Buck-boost converter control circuits and techniques

1. A controller for a Buck-Boost converter comprising a Buck switch coupled between an input voltage and an inductor and a Boost switch coupled to the inductor and through an output switch to an output of the converter at which an output voltage is provided, the controller comprising:

a feedback control circuit responsive to the converter output and configured to generate a first control signal for one of the Buck switch and the Boost switch; and

a feed forward control circuit responsive to the input voltage and configured to generate a second control signal for the other one of the Buck switch and the Boost switch, wherein the feed forward control circuit comprises a programmable device selected based on a predetermined level for the output voltage, wherein the feedback control circuit is configured to generate the first control signal for the Buck switch and the feed forward control circuit is configured to generate the second control signal for the Boost switch, and wherein a duty cycle of the Boost switch is based on the value of the input voltage, a desired duty cycle of the Buck switch when the converter transitions between a Buck mode of operation and a Buck-Boost mode of operation, and the predetermined level for the output voltage.

2. The controller of claim 1 wherein the feedback control circuit is configured to generate the first control signal for the Buck switch and the feed forward control circuit is configured to generate the second control signal for the Boost switch.

3. The controller of claim 1 wherein the programmable device comprises a resistor.

4. The controller of claim 2 wherein the duty cycle D boost of the Boost switch is substantially given by:

D

boost

≅

1

-

D

buck

0

V

out

0

+

V

fb

×

Vin

where Vin is the value of the input voltage, V fb is a diode drop, D buck 0 is a desired duty cycle of the Buck switch when the converter transitions between a Buck mode of operation and a Buck-Boost mode of operation, and V out 0 a predetermined level for the output voltage.

5. The controller of claim 1 wherein the feedback control circuit comprises:

an error amplifier responsive to a feedback signal from the converter output and to a reference signal to generate an error signal proportional to a difference between the feedback signal and the reference signal; and

a comparator responsive to the error signal and to a PWM ramp signal to generate the first control signal.

6. The controller of claim 5 wherein the feedback control circuit implements voltage mode control.

7. The controller of claim 5 wherein the PWM ramp signal is based in part on a current through the inductor and the feedback control circuit implements current mode control.

8. A Buck-Boost converter to supply a regulated output voltage at an output node, the converter comprising:

an inductor having first and second terminals;

a first switch coupled between an input voltage and the first terminal of the inductor;

a second switch coupled between the first terminal of the inductor and a reference voltage;

a third switch coupled between the second terminal of the inductor and the output node;

a fourth switch coupled between the second terminal of the inductor and the reference voltage;

a feedback control circuit responsive to the output voltage and configured to generate a control signal for the first switch; and

a feed forward control circuit responsive to the input voltage and configured to generate a control signal for the fourth switch, wherein the feed forward control circuit comprises a programmable device selected based on a predetermined level for the output voltage, wherein the feedback control circuit is configured to generate the first control signal for the first switch and the feed forward control circuit is configured to generate the second control signal for the fourth switch, and wherein a duty cycle of the fourth switch is based on the value of the input voltage, a desired duty cycle of the first switch when the converter transitions between a Buck mode of operation and a Buck-Boost mode of operation, and the predetermined level for the output voltage.

9. The converter of claim 8 wherein the converter is operable in (1) the Buck mode of operation in which the first switch is turned on and off according to a Buck duty cycle and the fourth switch is off; or (2) in the Buck-Boost mode of operation in which the first and fourth switches are on during a first phase and the first and fourth switches are off during a second phase.

10. The converter of claim 9 wherein the duty cycle of the fourth switch is substantially given by:

D

boost

≅

1

-

D

buck

0

V

out

0

+

V

fb

×

Vin

where Vin is the value of the input voltage, V fb is a forward voltage drop of the third switch, D buck 0 is a desired duty cycle of the Buck switch when the converter transitions between the Buck mode of operation and the Buck-Boost mode of operation, and V out 0 a predetermined level for the output voltage.

11. The converter of claim 9 wherein the Buck-Boost mode of operation further comprises a third phase in which the first switch is on and the fourth switch is off.

12. The converter of claim 8 wherein the programmable device comprises a resistor.

13. The converter of claim 8 wherein the feedback control circuit comprises:

an error amplifier responsive to a feedback signal from the converter output and to a reference signal to generate an error signal proportional to a difference between the feedback signal and the reference signal; and

a comparator responsive to the error signal and to a PWM ramp signal to generate the control signal for the first switch.

14. A controller for a Buck-Boost converter comprising a Buck switch coupled between an input voltage and an inductor and a Boost switch coupled to the inductor and through an output switch to an output of the converter at which an output voltage is provided, the controller comprising:

a feedback control circuit responsive to the converter output and configured to generate a first control signal for the Buck switch; and

a feed forward control circuit responsive to the input voltage and configured to generate a second control signal for the Boost switch, wherein a duty cycle D boost of the Boost switch is substantially given by:

D

boost

≅

1

-

D

buck

0

V

out

0

+

V

fb

×

Vin

where Vin is the value of the input voltage, V fb is a diode drop, D buck 0 is a desired duty cycle of the Buck switch when the converter transitions between a Buck mode of operation and a Buck-Boost mode of operation, and V out 0 is a predetermined level for the output voltage.

15. A Buck-Boost converter to supply a regulated output voltage at an output node, the converter comprising:

an inductor having first and second terminals;

a first switch coupled between an input voltage and the first terminal of the inductor;

a second switch coupled between the first terminal of the inductor and a reference voltage;

a third switch coupled between the second terminal of the inductor and the output node;

a fourth switch coupled between the second terminal of the inductor and the reference voltage;

a feedback control circuit responsive to the output voltage and configured to generate a control signal for the first switch; and

a feed forward control circuit responsive to the input voltage and configured to generate a control signal for the fourth switch, wherein the converter is operable in (1) a Buck mode of operation in which the first switch is turned on and off according to a Buck duty cycle and the fourth switch is off; or (2) in a Buck-Boost mode of operation in which the first and fourth switches are on during a first phase and the first and fourth switches are off during a second phase and wherein a duty cycle of the fourth switch is substantially given by:

D

boost

≅

1

-

D

buck

0

V

out

0

+

V

fb

×

Vin

where Vin is the value of the input voltage, V fb is a forward voltage drop of the third switch, D buck 0 is a desired duty cycle of the Buck switch when the converter transitions between the Buck mode of operation and the Buck-Boost mode of operation, and V out 0 is a predetermined level for the output voltage.