, but can also be configured to serve as analog switches with varying degrees of on or off. In the latter case, the switch can be considered off but still be conducting to some (generally small) degree (e.g., a transistor in subthreshold/cutoff mode). In this regard, a switch can also be considered as a voltage-controlled resistor or a voltage-controlled current source.
Each of the operation, disable, and reset modes of the soft-start circuit will now be described in more detail. A switch voltage vsw (105), which is applied to a control terminal of the switch (110) (e.g., a gate of a field effect transistor), can determine whether the soft-start circuit is in operation mode and disable mode or in reset mode. The switch (110) can be connected to ground (190). The switch voltage vsw (105) is generally a digital signal that can be at a voltage VDD, associated with a logic high state, or at a voltage VSS (e.g., 0 V), associated with a logic low state, where in general |VDD|>|VSS|. When the switch voltage vsw (105) is at VDD, the switch (110) is generally closed (turned on) and the soft-start circuit is in reset mode, where the ramp voltage vramp (125) transitions toward ground (190). In contrast, when the switch voltage vsw (105) is at VSS, the switch (110) is generally open (turned off) and the soft-start circuit is in operation mode or disable mode, where the ramp voltage vramp (125) transitions away from ground (190). As a result, the switch voltage vsw (105) can also be referred to as a reset voltage or reset signal.
Specifically, when in operation mode and disable mode, operation of the ramp generator (120) takes the ramp voltage vramp (125) and gradually increases it from ground (190) (e.g., vGND=0 V) toward a supply voltage VCC provided to the ramp generator (120). When in reset mode, operation of the ramp generator (120) takes the ramp voltage vramp (125) and decreases it toward ground (190).
The DC-DC converter (100) can further comprise a DC-DC conversion unit (180) that can be connected with the control loop (160) and can be configured to generate a voltage vPWM (165) based on the soft-start voltage vr-ref (172) from the control loop (160). From the voltage vPWM (165), a substantially DC voltage vOUT (185) can be generated. The DC-DC conversion unit (180) can take as input a voltage vIN (181), which is a supply voltage that is sufficient for sustaining operation of the DC-DC conversion unit (180). The voltage vPWM (165) may be a function of the input voltage vIN (181). An example range of values for input voltage vIN (181) can be 5 V±10%.
The DC-DC conversion unit (180) can generate a voltage vPWM (165) based on comparing the soft-start voltage vr-ref (172) to a feedback voltage from a resistor Rfb (175). By operating the control loop (160), the soft-start voltage vr-ref (172) is adapted to approach the reference voltage VREF (145), which can be internally generated (e.g., within the soft-start circuit). Operation of the control loop (160) for generation of the soft-start voltage vr-ref (172) will be discussed in more detail with reference to
As provided in
With resistors Rsh (170) and Rfb (175) coupled with the control loop (160), an equation for the DC-DC converter output voltage vOUT (185) can be given by vOUT=min(VREF, vramp)(Rfb/Rsh+1). Once the soft-start voltage vr-ref (172) has reached regulation, the soft-start voltage vr-ref (172) stays at VREF, the DC-DC converter output voltage vOUT (185) can be given by vOUT=VREF(Rfb/Rsh+1). Although in
More specifically, with reference to
It should be noted that the DC-DC conversion unit (180) can have internal control circuitry that can be configured to generate the square wave voltage vPWM (165) with a duty cycle vOUT/vIN such that vOUT=min(VREF, vramp)(Rfb/Rsh+1). The square wave voltage vPWM (165) can have a low value of 0 V and a high value of vIN (181).
During normal operation of circuits connected with a soft-start circuit, such as the DC-DC converter (100) of
Although shown as separate components, the components shown in
An exemplary method to generate a voltage reference is through an internal bandgap voltage reference. A bandgap voltage reference is a temperature independent voltage reference circuit that can be used to output a constant voltage VBG-REF. An exemplary voltage output by the bandgap voltage reference is 1.25 V.
An exemplary voltage for the reference voltage VREF (245) is 1 V. To obtain a VREF (245) of 1 V from VBG-REF, a voltage divider can be used. With reference to
Similar to that previously described in
Consider that the switch voltage vsw (305) is at a high logic state and thus the soft-start circuit (300) is in reset mode. For purposes of discussion, the ramp generator (320) can be assumed to be an RC ramp generator (320). The switch (310) is closed (turned on) and a voltage across capacitor C, referred to as a ramp voltage vramp (325), is discharged toward ground through resistor R2. When the switch voltage vsw (305) is in a low logic state and thus the soft-start circuit (300) is in operation mode or disable mode, the ramp voltage vramp (325) increases due to current flow from the VCC supply through resistor R1 and charging of the capacitor C via the current flow. In cases where resistance of resistor R2 is lower than resistance of resistor R1, transition time of the ramp voltage vramp (325) toward ground is faster than transition time of the ramp voltage vramp (325) from ground toward VCC. It should be noted that resistor R2 can limit maximum current during discharge to improve reliability of the soft-start circuit (300). Current that is too high may exceed an elecromigration limit that metal routing in the soft-start circuit (300) can support.
Although the ramp generator (320) may comprise an RC ramp generator, as depicted in
With reference to
Rate of increase of voltage is a function of a time constant τ=RC associated with the ramp generator (320). Example time constants associated with the ramp generator (320) are 24 μs (e.g., 1.6 MΩ×15 pF) and 0.16 s (e.g., 1.6 MΩ×0.1 μF). Time constants associated with the ramp generator (320) can vary and depend on particular applications/systems within which the ramp generator (320) is utilized. An example value of VCC is 3 V.
The ramp voltage vramp (325) can be applied as an input into an inverting terminal of a comparator (362). The soft-start voltage vr-ref (372) tied to a drain of the transistor switch (370) can be applied as an input into a noninverting terminal of the comparator (362). The comparator output voltage vcomp (374) takes the voltage inputs vramp (325) and vr-ref (372) and outputs vcomp=A(vr-ref−vramp), where A is an open-loop gain (can be greater than 60 dB) of the comparator (362). The comparator output voltage vcomp (374) can be applied to a control node (e.g., gate) of the transistor switch (370).
With reference to voltages shown in
By the time the ramp voltage vramp (325) has increased to VREF, the transistor switch (370) has turned off (with its gate voltage either at 0 V or otherwise smaller in magnitude than threshold voltage VT of the transistor switch (370)). Once the comparator output voltage vcomp (374) has decreased sufficiently close to 0 V such that the transistor switch (370) is in a subthreshold region and thus conducts little (if any) current, the soft-start voltage vr-ref (372) has reached steady-state of around VREF. At this point, the soft-start circuit (300) has been disabled (i.e., has entered disable mode). It should be noted that, although 0 V is utilized as a reference ground voltage VGND, other ground voltages can be utilized.
When vramp<VREF, the comparator output voltage vcomp (374) is positive and of a value such that the transistor switch (370), which is depicted as an NMOSFET in
When vramp>VREF, the comparator output voltage vcomp (374) is sufficiently low such that the transistor switch (370) is disabled (turned off) and thus the soft-start circuit (300) is in disable mode. Specifically, the transistor switch (370) gradually turns off (e.g., enters into cutoff mode or subthreshold region in the case of a FET) once Vramp exceeds VREF. In this case, the soft-start voltage vr-ref (372) follows the reference voltage VREF. As a result and as previously mentioned, the soft-start voltage vr-ref (372) can be referred to as a ramp-up reference voltage since vr-ref (372) gradually increases from ground voltage toward the reference voltage VREF. Rate at which vr-ref (372) increases from ground voltage toward VREF is a function of a time constant τ=R1C. The reference voltage VREF can be generated using a reference generator (240) such as that described in
Gradual (voltage) changes are a function of ramp-up time of the ramp generator (320). It should be noted that time for gradual (voltage) changes, as referred to in “gradually turning off” (e.g., gradually transitioning into subthreshold region) and “gradually turning on” the transistor switch (370) and gradually increasing of the soft-start circuit vr-ref (372) from ground voltage toward VREF, can be adjusted by utilizing one or more external soft-start resistances and/or capacitances (not shown).
Specifically, resistances and capacitances within the ramp generator (320) are generally fixed, and thus external resistances and/or capacitances can be connected with the ramp generator (320) to adjust an RC constant associated with the ramp generator (320). For example, an external soft-start capacitance can be disposed in parallel with capacitor C of the ramp generator (320) and can affect an RC constant associated with the ramp generator (320), thus affecting ramp-up time of the ramp generator (320). Actual capacitance value of the external soft-start capacitance can be selected based on desired ramp-up time. Gradual increase of the soft-start voltage vr-ref (372) can vary depending on design specification and application of the soft-start circuit (300), and an example range of time involved in such gradual increase can be in the tens of microseconds to hundreds of milliseconds.
As described in relation to
With reference back to
Specifically, once the transistor (370) has been turned off, which generally occurs after the ramp voltage vramp (325) exceeds the reference voltage VREF, the voltage vr-ref=VREF can be used by the DC-DC conversion unit (180 in
Operation of the soft-start circuit (500) shown in
In operation mode and disable mode, ramp voltage vramp (525) is ramped up from ground towards a supply voltage VCC (shown as 3 V in
When the IP source follower (564) operates in saturation mode, source voltage of the IP source follower (564) follows voltage of gate voltage but by a fixed voltage offset determined by VT of the IP source follower (564). The IP source follower (564) gradually disables when the ramp voltage vramp (525) approaches the reference voltage VREF and generally has completely disabled when the ramp voltage vramp (525) exceeds VREF. When vramp (525) has exceeded VREF, the soft-start circuit (500) is operating in disabled mode, as previously discussed.
Alternatively, the soft-start circuit (500) can comprise the switch (510), the ramp generator (520), and the IP source follower (564) and without the transistor switch (570) and the comparator (562). The soft-start voltage vr-ref (572) can follow the ramp voltage vramp (525) but with an offset given by VGS of the IP source follower (564).
Whereas the control loop (660) can control rate at which an output voltage vr-ref (672) of the soft-start circuit (600) approaches a reference voltage, an output voltage VEAOUT (684) of the OTA (682), which is supplied to a terminal denoted as ISET (686), can control maximum current level that a system (e.g., DC-DC converter) connected with the voltage soft-start circuit (600) can provide. Specifically, the terminal ISET (686) can contain devices that can map the output voltage VEAOUT (684) to an output current IOUT (not shown).
The OTA (682) takes as input a reference voltage VREF (645) and a voltage feedback. With reference to
It should be noted that a particular embodiment of a soft-start circuit selected for use in any application depends on design specifications and applications. For instance, limiting current during start-up may be desirable in some systems whereas such limiting of current may be undesirable in other systems.
Furthermore, it should be noted that although soft-start voltage vr-ref (172 in
The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments of the methods and apparatuses for a soft-start function with auto-disable of the disclosure, and are not intended to limit the scope of what the inventor regard as his disclosure. Modifications of the above-described modes for carrying out the disclosure may be used by persons of skill in the art, and are intended to be within the scope of the following claims.
It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” includes two or more referents unless the content clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains.
A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims.
