Area-friendly method for providing duty cycle inverse to supply voltage

1. An oscillator for use with an inductive converter, the oscillator comprising:

input power terminals that receive an input voltage;

a capacitor that is alternately coupled to a charging current source and a discharging current source, the charging current source operating to charge the capacitor at a rate proportional to the input voltage; and

a comparator that asserts an output signal when the capacitor charges to a first threshold voltage and deasserts the output signal when the capacitor discharges to a second threshold voltage,

wherein the first charging current source is one of a plurality of selectable charging current sources, the plurality including a second charging current source that charges the capacitor at a fixed rate.

2. The oscillator of claim 1 , wherein the discharging current source operates to discharge the capacitor at a fixed rate.

3. The oscillator of claim 2 , wherein the output signal has a duty cycle of 1/(S+1) and a period proportional to (S+1)/S, where S is the input voltage.

4. The oscillator of claim 1 , further comprising a swapping voltage reference that alternately supplies the first and second threshold voltages to the comparator.

5. The oscillator of claim 1 , further comprising CMOS transistors that, in response to the output signal, alternately couple the capacitor to the discharging current source and a selected one of the plurality of selectable charging current sources.

6. An oscillation method that comprises:

receiving an input voltage at input power terminals;

selecting a charging current source from a plurality of selectable charging current sources;

alternately coupling a capacitor to the selected charging current source and a discharging current source, the plurality including:

a first charging current source that operates to charge the capacitor at a rate proportional to the input voltage, and

a second charging current source that operates to charge the capacitor at a fixed rate;

asserting an output signal when a comparator determines that the capacitor has charged to a first threshold voltage; and

deasserting the output signal when the comparator determines that the capacitor has discharged to a second threshold voltage.

7. The oscillation method of claim 6 , wherein the discharging current source operates to discharge the capacitor at a fixed rate.

8. The oscillation method of claim 7 , wherein the output signal has a duty cycle of 1/(S+1) and a period proportional to (S+1)/S, where S is the input voltage.

9. The oscillation method of claim 6 , further comprising: alternately supplying the first and second threshold voltages to the comparator with a swapping voltage reference.

10. The oscillation method of claim 6 , wherein said alternately coupling employs CMOS transistors that alternately couple the selected charging current source and discharging current source to the capacitor in response to the output signal.

11. An oscillator that comprises:

a capacitor that is alternately coupled to a charging current source and a discharging current source, a first one of said charging and discharging current sources setting a capacitor current proportional to an input voltage, and a second one of said charging and discharging current sources setting the capacitor current at a preset constant value;

a comparator that asserts an output signal when the capacitor charges to a first threshold voltage and deasserts the output signal when the capacitor discharges to a second threshold voltage; and

a swapping voltage reference that alternately supplies the first and second threshold voltages to the comparator.

12. The oscillator of claim 11 , wherein the output signal has a period proportional to (S+1)/S, where S is the input voltage.

13. The oscillator of claim 11 , wherein the charging current source is one of a plurality of selectable charging current sources, one of which sets the capacitor current proportional to the input voltage and another of which sets the capacitor current at a preset constant value.

14. The oscillator of claim 11 , wherein the discharging current source is one of a plurality of selectable discharging current sources, one of which sets the capacitor current proportional to the input voltage and another of which sets the capacitor current at a preset constant value.

15. The oscillator of claim 11 , further comprising CMOS transistors that alternately couple the charging current source and discharging current source to the capacitor in response to the output signal.

16. An oscillator that comprises:

a capacitor that is alternately coupled to a charging current source and a discharging current source, the charging current source setting the capacitor current at a preset constant value and the discharging current source being a selectable one of a plurality of discharging current sources, one of which sets the capacitor current proportional to an input voltage and another of which sets the capacitor current at a preset constant value; and

a comparator that asserts an output signal when the capacitor charges to a first threshold voltage and deasserts the output signal when the capacitor discharges to a second threshold voltage.

17. The oscillator of claim 16 , wherein the output signal has a period proportional to (S+1)/S, where S is the input voltage.