Seamless DCM-PFM transition for single pulse operation in DC-DC converters

1. A converter operable to convert an input voltage at an input node to an output voltage at an output node by switching on and off a transistor at a switching frequency, the converter comprising:

an error amplifier circuit having a first input coupled to a reference voltage, a second input coupled to the output node through a resistive divider, a first output operable to output a control current and a second output operable to output a current equivalent to the control current;

a peak current comparator circuit having a first input coupled to the second output of the error amplifier circuit, a second input and an output, the second input of the peak current comparator is adapted to be coupled to the input node through an inductor;

an off-time timer circuit having an input coupled to the first output of the error amplifier circuit and having an output, the off-time timer circuit operable to set the switching frequency based on the control current; and

a control circuit having a first input coupled to the output of the peak current comparator circuit, a second input coupled to the output of the off-time timer circuit and an output coupled to a control terminal of the transistor.

2. The converter of claim 1 , wherein the converter is operable to operate in a mode of operation selected from the group consisting of: continuous conduction mode (CCM), discontinuous conduction mode (DCM), pulse frequency mode (PFM) and pulse width modulation mode (PWM).

3. The converter of claim 2 , wherein the converter is operable to switch from one mode operation to another mode of operation.

4. The converter of claim 3 , wherein the converter is operable to reduce voltage ripple on the output voltage during a transition from one mode of operation to another mode of operation.

5. The converter of claim 1 , wherein the output current is modulated from a valley current value to a peak current value at the switching frequency.

6. The converter of claim 5 , wherein the peak current comparator circuit is operable to set the peak current value based on the control current.

7. The converter of claim 1 , wherein the error amplifier circuit includes:

an error amplifier having inputs coupled to the first and second inputs of the error amplifier circuit and an output;

a first transistor having a first current terminal coupled to a supply voltage, a second current terminal coupled to the output of the error amplifier and a control terminal coupled to the second current terminal, the first transistor having a transistor current equivalent to the control current;

a second transistor having a first current terminal coupled to the supply voltage, a second current terminal coupled the input of the off-time timer circuit and a control terminal coupled to the control terminal of the first transistor, the first transistor and the second transistor form a first current mirror; and

a third transistor having a first current terminal coupled to the supply voltage, a second current terminal coupled the first input of the peak comparator circuit and a control terminal coupled to the control terminal of the first transistor, the first transistor and the third transistor form a second current mirror.

8. The converter of claim 7 , wherein a current from the first current terminal to the second current terminal of the second transistor is equivalent to the control current and a current from the first current terminal to the second current terminal of the third transistor is equivalent to the control current.

9. A control circuit operable to switch a transistor on and off at a switching frequency to convert an input voltage at an input node to an output voltage at an output node, the control circuit comprising:

an error amplifier circuit operable to generate a control current and having a first output operable to output a first output current and having a second output operable to output a second output current, the first output current and the second output current are equivalent to the control current;

a peak current comparator circuit having a first input coupled to the second output of the error amplifier circuit, a second input and an output, the second input is adapted to be coupled to the input node through an inductor;

an off-time timer circuit having an input coupled to the first output of the error amplifier circuit and an output, the off-time timer circuit operable to set the switching frequency based on the control current; and

a control circuit having a first input coupled to the output of the peak current comparator circuit, a second input coupled to the output of the off-time timer circuit and an output coupled to a control terminal of the transistor.

10. The control circuit of claim 9 , wherein, based on control signals from the control circuit, the converter is operable to operate in a mode of operation selected from the group consisting of: continuous conduction mode (CCM), discontinuous conduction mode (DCM), pulse frequency mode (PFM) and pulse width modulation mode (PWM).

11. The control circuit of claim 10 , wherein the converter is operable to switch from a first mode operation to a second mode of operation.

12. The control circuit of claim 11 , wherein current through the inductor is modulated from a valley current value to a peak current value at the switching frequency.

13. The control circuit of claim 12 , wherein during the first mode of operation the switching frequency is fixed and the peak current value is variable.

14. The control circuit of claim 12 , wherein during the second mode of operation the switching frequency is variable and the peak current value is fixed.

15. The control circuit of claim 12 , wherein the converter is operable to operate in a transition mode of operation after the first mode of operation and before the second mode of operation, and the switching frequency and the peak current value are variable during the transition mode of operation.

16. The control circuit of claim 9 , wherein current through the inductor is modulated from a valley current value to a peak current value at the switching frequency.

17. The control circuit of claim 16 , wherein the peak current comparator circuit is operable to set the peak current value based on the control current.

18. A method of operating a converter operable to convert an input voltage at an input node to an output voltage at an output node, the method comprising the steps of:

switching a transistor on and off during a heavy load period at a fixed switching frequency to provide pulses of current to the load and varying a maximum current per pulse of current;

switching the transistor on and off during a light load period at a fixed maximum current per pulse of current and varying the switching frequency; and

switching the transistor on and off during a period between the heavy load period and the light load period by varying the switching frequency and the maximum current per pulse of current.

19. The method of claim 18 , wherein the maximum current per pulse of current is I PEAK .

20. The method of claim 18 , where in switching frequency is F SW .