Battery operated devices, such as personal electronic devices, robots, electric cars, industrial equipment, medical equipment and wearable devices, require a stable and consistent power source. In addition, longer battery life is desirable so high efficiency (especially during light loads) is important. Highly efficient voltage regulators may be used in these applications to provide a regulated voltage from the battery to the device while requiring less battery power to operate. In addition, display devices (such as organic light-emitting diode displays and LCD displays) also incorporate voltage regulators. A stable output voltage with minimal ripple is important for the proper operation of these display devices.
Switching regulators, also referred to as DC-DC converters, are used to convert or regulate an input voltage to an output voltage. The input voltage can be greater than, less than or equal to the output voltage. If the input voltage is greater than the output voltage, the converter/regulator may be referred to as a “step-down” converter/regulator or a “buck converter”.
Generally, switching regulators include at least one power switch and one or more energy storage devices, such as an inductor and a capacitor. The power switch can be implemented using a metal-oxide-silicon field-effect transistor (MOSFET), bipolar junction transistor (BJT) or other type of power transistor. The switching regulator may include a high-side switch and a low-side switch or a single power switch and a diode.
As discussed above, each of the converters illustrated in
A converter having an input adapted to be connected to an input voltage and an output adapted to supply an output voltage, the converter comprising: a high-side switch having a first current terminal, a second current terminal and a first control terminal, the first current terminal is coupled to the input voltage and the second current terminal is coupled to a switching node; a high-side driver circuit having an input, a first supply input, a second supply input and an output coupled to the first control terminal, the second supply input is coupled to the switching node; a bootstrap capacitor having a first terminal and a second terminal, the first terminal coupled to the first supply input and the second terminal coupled to the second supply input; a switch having a first terminal and a second terminal, the first terminal of the switch is coupled to the first terminal of the bootstrap capacitor and the second terminal of the switch is connected to a supply voltage (VDD volts above ground). In an embodiment, the converter further includes: a low-side switch having a third current terminal, a fourth current terminal and a second control terminal, the third current terminal is coupled to the switching node and the fourth current terminal is coupled to the output voltage; and/or an inductor having a first terminal coupled to the switching node and a second terminal coupled to ground. In an embodiment, the switch is operable to connect the first terminal of the bootstrap capacitor to the supply voltage during a time period while the high-side switch and the low-side switch are turned off (for an off-time duration). In an embodiment, the time period is shorter than the off-time duration, but is long enough to recharge the bootstrap capacitor by the supply voltage during the time period. In an embodiment, the bootstrap capacitor is recharged by the supply voltage when the switch is closed, and the switch closed for a recharge period: at start-up of the converter; and/or if the high-side switch and the low-side switch are turned off.
Another embodiment is an inverting buck-boost converter adapted to be coupled to a first supply terminal of a voltage source having a second supply terminal connected to ground, the inverting buck-boost converter comprising: a high-side switch having a first current terminal, a second current terminal and a first control terminal, the first current terminal is adapted to be coupled to an input voltage and the second current terminal is coupled to a switching node; a low-side switch having a third current terminal, a fourth current terminal and a second control terminal, the third current terminal is coupled to the switching node and the fourth current terminal is coupled to an output of the inverting buck-boost converter; a high-side driver having a first input, a first supply input, a second supply input and an output coupled to the first control terminal; a bootstrap capacitor having a first terminal coupled to the first supply input and a second terminal coupled to the second supply input and the switching node; and wherein the first terminal of the bootstrap capacitor is adapted to be coupled to the first supply terminal of the voltage source for a first time period. In an embodiment, the first time period occurs: at start-up of the inverting buck-boost converter; or for a portion of time that the high-side switch and the low-side switch are turned off. In an embodiment, the bootstrap capacitor is recharged by the voltage source during the first time period. The first supply terminal has a voltage potential (VDD) with reference to ground.
Another embodiment is an inverting buck-boost comparator adapted to be coupled to a voltage supply having a first voltage supply terminal and a ground terminal connected to ground, the inverting buck-boost converter comprising: a high-side transistor having a gate, a drain coupled to an input voltage and a drain coupled to a switching node; a high-side gate driver having a first supply input, a second supply input coupled to the switching node and an output coupled to the gate of the high-side transistor; a bootstrap capacitor having a first end coupled to the first supply input of the high-side gate driver and a second end coupled to the second supply input of the high-side gate driver; and a switch having a first end connected to the first voltage supply terminal and a second end coupled to the first end of the bootstrap capacitor. The first voltage supply terminal supplies a first voltage value with respect to ground. In an embodiment, the switch is operable to connect the first voltage supply terminal to the bootstrap capacitor to recharge the bootstrap capacitor when the switch is closed.
For a detailed description of various examples, reference will now be made to the accompanying drawings in which:
Whenever an n-type transistor is used as the high-side (HS) switch in the power stage of an inverting buck-boost converter, a potential above the input voltage (VIN) should be generated in order to turn this transistor on. One approach for producing this potential (VBST) is to employ a bootstrap capacitor (CBST) which acts as a floating supply for the high-side switch driver and which is connected between the switching node (LX) and the VBST voltage.
In some example embodiments, the charge replenishment of CBST is achieved by means of a voltage supply VNDRV_HS, which is VDD volts above VOUT, and a diode connected between the VNDRV_HS supply and VBST. Specifically, when the low-side (LS) switch is turned on, the LX node is pulled to VOUT and this causes the bootstrap diode to be forward biased and CBST to be recharged through VNDRV_HS up to VDD volts (with respect to VOUT).
The bootstrap capacitor will be properly refreshed when the LX node is at VOUT. In many applications, however, a “high impedance” state may occur after the period when the low-side switch is on and the high-side switch is off. The “high impedance” state may occur at other times, too. During the “high impedance” state, the low-side switch and the high-side switch are off and the inductor is discharged. This causes the LX node to be at (or near) a ground potential. During the high impedance state, the normal refresh mechanism through VNDRV_HS and the bootstrap diode will not occur.
If the converter remains in the “high impedance” state for an extended duration, the bootstrap capacitor might discharge (either by bias or by leakage currents) to a voltage level that does not ensure the correct operation of the high-side driver. If this occurs, the high-side driver may not able to turn on the high-side switch.
In an example embodiment, the low-side switch can be turned on for a short duration after the high impedance state. The short duration should be long enough to “recharge” the bootstrap capacitor. While this approach may be effective for recharging the bootstrap capacitor, it can cause excessive output ripple. In addition, converter efficiency may decrease during light loads (especially if the duration of the on-time for the low-side switch is too long).
Referring to
HS switch 234 is “driven” (meaning to turn on or off) by drive signal VG_HS, which is the output of HS gate driver 228. The output of HS gate driver 228 is based on the output of HS level shifter 220, which is based on a control signal HSD 214 received by HS level shifter 230. Control signal 214 defines which mode of operation (such as continuous conduction mode (CCM), discontinuous conduction mode (DCM), pulse frequency modulation mode (PFM), pulse width modulation mode (PWM) or modes of operation used by conventional converters) the HS switch 234 is operating in and controls whether the HS switch is conducting (on) or non-conducting (off). Similarly, LS switch 254 is driven (turned on, conducting, or off, non-conducting) by drive signal VG_LS, which is the output of LS gate driver 250. LS level shifter 244 receives a low-side control signal (LSD) 240 and outputs a level-shifted signal to LS gate driver 250. LSD control signal 240 controls the state of operation (such as CCM, DCM, PFM, PWM or modes of operation used by conventional converters) for LS switch 250. In most modes of operation, either HS switch 234 is on (with LS switch 254 turned off) or LS switch 254 is on (with HS switch 234 turned off). However, as discussed above, an issue can arise if both HS switch 234 and LS switch 254 are off (also referred to as being in a “tri-state mode” or a “high impedance mode”). This can occur during a PWM mode of operation where the switching frequency is low (such as during light load conditions) or during startup of the converter and can cause insufficient charging of the bootstrap capacitor.
HS level shifter 220 and LS level shifter 244 are connected to first supply 218, VDD, and second supply 222, ground. In addition, HS level shifter 220 and HS gate driver are connected to VBST 224 and rail 225 (which is equivalent to the potential at switch node 258). Bootstrap capacitor, CBST, causes VBST to remain at a potential that is around the potential of the switching node, VLX, plus VDD. VBST and VLX are voltage supplies for HS gate driver 228. LS level shifter 244 and LS gate driver 250 are connected to VNDRV_LS supply 246 and VOUT 260. VNDRV_LS supply 246 and VOUT are voltage supplies for LS gate driver 250.
VNDRV_HS supply 206 is supplied to the anode of diode 208. VNDRV_HS supply 206 is set to a value around VOUT plus VDD. Since converter 200 is an inverting buck-boost converter, the value of VOUT will have the opposite polarity as VIN. Hence, the value at the cathode of diode 208 may be less than VDD. However, the potential at node 210 may be increased (to, for example, refresh the charge on the bootstrap capacitor 212) if SAUX switch 204 is closed, thereby connecting node 210 (which is connected on one plate of the bootstrap capacitor 212) to VDD_AUX supply 202. The value of VDD_AUX supply 202 is around VDD plus ground (roughly VDD). Diode 208 may be replaced with a switch (not shown) to eliminate the forward voltage drop of the diode.
Referring to 2B, as discussed above, inverting buck-boost converters (such as inverting buck-boost converter 200) may have an issue if the charge stored in bootstrap capacitor 212 is insufficient to turn on HS switch 234. Insufficient charge in bootstrap capacitor 212 may occur at start-up of converter 200, during PFM mode of operation for light loads (causing the switching frequency to be low) or following a period when both HS switch 234 and LS switch 254 are turned off.
In some example embodiments switch 204 is included in converter 200. Switch 204 can be implemented by a two-pole switch or a transistor that will provide sufficient isolation between supply VDD_AUX 202 and node 210. If the charge on the bootstrap capacitor 212 is too low, switch 204 may be closed thereby connecting node 210 to VDD_AUX 202 (equal to VDD+ground or VDD with reference to ground). During the period that switch 204 is closed, current ICHARGE 216 flows from VDD_AUX source 202 through bootstrap capacitor 212 and inductor 262 to ground. During this period, the bootstrap capacitor 212 is charged to a potential near VDD. Hence, VBST supply 224 will be around VDD+VLX, and VLX will be around ground potential while HS switch 234 and LS switch 254 are turned off. Once the bootstrap capacitor 212 is sufficiently charged or a predetermined time period (determined to be long enough for bootstrap capacitor 212 to be sufficiently charged) has expired, switch 204 may be opened. Once, HSD signal 214 goes high (logical “1”), bootstrap capacitor 212 will be sufficiently charged to drive HS gate driver 238 to turn on HS switch 234. Charging bootstrap capacitor 212 using switch 204 will cause less VOUT ripple as compared to toggling LS switch 254 to charge bootstrap capacitor 212.
Switch 204 may be switched open (disconnecting VDD_AUX 202 from node 210) or closed (connecting VDD_AUX 202 to node 210) using “closed-loop control” or “open-loop control”. If closed-loop control is used, the voltage across the bootstrap capacitor 212 is monitored (such as by an under voltage lock out circuit, UVLO circuit). If the voltage across bootstrap capacitor goes too low (below VBST_MIN—signifying that the charge is getting too low), a refresh circuit (including switch 202) would be enabled to recharge bootstrap capacitor 212. Closed-loop control is more suitable than open-loop control where the capacitance of bootstrap capacitor 212 is not known (such as when an external capacitor is used instead of capacitor incorporated into the same semiconductor die as the converter) or if the discharge current, IDISCH, through the capacitor is not known.
In instances where the capacitance of bootstrap capacitor 212, CBST, and the value of IDISCH are known, open-loop control may be used. In order to implement open-loop control, a value for the maximum tolerated voltage drop, ΔVBST_MAX, across the bootstrap capacitor 212 should be determined. This value can be the lower supply voltage limit below which the operation of the HS gate driver 228 will no longer reliably turn on HS switch 234. The maximum tolerated discharge time, tDISCH_MAX, can be computed from:
After the expiration of tDISCH_MAX, switch 202 would be closed for a period of time, tPULSE, calculated to be sufficient to recharge the bootstrap capacitor 212 so that the voltage across the bootstrap capacitor is around VDD.
The following description is with reference to the timing diagrams of
Referring to
In the foregoing discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” An element or feature that is “configured to” perform a task or function may be configured (e.g., programmed or structurally designed) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or re-configurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof. Additionally, uses of the phrases “ground” or similar in the foregoing discussion are intended to include a chassis ground, an Earth ground, a floating ground, a virtual ground, a digital ground, a common ground, and/or any other form of ground connection applicable to, or suitable for, the teachings of the present disclosure. Unless otherwise stated, “approximately” preceding a value means +/−10 percent of the stated value. As used herein, the term “modulate” shall also mean “to vary” or “to change.” The terms “node”, “terminal”, “pin” and “interconnection”, for example, are interchangeably used and referred to any connection (or interconnection) between features. These terms are not meant to be limiting with respect to a certain type of physical structure. For example, the “terminals” of a circuit element are meant to be each connection to such circuit element. Hence, an integrated resistor would be referred to have two terminals (ends) even though these “terminals” are just the two connections to the integrated resistor.
The above discussion is meant to be illustrative of the principles and various embodiments of the present disclosure. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.
The application claims priority to US Provisional Application No. 62/926,247, filed Oct. 25, 2019 entitled “Bootstrap Capacitor Refreshing for Inverting Buck-Boost DC-DC Converters”, which is hereby incorporated herein by reference in its entirety.
Number | Date | Country | |
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62926247 | Oct 2019 | US |