Constant frequency current-mode buck-boost converter with reduced current sensing

1. A method of controlling a constant frequency current-mode buck-boost converter, the buck-boost converter including an inductor, a first switch connected between an input terminal of the converter and a first end of the inductor, a first current control means connected between the first end of the inductor and ground, a second current control means connected between a second end of the inductor and an output terminal of the converter, and a second switch coupled between the second end of the inductor and ground; the method comprising:

deriving a feedback signal from the output terminal;

sensing a signal representative of current in the inductor to provide a sensed current signal;

providing first and second ramp signals, the first ramp signal for use in controlling buck operation through at least the first switch, the second ramp signal for use in controlling boost operation through at least the second switch; and

generating drive signals to repeatedly open and close at least the first switch and second switch at individual duty cycles determined on the basis of the feedback signal, the sensed current signal and the first and second ramp signals; wherein

the first and second ramp signals are mutually offset to one another in time, in such a way as to allow said sensing step to be performed only while the first switch is closed.

2. The method according to claim 1 , wherein the first and second ramp signals are mutually offset by 50% of a repetition period of operation of the switches.

3. The method according to claim 1 , wherein the drive signals are adapted to perform peak-valley current mode control of the converter.

4. The method according to claim 3 , wherein said sensing step is performed by sensing the current flowing in the first switch at a timing immediately before closing another said switch of the converter.

5. The method according to claim 1 , which is applied during buck-boost operation of the converter with a high buck duty cycle and a low boost duty cycle.

6. The method according to claim 1 , wherein each of the ramp signals is a periodic sawtooth signal, the slope of the first ramp signal being opposite to that of the second ramp signal.

7. The method according to claim 6 wherein the sloping portion of each sawtooth waveform occupies substantially the whole cycle period of the ramp signals.

8. The method according to claim 6 wherein the sloping portion of the sawtooth waveform is preceded or followed by a flat portion in each cycle.

9. The method according to claim 1 , wherein said sensing step is performed using an amplifier in parallel with the first switch and the timing of performing the sensing step takes account of a settling time of the amplifier.

10. The method according to claim 1 , wherein the step of deriving a feedback signal includes comparing a voltage at the output terminal with a reference voltage to provide an error voltage for use in said step of generating drive signals.

11. The method according to claim 10 , wherein said step of generating drive signals includes supplying the error voltage, the sensed current signal and the first ramp signal to a buck comparator, and supplying the error voltage, the sense current signal and the second ramp signal to a boost comparator, the outputs of the buck comparator and boost comparator being processed to provide the first drive signal for buck operation and the second drive signal for boost operation.

12. A controller for a constant-frequency current-mode buck-boost converter, the converter including an inductor, a first switch connected between an input terminal of the converter and a first end of the inductor, a first current control means connected between the first end of the inductor and ground, a second current control means connected between a second end of the inductor and an output terminal of the converter, a second switch coupled between the second end of the inductor and ground, and current sensing means for sensing a signal representative of current in the inductor to provide a sensed current signal, the controller comprising:

feedback means for deriving a feedback signal from the output terminal;

a waveform generator for generating first and second ramp signals, the first ramp signal for use in controlling buck operation through at least the first switch, the second ramp signal for use in controlling boost operation through at least the second switch; and

control logic for generating drive signals to repeatedly open and close at least the first switch and second switch at individual duty cycles determined on the basis of the feedback signal, the sensed current signal and the first and second ramp signals; wherein

the waveform generator is arranged to provide the first and second ramp signals mutually offset with respect to one another in time; and

the control logic is arranged to employ the sensed current signal only while the first switch is closed.

13. The controller according to claim 12 , wherein the waveform generator is arranged to generate the first and second ramp signals mutually offset by 50% of a repetition period of operation of the switches.

14. The controller according to claim 12 , wherein the waveform generator is arranged to generate each of the ramp signals in the form of a periodic sawtooth signal, the slope of the first ramp signal being opposite to that of the second ramp signal.

15. The controller according to claim 12 , wherein the feedback means includes means for comparing a voltage at the output terminal with a reference voltage to provide an error voltage.

16. The controller according to claim 15 , wherein the control logic includes a buck comparator arranged to receive the error voltage, the sensed current signal and the first ramp signal and a boost comparator arranged to receive the error voltage, the sensed current signal and the second ramp signal, and drive circuitry responsive to the outputs of the buck comparator and boost comparator to provide the first drive signal for buck operation and the second drive signal for boost operation.

17. A constant-frequency current-mode buck-boost converter comprising an inductor, a first switch connected between an input terminal of the converter and a first end of the inductor, a first current control means connected between the first end of the inductor and ground, a second current control means connected between a second end of the inductor and an output terminal of the converter, a second switch coupled between the second end of the inductor and ground, and an inductor current sensing means, and further comprising the controller according to claim 12 .

18. The current-mode buck-boost converter according to claim 17 , wherein the inductor current sensing means comprises an amplifier in parallel with the first switch and the control logic is arranged to receive the current sense signal at a timing which takes account of a settling time of the amplifier.

19. A system incorporating the current mode buck-boost converter according to claim 17 .

20. The system according to claim 19 , wherein the system is a portable electronic device.

21. The system according to claim 19 , wherein the system is a hard-disk based electronic device.