Method and apparatus to allow rapid adjustment of the reference voltage in a switching regulator

[0001] The present invention relates to switching voltage regulators, and, in particular, to a circuit for enabling a rapid adjustment of a reference voltage during a fault condition.

[0002] A power conversion circuit (e.g., a switching regulator) may accept a Direct Current (DC) voltage source at one level and output a desired DC voltage at another level. The switching regulator may include one or more switches, which can be implemented by Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFETs), and the like. The switches may alternate between connecting and disconnecting the voltage source to subcircuits that drive an output. Typically, a duty cycle of the switching determines the output voltage level. The switching is generally controlled by a Pulse-Width Modulation (PWM) circuit.

[0003] Switching regulators are useful in high current applications, such as high power microprocessors, notebook computers, desktop computers, network servers, large memory arrays, workstations and DC high power distribution systems, and the like. The switching regulator can have multiple parallel channels to process one or more voltage sources to drive a common output. A transient response of typical switching regulators may not be satisfactory in applications with demands for high slew rates of output currents. The switching regulator may regulate the output voltage employing a relatively slow feedback circuit which continuously adjusts a control parameter, such as a duty cycle. The duty cycle may be adjusted in accordance with differences between the output voltage of the switching regulator and a nominal value provided by a reference voltage.

[0004] Although the switching regulator can theoretically achieve faster transient responses by operating at higher frequencies, practical switching devices limit the operating frequencies of the switching regulator. For instance, the inherent impedance of the switching devices reduces efficiency at high switching frequencies.

[0005] Thus, it is with respect to these considerations and others that the present invention has been made.

[0012] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the present invention may be embodied as methods or devices. Accordingly, the present invention may take the form of an entirely hardware embodiment or an embodiment combining software and hardware aspects. The following detailed description is, therefore, not to be taken in a limiting sense.

[0013] Briefly stated, the present invention is related to a circuit for enabling a rapid adjustment of a reference voltage during a disable period such as a fault condition, shutdown condition, and the like of a voltage regulator.

[0014] A common method of soft starting a voltage regulator is to ramp up the reference voltage that is used to regulate an output voltage from zero volts each time the switching voltage regulator is enabled. Rapid disable and enable cycles may occur in response to an overload condition, short-circuit of the output, and the like. In one implementation, the switching voltage regulator may have an LC filter at its output stage that may be opened when the circuit is disabled. This mode of operation may allow the output voltage to decay towards zero volts based on a time constant of a load and the LC filter capacitor. An undesirable result of this configuration may be high discharge current values due to the output voltage not being able to decay fully to zero volts during short disable periods.

[0015] Another configuration that is arranged to reduce large current and voltage transients following short disable periods may employ tracking the decaying output voltage during disable condition and starting the reference voltage at the tracked output voltage when an enable signal triggers an end of the short disable period. This modified soft start configuration may be implemented by adding a switch to couple a soft start capacitor through a voltage divider to the output voltage. This configuration may, however, result in longer discharge time for the capacitor limiting a choice of values for the voltage divider.

[0016] The present invention provides a soft start circuit that is arranged to couple the switch of the modified soft start configuration through a second switch to a low resistance voltage divider that provides a path for the reference node capacitance to rapidly discharge. The faster discharge may allow the reference voltage to match the output voltage even when the switch is activated for a very brief interval. This in return allows the output voltage to start up in a controlled manner after the disable period.

[0017] While a preferred embodiment of the present invention may be implemented in a switching voltage regulator circuit, the invention is not so limited. The described circuit and parts of it may be employed as part of virtually any switching or linear voltage regulator circuit known to those skilled in the art.

[0018] FIG. 1 illustrates a block diagram of an embodiment of voltage regulator circuit 100 according to the present invention along with power source 102 and load 114. Power source 102 may include any DC power source such as an AC/DC converter, a DC/DC converter, a battery, and the like.

[0019] Voltage regulator circuit 100 includes output stage 104, feedback circuit 106, reference source 108, soft start circuit 110, and error amplifier 112. FIG. 1 shows a particular arrangement of inputs and outputs of the various components. In one embodiment, all of the components of voltage regulator circuit 100 may be included in the same chip. Alternatively, one or more of the components of circuit 100 may be off-chip.

[0020] Output stage 104 is configured to receive input voltage Vin from power source 102 and provide regulated output voltage V_out to load 114 in response to V_in and error voltage V_err from error amplifier 112.

[0021] Feedback circuit 106 is arranged to receive output voltage V_out and provide feedback voltage V_fb to an input of error amplifier 112. In one embodiment, feedback circuit 106 may include a voltage divider.

[0022] Reference source 108 is arranged to provide a reference voltage V_ref for regulation of the output voltage. In a switching voltage regulator that does not include a soft start feature, V_ref may be provided to another input of error amplifier 112 resulting in the regulation of output voltage V_out based on a difference between V_fb and V_ref. This may lead to undesirably high discharge currents, and the like after short disable periods as described above.

[0023] In one embodiment of the present invention, soft start circuit 110 is arranged to receive V_ref and to provide modified reference voltage V_ref′ to the other input of error amplifier 112 such that transient currents and voltages after short disable periods are reduced while a discharge time of the soft start capacitance is not relatively short.

[0024] Error amplifier 112 is arranged to receive V_ref′ and V_fb and provide error voltage V_err to output stage 104 for regulation of output voltage V_out. In another embodiment, a pulse-width-modulation (PWM) comparator may be employed between error amplifier 112 and output stage 104.

[0025] FIG. 2 schematically illustrates an embodiment of switching voltage regulator circuit 200. Switching voltage regulator circuit 200 is arranged to provide a regulated output voltage V_out based, in part, on input voltage V_in. Reference voltage V_ref is employed to modify V_out to have a soft starting slope.

[0026] Switching voltage regulator circuit 200 includes reference voltage source 208, soft start circuit 210, feedback circuit 206, error amplifier 224, feedback impedance Z_C, pulse-width-modulation (PWM) comparator 232, and output stage 204. A ramping voltage source providing V_ramp and load resistance R_O are also shown in FIG. 2.

[0027] In a typical operation, reference voltage source 208 provides reference voltage V_ref. Reference voltage source 208 may be off chip in one embodiment. Feedback circuit 206 includes primary voltage divider comprising resistors R_X and R_Y. Primary voltage divider is arranged to provide output voltage V_out to a inverting input of error amplifier 224, which is coupled to a node between resistors R_X and R_Y.

[0028] Modified reference voltage V_ref′ is provided to a non-inverting input of error amplifier 224 at reference node N241 as described below. Feedback impedance Z^C is arranged to couple a inverting input of error amplifier 224 to an output of the same amplifier. The output of error amplifier 224 is coupled to an input of PWM comparator 232. PWM comparator 232 is arranged to compare an output voltage of error amplifier 224 with ramping voltage V_ramp, creating a pulse width modulated signal that represents the difference between the output voltage and the reference voltages.

[0029] Output stage 204 is arranged to receive an output of PWM comparator 232 and to provide regulated output voltage V_out to a load, represented by load resistance R^O in FIG. 2. Output stage 204 includes drivers 234, regulation transistor 236, and output LC filter comprising series inductor L and parallel capacitor C. In one embodiment, output stage 204 may include synchronous regulation transistor M238 as well. While a synchronous regulation circuit is depicted in FIG. 2, the present invention is not so limited. In another embodiment, the circuit may employ asynchronous regulation. Output voltage V_out is also provided to feedback circuit 206 and soft start circuit 210 for regulation and modification of reference voltage V_ref.

[0030] Soft start circuit 210 is arranged to modify reference voltage V_ref such that V_ref has a rapid decay following a disable condition and a slow rising slope when the circuit is re-enabled. Soft start circuit 210 includes soft start resistor R_S and soft start capacitor C_SS, which are coupled at node N241, switches 222 and 230, one-shot device 228, primary voltage divider comprising resistors R_X and R_Y, and auxiliary voltage divider comprising resistors R_XA and R_YA. Auxiliary voltage divider comprising resistors R^XA and R_YA is arranged to provide a low resistance path between V_ref′ and the output when switches 222 and 230 are closed. Switch 222 is arranged to be controlled by an output of one-shot device 228. One-shot device 228, receives a fault enable signal V_EN and then activates switch 222 to couple the reference node to a node between resistors R_XA and RYA. In one embodiment, V_EN may be associated with a disable condition, while in another embodiment V_EN may be indicative of re-enabling of the circuit following a disable condition.

[0031] The auxiliary voltage divider is arranged to be coupled to ground when switch 230 is closed. Switch 230 is controlled by an output of one-shot device 228. One-shot device 228 is also arranged to receive the enable signal V_EN. One-shot device 228 essentially turns on switches 222 and 230 when the enable signal V_EN is applied after a disable condition, and turns the switch off when the circuit is operating normally. In one embodiment, one-shot device 228 maybe a monostable multivibrator, a distable multivibrator, and the like.

[0032] Switches 222 and 230 are essentially low resistance switches and may include switch transistors. In one embodiment, switches 222 and 230 may include a BJT, an NMOS FET, and a PMOS FET transistor.

[0033] A discharge constant of capacitance at the reference node (node N241) may be determined by a value of R_S, a capacitance of C_SS, output capacitor C, and a value of R_XA and R_YA when both switches are closed. To achieve a rapid discharge of the reference node capacitance after a disable condition, R_XA and R_YA may be preselected with a low resistance: Rapid discharge of the reference node capacitance may result in slower increase of modified reference voltage V_ref′, and accordingly output voltage V_out, preventing undesirable output spikes or large discharge currents after a disable condition.

[0034] In a typical operation, a fault condition causes the enable signal V_EN to transition from low to high. This change closes switches 222 and 230 resulting in modified reference voltage V_ref′ being discharged rapidly while the output of switching voltage regulator circuit 200 is open circuit. Output voltage V_out may also be discharged through the low resistance path provided by the auxiliary voltage divider and switch 230. Some time later, when the switching voltage regulator circuit is turned on again, modified reference voltage V_ref′ may begin increasing again. As V_ref′ increases output voltage V_out may follow V_ref′ because of feedback impedance and the primary voltage divider around error amplifier 224. Typically, it is desirable for V_out to follow V_ref′ because this prevents abrupt changes in V_out allowing a controlled start-up. If V_out changes abruptly start-up is not controlled and transients may be generated adversely affecting other circuits that are supplied by switching voltage regulator circuit 200.

[0035] As mentioned before, a rate at which V_ref′ changes is governed by the total capacitance at the non-inverting input of error amplifier 224 and a value of resistors R_XA and R_YA that make up the auxiliary voltage divider. By providing V_ref′ to node N242 between resistors R_XA and R_YA of the auxiliary voltage divider, which are arranged to have very low values, the rate at which V_ref′ decays may be much greater than without the auxiliary voltage divider. The faster decay may allow V_ref′ to match V_out even when V_EN is held low for very brief intervals. This may allow output voltage V_out to always start up in a controlled manner.

[0036] While FIG. 2 shows the regulator circuit as a buck switching regulator, other regulation topologies such as fly-back, boost, switched capacitor, and the like, may also employ the soft starting method described above without departing from the scope and spirit of the present invention.

[0037] FIG. 3 schematically illustrates an embodiment of a linear voltage regulator circuit 300 according to the present invention. Linear voltage regulator circuit 300 includes reference voltage source 308, soft start circuit 310, feedback circuit 306, error amplifier 324, feedback impedance Z_C, and output stage 304.

[0038] Reference voltage source 308, soft start circuit 310, feedback circuit 306, error amplifier 324, and feedback impedance Z_C essentially include same components and operate as the same way as described in FIG. 2.

[0039] Output stage 304 is, however, different. Instead of the drivers and switching transistors of the switching voltage regulator of FIG. 2, linear voltage regulator 300 includes power pass transistor 342 that is arranged to be operated in its linear region. A gate of power pass transistor 342 is controlled by an output of error amplifier 324. An output of power pass transistor 342 is coupled to an LC filter similar to the output stage of switching voltage regulator 200 of FIG. 2.

[0040] FIG. 4 illustrates voltage diagrams 450, 460, and 470 of enable, reference and output voltages V_EN, V_ref′, and V_out of an embodiment of switching voltage regulator circuit 200 of FIG. 2. A vertical axis of each voltage diagram represents voltage, while a horizontal axis represents time.

[0041] Voltage diagram 450 includes waveform 451 of V_EN during an exemplary operation of the switching voltage regulator circuit. The operation begins at time point t₁ when V_EN changes to a high level. After a period of normal operation a disable condition such as a fault condition, a shutdown, and the like occurs at time point t₂, when V_EN drops to a low level. Following a short duration, the circuit is enabled again at time point t₃ when V_EN goes high again.

[0042] Voltage diagram 460 includes waveforms 461 and 462. Waveform 461 represents modified reference voltage V_ref′ under the conditions described above without the switch being enabled. Waveform 462 represents modified reference voltage V_ref′ under the same conditions when the effect of the enabled switch according to the present invention is included.

[0043] Both waveforms begin to rise slowly at t₁ following a linear slope and reach nominal voltage V_nom. The relatively high resistance of the primary voltage divider of switching voltage regulator circuit 200 may prevent a rapid discharge of capacitance at reference node N241 of FIG. 2. Therefore, following a disable condition at t₂, waveform 461 begins to decay slowly. Accordingly, V_ref′ drops to V₂ at t₃. When the circuit is enabled again, waveform 461 rises rapidly from V₂ to V_nom.

[0044] Waveform 462 is virtually identical to waveform 461 until t₂. However, waveform 462 decays at a relatively faster rate between t₂ and t₃ due to a low resistance discharge path provided by the auxiliary voltage divider of switching voltage regulator circuit 200 of FIG. 2. This low resistance path may decrease an RC time constant at reference node N241 resulting in a faster decay rate for V_ref′. Because the decay rate is faster, V_ref′ reaches V₁, which may be significantly lower than V₂ at t₃. When the circuit is enabled at t₃, waveform 462 begins to rise slowly and reaches nominal reference voltage V_nom much later than waveform 461.

[0045] Voltage diagram 470 includes waveforms 471 and 472. Similar to voltage diagram 460, Waveform 471 represents output voltage V_out under the conditions described above without an effect of the enabled switch. Waveform 472 represents V_out under the same conditions when the effect of the enabled switch according to the present invention is included.

[0046] Both waveforms begin to rise slowly at t₁ following a linear slope and reach regulation voltage V_reg. When the disable condition occurs at t₂, both waveforms begin to decay according to an exponential capacitor decay curve of an output capacitor dropping to V₃ at t₃. When the circuit is enabled again, waveform 471 rises rapidly to V_reg potentially generating voltage and current transients.

[0047] Waveform 472, however, rises at a slower rate after t₃ due to slowly rising modified reference voltage V_ref′. V_out of waveform 472 reaches V_reg later, but voltage and current transients are prevented.

[0048] 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.