Power supply controller having frequency foldback and volt-second duty cycle clamp features

Claims

We claim:

1. A power supply controller for controlling the duty cycle of a power supply switch as a function of an oscillator ramp signal generated across a timing capacitor in order to provide a regulated output voltage, said power supply controller comprising:

a first current source for charging the timing capacitor with a first fixed current; and

a second current source for discharging the timing capacitor with a second fixed current when the regulated output voltage is greater than a predetermined level and for discharging the timing capacitor with a variable current when the regulated output voltage is less than said predetermined level.

2. The power supply controller recited in claim 1 wherein said variable current decreases as the regulated output voltage decreases below said predetermined level.

3. The power supply controller recited in claim 1 further comprising:

a first operational amplifier having a first input responsive to a reference voltage and a second input coupled to a first resistor and to an output of said first operational amplifier, wherein said first current source is controlled by a current through said first resistor; and

a second operational amplifier having a first input responsive to a voltage proportional to the regulated output voltage and a second input coupled to a second resistor and to an output of said second operational amplifier, wherein said second current source is controlled by a current through said second resistor, said current through said second resistor being a function of said voltage proportional to the regulated output voltage.

4. The power supply controller recited in claim 1 wherein said first current source charges the timing capacitor for a fixed duration and said second current source discharges the timing capacitor for a variable duration.

5. The power supply controller recited in claim 4 further comprising:

a first comparator for comparing the oscillator ramp signal to a first reference voltage to provide a first comparator output signal;

a second comparator for comparing the oscillator ramp signal to a second reference voltage to provide a second comparator output signal;

a latch set in response to said first comparator output signal and reset in response to said second comparator output signal to provide a latch output signal;

a first switch coupled in series between said first current source and the timing capacitor, said first switch responsive to said latch output signal for selectively coupling and decoupling said first current source to and from the timing capacitor to control said fixed duration during which said first current source charges the timing capacitor; and

a second switch coupled in series between the timing capacitor and said second current source, said second switch responsive to an inverted version of said latch output signal for selectively coupling and decoupling said second current source to and from the timing capacitor to control said variable duration during which said second current source discharges the timing capacitor.

6. A power supply comprising:

a power switch operable at a variable duty cycle to convert an input voltage to a regulated output voltage;

a timing capacitor across which an oscillator ramp signal is generated, wherein said variable duty cycle is a function of said oscillator ramp signal; and

a power supply controller responsive to said oscillator ramp signal for providing a drive signal to said power switch to control said variable duty cycle, said power supply controller comprising:

a first current source for charging said timing capacitor with a fixed current; and

a second current source for discharging said timing capacitor with a fixed current when said regulated output voltage is greater than a predetermined level and for discharging said timing capacitor with a variable current when said regulated output voltage is less than said predetermined level.

7. The power supply recited in claim 6 wherein said variable current decreases as said regulated output voltage decreases below said predetermined level.

8. The power supply recited in claim 6 further comprising:

a first operational amplifier having a first input responsive to a reference voltage and a second input coupled to a first resistor and to an output of said first operational amplifier, wherein said first current source is controlled by a current through said first resistor; and

a second operational amplifier having a first input responsive to a voltage proportional to said regulated output voltage and a second input coupled to a second resistor and to an output of said second operational amplifier, wherein said second current source is controlled by a current through said second resistor, said current through said second resistor being a function of said voltage proportional to said regulated output voltage.

9. The power supply recited in claim 6 wherein said first current source charges said timing capacitor for a fixed duration and said second current source discharges said timing capacitor for a variable duration.

10. The power supply recited in claim 9 further comprising:

a first comparator for comparing said oscillator ramp signal to a first reference voltage to provide a first comparator output signal;

a second comparator for comparing said oscillator ramp signal to a second reference voltage, higher than said first reference voltage to provide a second comparator output signal;

a latch set in response to said first comparator output signal and reset in response to said second comparator output signal to provide a latch output signal;

a first switch coupled in series between said first current source and said timing capacitor, said first switch responsive to said latch output signal for selectively coupling and decoupling said first current source to and from said timing capacitor to control said fixed duration during which said first current source charges said timing capacitor; and

a second switch coupled in series between said timing capacitor and said second current source, said second switch responsive to an inverted version of said latch output signal for selectively coupling and decoupling said second current source to and from said timing capacitor to control said variable duration during which said second current source discharges said timing capacitor.

11. The power supply recited in claim 8 further comprising a peak detector circuit responsive to said regulated output voltage for generating said voltage proportional to said regulated output voltage.

12. The power supply recited in claim 6 wherein said power supply controller is a peak current mode controller.

13. The power supply recited in claim 6 further comprising a volt-second duty cycle clamp circuit for limiting said variable duty cycle of said power switch to a maximum duty cycle which varies in accordance with said input voltage.

14. A power supply controller for controlling the duty cycle of a power supply switch which is operable to convert an input voltage to a regulated output voltage, said power supply controller comprising:

a volt-second duty cycle clamp circuit for limiting the duty cycle of the power supply switch to a maximum duty cycle set as a function of the voltage across a capacitor charged by a charging current inversely proportional to the input voltage, wherein said capacitor has a tolerance associated therewith and said maximum duty cycle is independent of said tolerance of said capacitor.

15. The power supply controller recited in claim 14 wherein said volt-second duty cycle clamp circuit comprises:

a capacitor;

a comparator having a first input receiving an oscillator ramp signal, a second input coupled to sa