Apparatus and method for step-down switching voltage regulation

[0001] This application is a continuation-in-part of the application having the application Ser. No. 10/835,616, filed on Apr. 29, 2004, of which the benefit of the earlier filing date is hereby claimed under 35 U.S.C. § 120, and which is hereby incorporated by reference.

[0002] The invention is related to voltage regulation, and in particular, to an apparatus and method for a step-down switch voltage regulator circuit that samples a sensed current of the low-side switch.

[0003] 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 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. In current mode switching regulators, current sensing is also employed. Previously, current sensing has been accomplished by coupling a resistor in series with the inductor, and sensing a voltage drop across the resistor.

[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] Briefly stated, the invention is related to a current mode PWM buck regulator. The regulator includes a top-side transistor, a bottom-side transistor, an inductor, a sample-and-hold circuit, a ramp generator, a PWM comparator, an error amplifier, and a current sense amplifier. A current through the low-side transistor is sensed by the current sense amplifier. Also, a current sense voltage provided by the current sense amplifier is sampled when the low-side transistor is on, and held when the low-side transistor is off.

[0013] The ramp generator is arranged to generate a voltage ramp that emulates the upslope of the inductor current. Additionally, the sampled low-side transistor current is combined with the voltage ramp. The PWM comparator is arranged to provide a PWM signal by comparing the voltage ramp to a comparison signal provided by the error amplifier.

[0014] FIG. 1 illustrates a block diagram of an embodiment of regulator circuit 100. Regulator circuit 100 includes top-side switch circuit 151, bottom-side switch circuit 152, inductor circuit L1, capacitor circuit C1, current-sense amplifier circuit A1, sample-and-hold circuit 110, and switch control circuit 120.

[0015] Inductor circuit L1 is arranged to provide current I1. More specifically, inductor circuit L1 is arranged such that a voltage substantially given by Vin−Vout is across inductor LI when switch circuit 151 is closed. Accordingly, dI1/dt is substantially given by (Vin−Vout)/L1 when switch circuit 151 is closed.

[0016] Also, capacitor circuit C1 is configured to provide output voltage Vout from current I1. Current-sense amplifier A1 is configured to sense a current across switch circuit 152 and provide current sense voltage Vcs in response to the sensed current. Additionally, sample-and-hold circuit 110 is configured to sample voltage Vcs when switch circuit 152 is closed, and further configured to hold voltage Vcs when switch circuit 152 is open. Switch control circuit 120 is configured to provide signals DRV1 and DRV2 based in part on signals Vout and Vsh. Further, switch control circuit 120 is configured to provide signal DRV2 as an inverse of signal DRV1. Switch circuit 151 is configured to open and close responsive to signal DRV1, and switch circuit 152 is configured to open and close responsive to signal DRV2.

[0017] FIG. 2 shows a block diagram of an embodiment of regulator circuit 200 that includes a current source circuit for emulating the inductor current. Components in regulator circuit 200 may operate in a substantially similar manner to similarly named components in regulator circuit 100, and may operate in a different manner in some ways. Regulator circuit 200 further includes current source circuit 230 and resistor R0. In regulator circuit 200, top-side switch circuit 251 includes transistor M1, and bottom-side switch circuit 252 includes transistor M2.

[0018] In operation, current source circuit 230 is configured to provide a voltage that is substantially equal output voltage Vout at node N1. Accordingly, resistor R0 has a voltage substantially equal to Vin−Vout across it. Resistor R0 is arranged to provide current 12, where current 12 is substantially given by (Vin−Vout)/R0. Also, current source circuit 230 is further configured to provide current 13 from current 12.

[0019] Additionally, ramp generator circuit 240 provides voltage Vramp from current 13 and voltage Vsh. By proper pre-selection of a resistance that is associated with resistor R0, the upslope associated with voltage Vramp is substantially similar to the upslope associated with current I1. Also, the voltage ramp is offset according to voltage Vsh. Accordingly, voltage Vramp substantially emulates current I1. Switch control circuit 220 is arranged to provide signals DRV1 and DRV2, based in part on signals Vramp and Vout.

[0020] FIG. 3 schematically illustrates an embodiment of regulator circuit 300. Components in regulator circuit 300 may operate in a substantially similar manner to similarly named components in regulator circuit 300, and may operate in a different manner in some ways. Current source circuit 330 may include transistors Q1–Q4 and operational amplifier circuit A2. Sample-and-hold circuit 310 includes switch circuit S1 and capacitor circuit C2. Also, ramp generator circuit 340 may include capacitor circuit C3, transistor M3, and operational amplifier circuit A3. Switch control circuit 320 may include resistors R1 and R2, error amplifier A4, PWM comparator circuit 360, Inverter Inv1, and buffers B1 and B2.

[0021] In operation, operational amplifier circuit A2 may adjust the base voltage of transistors Q1 and Q2 such that the voltage at node N1 is substantially equal to output voltage Vout. Transistors Q1 and Q2 may be arranged as a current source to mirror current I2 to the collector of transistor Q3. Similarly, transistors Q3 and Q4 may be arranged as another current source that is configured to mirror the collector current of transistor Q3 to provide current I3.

[0022] Also, transistor M3 is arranged as a switch that is responsive to signal Reset. Amplifier circuit A3 may be arranged as a follower, such that sensed signal Vc is substantially equal to voltage Vsh. When signal Reset is low, signal Ramp begin linearly increasing from signal Vc. In this way, signal Vc is combined with signal Ramp. Signal Ramp continues to linearly increase until signal Reset changes to high. When signal Reset changes to high, capacitor C3 discharges, and signal Ramp returns to voltage Vc.

[0023] Further, the upslope associated with signal Ramp is substantially given by dV/dt=(Vin−Vout)/(R0*C3). Accordingly, by properly pre-selecting the resistance of resistor R0, the upslope of the voltage ramp is substantially similar to the upslope of inductor current I1.

[0024] Additionally, in one embodiment, if Vin is substantially greater than Vout, the value of Vout can be ignored, e.g., where Vin is relatively equivalent to 10 volts and Vout is relatively equivalent to 1 volt. In this embodiment, the upslope associated with signal Ramp can be represented by dV/dt=Vin/(R0*C3).

[0025] Also, resistors R1 and R2 may be arranged as a voltage divider that is configured to provide voltage Vfb from voltage Vout. Error amplifier A4 may be configured to provide signal Comp from signals Vfb and Vref. PWM comparator circuit 360 is configured to provide signal PWM from in response to a comparison of signals Comp and Ramp. Buffer B1 may be configured to provide signal DRV1 from signal PWM. Also, inverter circuit Inv1 may be arranged to invert signal DRV1. Buffer B2 may be arranged to buffer the output signal of inverter circuit Inv1 to provide signal DRV2.

[0026] Although a particular arrangement regulator circuit 300 is illustrated in FIG. 3, other embodiments may include other arrangements of circuit 300. For example, buffers B1 and B2 and operational amplifier A3 are optional circuit components that need not be included in regulator circuit 300. Additionally, current source circuit 330 may be arranged in a different manner than shown in FIG. 3. Also, although operational amplifier A2 is employed to provide a voltage that is substantially equal to Vout at node N1, operational amplifier A2 may be replaced with a different circuit that is arranged to provide a voltage that is substantially equal to Vout at node N1. Although FIG. 3 illustrates an embodiment in Inverter Inv1 is employed to provide signal DRV2 as an inverse of signal DRV1, in other embodiments, other arrangements may be employed to provide signal DRV2 as an inverse of signal DRV1.

[0027] FIG. 4 illustrates another embodiment of the invention where the operation of a ramp generator capacitive element and a sample and hold capacitive element are combined in one capacitive element C1. In this embodiment, if S1 is closed, the combined capacitor operates as the sample and hold capacitor discussed above. Similarly, if S2 is closed, the combined capacitor operates as the ramp generator capacitor discussed above. Further, when the low side switch is on, S1 is on and S2 is off.

[0028] FIG. 5 illustrates an embodiment of regulator circuit 500 arranged in accordance with aspects of the present invention. Regulator circuit 500 includes top-side switch circuit M577, bottom-side switch circuit M578, inductor circuit L579, capacitor circuit C580, resistor circuit R576, current-sense amplifier circuit 595, sample-and-hold circuit 574, oscillator 570, current source circuit 571, ramp generator circuit 572, current multiplier 575, and switch control circuit 573.

[0029] Inductor circuit L579 is arranged to provide current 11. More specifically, inductor circuit L579 is arranged such that a voltage substantially given by Vin−Vout is across inductor L579 when switch circuit M577 is closed. Accordingly, dIl/dt is substantially given by (Vin−Vout)/L579 when switch circuit M577 is closed.

[0030] Also, capacitor circuit C580 is configured to provide output voltage Vout based on current I1. Current-sense amplifier 595 is configured to sense a current across switch circuit M578 and provide current sense voltage Vcs in response to the sensed current. Additionally, sample-and-hold circuit 574 is configured to sample voltage Vcs when switch circuit M578 is closed, and further configured to hold voltage Vcs when switch circuit M578 is open. Switch control circuit 573 is configured to provide a first signal and a second signal to switch circuits M577 and M578 based in part on Vramp. Further, switch control circuit 573 is configured to provide the second signal as an inverse of the first signal. Switch circuit M577 is configured to open and close responsive to the first signal, and switch circuit M578 is configured to open and close responsive to the second signal.

[0031] Switch control circuit 573 is arranged to be controlled by Vramp generated by ramp generator circuit 572. Ramp generator circuit 572 is arranged to receive current 15 from current multiplier 575 and voltage Vsh from sample and hold circuit 573. Current multiplier 575 is arranged to determine I5 based on currents I4 from oscillator 570 and I3 from current source circuit 571 such that I5=(I3*I4)/I1, where I1 is reference current and 12 uA in this case. Current source circuit 571 is arranged to receive Vout and I2, where I2=(Vin−Vout)/R2. Current source circuit 571 is further arranged to provide 13 such that I3=(Vin−Vout)/R, where the R is 2 Mohm. Moreover, I4 is determined by oscillator 570 such that I4=fosc*K, where K is 6*10⁻¹², and fosc is a frequency of the oscillator. I1, R, and K may be different value depending upon current mirror ratio in analog multiplier circuit 575 and current source circuit 571, or a capacitor value in the ramp generator circuit 572.

[0032] By employing current source circuit 571 and oscillator 570 to determine I5, which is used to determine Vramp, a frequency and a magnitude of Vramp may be modified without using an external resistor. In one embodiment, oscillator 570 may comprise a current controlled oscillator enabling a user to modify Vramp by changing a value of a current provided to oscillator 570.

[0033] FIG. 6 schematically illustrates an embodiment of analog current multiplier 675 along with oscillator 681, current source circuit 682, and ramp generator circuit 692. Analog current multiplier 675 is one embodiment current multiplier 575 of the regulator circuit of FIG. 5. Analog current multiplier 675 includes transistors M683–M690 and current source 691.

[0034] Transistors M689 and M690 are arranged to operate as a current mirror providing I3 from current source circuit 682 to an emitter terminal of M684. I3 may be expressed as I3=(Vin−Vou)/R, where R is 2 Mohm, as described in conjunction with FIG. 5 above. M684 and M685 are cascaded with their collector terminals coupled to a supply voltage and the base terminals driven by I4 provided by oscillator 681, where I4=fosc*K, where K is 6*10⁻¹², as described above. Current source 691 is coupled to an emitter terminal of M685 and is arranged to provide I1 with a value of about 12 μA.

[0035] A collector terminal of transistor M688 is also coupled to the base-collector terminal of diode connected transistor M686 such that current I5 flows through a collector terminal of M688. I5 may expressed as I5=(I3*I4)/I1. A relationship between base-emitter voltages of transistors M683, M684, M685, and M688 may be expressed as:
V_BE(688)=V_BE(683)+V_BE(684)−V_BE(685), where
V_BE(683)=V_T*ln(I4/Is), V_BE(684)=V_T*ln(I3/Is), and V_BE(685)=V_T*ln(I1/Is).

[0036] where Is is satulation current of bipolor transistor.

[0037] From the above base-emitter voltages, V_BE(688) may be expressed as:

[0038] V_BE(688)=V_T*ln(I3*I4)/(I1*Is)=Vt*ln(I5/Is). Assuming I1 is about 12 μA as mentioned previously, I5 may be expressed as

[0039] $I5=I3*\frac{I4}{12*{10}^{-6}}.$
Transistors M686 and M687 are arranged to operate as a current mirror providing 15 to ramp generator circuit 692.

[0040] While analog current multiplier is shown employing BJT transistors and references to BJT technology are made throughout the specification and claims, other circuit technique such as Gilbert type analog multiplier may also be implemented without departing from the scope or spirit of the invention.

[0041] Each of the resistor circuits described may consist of a single resistor only, or may include a plurality of resistors that are coupled in series or in parallel, and the like. Similarly, each of the capacitor circuits describe may consist of a single capacitor only, or may include a plurality of capacitors that are coupled in series or in parallel, and the like. These and other embodiments are within the spirit and scope of the invention.

[0042] 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.