Single-shot duty cycle generator for a switched-mode power supply

1. A switched-mode power supply device comprising:

a power switch configured to transfer power from a supply line to a load in switched-mode; and

a duty cycle generator that includes:

a first oscillator configured to operate at a frequency proportional to a voltage of the supply line; and

a second oscillator configured to operate at a frequency proportional to a voltage of the load; wherein:

the duty cycle generator is configured to operate the power switch according to a duty cycle based on a ratio between the first and second oscillator frequencies.

2. The device of claim 1 , wherein the first and second oscillators have respective voltage-to-frequency responses that match each other.

3. The device of claim 1 , wherein the duty cycle generator includes:

a first counter configured to count cycles of the first oscillator;

a second counter configured to count cycles of the second oscillator;

a clock configured to reset the first and second counters at each clock cycle transition;

a latch configured to store content of the second counter at each clock cycle transition; and

a comparator configured to turn the power switch on while content of the first counter is smaller than the content of the latch.

4. The device of claim 1 , wherein:

the first oscillator comprises:

an odd number of first inverters connected in a first ring;

a first plurality of current sources respectively configured to supply the first inverters; and

a first adjustment circuit configured to set a current of each current source of the first oscillator to a value proportional to the voltage of the supply line; and

the second oscillator comprises:

an odd number of second inverters connected in a second ring;

a second plurality of current sources respectively configured to supply the second inverters; and

a second adjustment circuit configured to set a current of each current source of the second oscillator to a value proportional to the voltage of the load.

5. A device comprising:

a first oscillator configured to operate at a frequency proportional to a first voltage;

a first counter configured to count cycles of the first oscillator;

a second oscillator configured to operate at a frequency proportional to a second voltage smaller than the first voltage;

a second counter configured to count cycles of the second oscillator;

a clock circuit configured to reset the first and second counters at clock cycle transitions;

a latch configured to store content of the second counter at clock cycle transitions; and

a comparator configured to enable an output signal while content of the first counter is smaller than the content of the latch.

6. The device of claim 5 , wherein the first and second oscillators have respective voltage-to-frequency responses that match each other.

7. The device of claim 5 , wherein:

the first oscillator comprises:

an odd number of first inverters connected in a first ring;

a first plurality of current sources respectively configured to supply the first inverters; and

a first adjustment circuit configured to set a current of each current source of the first oscillator to a value proportional to the voltage of the supply line; and

the second oscillator comprises:

an odd number of second inverters connected in a second ring;

a second plurality of current sources respectively configured to supply the second inverters; and

a second adjustment circuit configured to set a current of each current source of the second oscillator to a value proportional to the voltage of the load.

8. A switched-mode power supply comprising:

an inductor configured to supply a load; and

a switched-mode power supply device configured to drive the inductor, the switched-mode power supply device including:

a power switch configured to transfer power from a supply line to the load in switched-mode; and

a duty cycle generator that includes:

a first oscillator configured to operate at a frequency proportional to a voltage of the supply line;

a second oscillator configured to operate at a frequency proportional to a voltage of the load; wherein:

the duty cycle generator is configured to operate the power switch according to a duty cycle based on a ratio between the first and second oscillator frequencies.

9. The switched-mode power supply of claim 8 , wherein the first and second oscillators have respective voltage-to-frequency responses that match each other.

10. The switched-mode power supply of claim 8 , wherein duty cycle generator includes:

a first counter configured to count cycles of the first oscillator;

a second counter configured to count cycles of the second oscillator;

a clock configured to reset the first and second counters at each clock cycle transition;

a latch configured to store content of the second counter at each clock cycle transition; and

a comparator configured to turn the power switch on while content of the first counter is smaller than the content of the latch.

11. The switched-mode power supply of claim 8 , wherein:

the first oscillator comprises:

an odd number of first inverters connected in a first ring;

a first plurality of current sources respectively configured to supply the first inverters; and

a first adjustment circuit configured to set a current of each current source of the first oscillator to a value proportional to the voltage of the supply line; and

the second oscillator comprises:

an odd number of second inverters connected in a second ring;

a second plurality of current sources respectively configured to supply the second inverters; and

a second adjustment circuit configured to set a current of each current source of the second oscillator to a value proportional to the voltage of the load.