Soft-start voltage regulator circuit

[0001] The invention is related to voltage regulator circuits, and in particular, to a soft-start voltage regulator circuit that is arranged such that the reference voltage seen by the error amplifier substantially tracks the output voltage during disable.

[0002] A step-down switching regulator may be configured to provide a regulated output voltage (Vout) in response to an input voltage (Vin). The switching regulator may include a top-side transistor and a bottom-side transistor. When the top-side transistor is on and the bottom-side transistor is off, Vin-Vout is typically applied across an inductor, causing the inductor current to ramp upwards. When the top-side transistor is off and the bottom-side transistor is on, the inductor current ramps downwards. The inductor current is applied to a capacitor to provide the output voltage. Also, feedback may be employed to control the duty cycle of the switching. The output voltage may be sensed to provide the feedback.

[0011] Various embodiments of the present invention will be described in detail with reference to the drawings, where like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the invention, which is limited only by the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the claimed invention.

[0012] Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meanings identified below are not intended to limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” includes plural reference, and the meaning of “in” includes “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. The term “coupled” means at least either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, charge, temperature, data, or other signal.

[0013] Briefly stated, the invention is related to a voltage regulator that includes a soft-start circuit, an error amplifier, an output stage, a voltage divider, and a soft-start switch. The output stage is arranged to provide an output voltage from an input voltage and an error signal. Also, the voltage divider is arranged to provide a feedback signal from the output voltage. The error amplifier is arranged to provide the error signal from a soft-start reference voltage and the feedback signal. Further, the soft-start circuit is arranged to provide the soft-start reference voltage from a reference voltage. If the voltage regulator is enabled, the soft-start reference voltage ramps toward the reference voltage until the soft-start reference voltage reaches the reference voltage. The soft-start switch is coupled across the inputs of the error amplifier. If the voltage regulator is disabled, the soft-start switch is closed. Closing the soft-start switch causes the soft-start reference voltage to substantially track the decaying output voltage during disable.

[0014] FIG. 1 shows a block diagram of an embodiment of voltage regulator circuit 100. Voltage regulator circuit 100 includes soft-start circuit 150, soft-start switch circuit S1, error amplifier circuit 120, output stage 130, and voltage divider circuit 140. Voltage divider circuit 140 includes impedance circuit 141 and impedance circuit 142.

[0015] Soft-start circuit 150 is configured to provide soft-start reference signal Vref_—ss from reference signal Vref. More specifically, soft-start circuit 150 is arranged to provide signal Vref_—ss at node N1. During normal operation, soft-start circuit 150 may be arranged to provide signal Vref_—ss such that a voltage associated with signal Vref_—ss is substantially the same as a voltage that is associated with signal Vref. However, when voltage regulator circuit 100 is enabled (by changing signal Enable from an inactive level to an active level), soft-start circuit 150 may be arranged to adjust the voltage associated with signal Vref_—ss towards the voltage associated with signal Vref relatively slowly. In one embodiment, when voltage regulator circuit 100 is enabled, soft-start circuit 150 is arranged to provide signal Vref_—ss such that the voltage associated with signal Vref_—ss ramps toward the voltage that is associated with signal Vref until the voltage associated with signal Vref_—ss reaches the voltage associated with signal Vref.

[0016] Additionally, error amplifier circuit 120 is configured to provide error signal Verr by amplifying the voltage difference between nodes N1 and N2. Output stage 130 is arranged to provide a regulated output voltage Vout from input signal Vin, responsive to signal Verr. In one embodiment, input signal Vin is an unregulated input signal. Output stage 130 may be arranged to provide substantially no power to node N3 when signal Enable is inactive. In one embodiment, output stage 130 is tri-stated when signal Enable is inactive.

[0017] Also, voltage divider circuit 140 is arranged to provide feedback signal Vfb from signal Vout.

[0018] Soft-start switch circuit S1 is coupled between nodes N1 and N2. Soft-start switch circuit S1 is configured to close if signal Enable corresponds to an inactive level, and to open if signal Enable corresponds to an active level. More specifically, soft-start circuit S1 may be arranged to adjust a resistance between nodes N1 and N2. Accordingly, if signal Enable corresponds to an inactive level, nodes N1 and N2 may be substantially shorted together. However, if signal Enable corresponds to an active level, soft-start circuit S1 may be substantially an open circuit.

[0019] FIG. 2 illustrates a block diagram of an embodiment of voltage regulator circuit 200, in which an embodiment of a soft-start circuit is illustrated. Components in voltage regulator circuit 200 may operate in a substantially similar manner to similarly-named components in voltage regulator circuit 100, and may operate in a different manner in some ways. Soft-start circuit 250 includes follower circuit 210 and soft-start capacitor circuit C1.

[0020] Follower circuit 210 is arranged to provide signal Vref_—ss from signal Vref. Follower circuit 210 has a limited output current drive. Soft-start capacitor circuit C1 is arranged to control a rate of change of a voltage that is associated signal Vref_—ss. Accordingly, the voltage associated with signal Vref_—ss follows the voltage that is associated with signal Vref. However, when voltage regulator circuit 200 is first enabled, the voltage associated with signal Vref_—ss ramps towards signal Vref until the voltage associated with signal Vref_—ss reaches the voltage that is associated with signal Vref. The output current limit of follower circuit 210 is well-controlled such that the ramp rate of signal Vref_—ss is well-controlled.

[0021] FIG. 3 shows a block diagram of an embodiment of voltage regulator circuit 300. Components in voltage regulator circuit 300 may operate in a substantially similar manner to similarly-named components in voltage regulator circuit 200, and may operate in a different manner in some ways. Voltage regulator circuit 300 further includes output capacitor circuit Cout. Output current-limited follower circuit 310 includes transconductance circuit A1. In other embodiment, transconductance circuit A1 may be replaced with a current-limited operational amplifier, and the like. Impedance circuit 341 includes resistor circuit R1. Similarly, impedance circuit 342 includes resistor circuit R2.

[0022] Also, switch circuit S1 is arranged such that, if voltage regulator circuit 300 is disabled (by changing signal Enable from an active level to an inactive level), nodes N1 and N2 are substantially shorted together. When signal Enable corresponds to an inactive level, voltage Vout overdrives the voltage at node N1, overpowering the current-limited current driver of follower circuit 310. This causes the voltage at node N1 to become substantially equal to the voltage associated with signal Vfb. Since output stage 330 is disabled when signal Enable is inactive, signal Vout decays at a rate that is based on capacitor circuit Cout and the output load (not shown). Signal Vref_—ss substantially tracks signal Vout as signal Vout decays.

[0023] The operation of voltage regulator circuit 300 may be more easily understood with reference to FIG. 4.

[0024] FIG. 4 illustrates a timing diagram of waveforms of embodiments of signals Enable, Vref_—ss, and Vout from FIG. 3. Waveform 460 is a waveform of an embodiment of signal Enable. Waveform 462 is a waveform of an embodiment of signal Vref_—ss. Waveform 464 is a waveform of an embodiment of signal Vout.

[0025] For the embodiment of signal Enable illustrated by waveform 460, signal Enable changes from an inactive level (e.g. low) to an active level (e.g. high) at time A. Accordingly, as explained above, voltage Vref_—ss ramps upwards towards voltage Vref. The operation of voltage regulator circuit 300 causes voltage Vout to be regulated such that voltage Vfb substantially tracks voltage Vref_—ss as voltage Vref_—ss ramps upwards. The feedback loop is active from time A to time B, but voltage Vref_—ss ramps up relatively slowly, so voltage Vfb tracks it.

[0026] At time B, voltage Vref_—ss substantially reaches voltage Vref. Accordingly, at this point, voltage Vref_—ss remains substantially the same as voltage Vref. Also, signal Vout is regulated such that signal Vout remains substantially at the desired output voltage level.

[0027] For the embodiment of signal Enable illustrated by waveform 460, signal Enable changes from the active level (e.g. high) to the inactive level (e.g. low) at time C. Accordingly, as explained above, voltage Vout decays, and voltage Vref_—ss substantially tracks voltage Vfb. Because voltage Vref_—ss substantially tracks voltage Vfb, excessive inrush currents may be prevented when signal Enable is asserted since signal Verr is relatively near zero.

[0028] For the embodiment of signal Enable illustrated by waveform 460, signal Enable again changes from an inactive level (e.g. low) to an active level (e.g. high) at time D. Similar to time A, voltage Vref_—ss ramps upwards towards voltage Vref. However, it is noteworthy that signal Vref_—ss does not ramp upwards from 0V at time D. Rather, voltage Vref_—ss ramps upwards from a voltage that is relatively close to voltage Vref. Accordingly, after signal Enable changes to the active level at time D, signal Vout may reach the desired output voltage relatively quickly, without causing excessive inrush currents.

[0029] Voltage regulator circuit 300 may be turned on and off rapidly for power management purposes and to reduce wasted power. By causing signal Vref_—ss to track the decaying output voltage when voltage regulator circuit 300 is disabled, start-up time may be substantially optimized for short disable events. Also, the slow-start topology shown in FIG. 3 may prevent large inrush currents which might otherwise exceed safe current ratings of devices in voltage regulator circuit 300, may prevent excessive energy from being wasted by capacitor circuit Cout.

[0030] FIG. 5 shows a block diagram of an embodiment of voltage regulator circuit 500 that includes a buffer circuit. Components in voltage regulator circuit 500 may operate in a substantially similar manner to similarly-named components in voltage regulator circuit 300, and may operate in a different manner in some ways. Voltage regulator circuit 500 further includes buffer circuit B1.

[0031] Buffer circuit B1 is arranged to provide error amplifier input signal Vfb_—buf at node N2 by buffering signal Vfb. This enables capacitor circuit C1 to be driven without the slew-limiting impedance of voltage divider circuit 540.

[0032] FIG. 6 schematically illustrates an embodiment switching voltage regulator circuit 600. Components in switching voltage regulator circuit 600 may operate in a substantially similar manner to similarly-named components in voltage regulator circuit 300, and may operate in a different manner in some ways. Output stage 630 includes pulse modulation control circuit 670, transistors M1–M2, and inductor L1.

[0033] Transistor M1 is arranged to operate is a high-side switch that opens and closes responsive to control signal C1. Similarly, transistor M2 is arranged to operate as a synchronous switch that opens and closes responsive to control signal C2. When transistor M1 is on and the transistor M2 is off, Vin-Vout is applied across inductor L1, causing inductor current I1 to ramp (e.g. increasing). When transistor M1 is off and the transistor M2 is on, inductor current I1 ramp in the reverse direction (e.g. decreasing).

[0034] Pulse modulation control circuit 670 is arranged to provide control signal C1 and C2 responsive to signal Verr. In one embodiment, pulse modulation control circuit 670 is a pulse width modulation (PWM) control circuit that is arranged to modulate the duty cycle of signal C1 responsive to signal Verr. In another embodiment, pulse modulation control circuit 670 is a pulse frequency modulation (PFM) control circuit that is arranged to modulate the frequency of signal C1 responsive to signal Verr. In one embodiment, pulse modulation control circuit 670 is arranged to provide signal C2 such that signal C2 is an inverse of signal C1 if signal enable is active. In one embodiment, pulse modulation control circuit 670 is further arranged to provide signal C2 such that signal C2 corresponds to a non-asserted level (i.e. low) if signal enable is inactive. This way, transistors M1 and M2 are both off during disable.

[0035] In one embodiment, as illustrated in FIG. 6, error amplifier circuit 620 is arranged to receive signal Enable. In this embodiment, error amplifier circuit 620 is arranged to provide signal Verr at a predetermined voltage if signal Enable is inactive. By providing signal Verr at a known voltage if signal Enable is inactive, error amplifier circuit 620 operates in a known operating condition that may make its operation predictable, and may make error amplifier 620 more responsive when signal Enable changes to the active logic level.

[0036] Although switching voltage regulator circuit 600 is shown as a buck regulator in FIG. 6, in other embodiments, switching voltage regulator circuit 600 may be a boost regulator, an inverting regulator, and the like. Additionally, although a synchronous switch (transistor M2) is shown and described in FIG. 6, the synchronous switch may be replaced with a diode in other embodiments, without departing from the scope and spirit of the invention.

[0037] FIG. 7 schematically illustrates an embodiment of linear voltage regulator circuit 700. Components in linear voltage regulator circuit 700 may operate in a substantially similar manner to similarly-named components in voltage regulator circuit 300, and may operate in a different manner in some ways. Output stage 730 includes may include transistor Q1. Transistor Q1 is arranged as a series pass transistor that configured to provide signal Vout from signal Vin, responsive to signal Verr.

[0038] Although transistor Q1 is shown as a bipolar junction transistor (BJT) in FIG. 7, transistor Q1 may also be a metal-oxide-semiconductor field effect transistor (MOSFET), and the like.

[0039] Also, although not shown in FIG. 7, output stage 730 may further include an active pulldown transistor. By causing signal Vref_—ss to track the decaying output voltage when voltage regulator circuit 700 is disabled, the active pulldown transistor can be protected during short disable events.

[0040] The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.