Power supply circuit, corresponding device and method

1. A circuit comprising:

a transformer having a primary winding coupled to an input port configured to receive an input voltage and a secondary winding configured to provide an output voltage at an output port;

controller circuitry configured to switch on and off a current through the primary winding so that energy is transferred to the secondary winding while switching on and off; and

supply circuitry connected to the controller circuitry,

wherein the supply circuitry is coupled to an auxiliary winding of the transformer and configured to produce a supply voltage for the controller circuitry,

wherein the controller circuitry is further configured to:

transition to a burst mode to switch on and off the current through the primary winding in first bursts during which the current through the primary winding is switched on and off, wherein the first bursts are separated by intervals during which switching on and off the current through the primary winding of the transformer by the first bursts is discontinued; and

provide second bursts during the intervals to keep the supply voltage of the controller circuitry between a lower bound value and an upper bound value while the output voltage ramps down to a requested valley value; or

provide the second bursts during the intervals after reaching a timeout limit to provide the supply voltage to the controller circuitry while the output voltage ramps down to the requested valley value; and

wherein the controller circuitry is further configured to:

receive a request for an electrical load to reduce the output voltage at the output port toward the requested valley value;

transition to the burst mode in response to ramping down the output voltage to the requested valley value;

determine whether the output voltage at the output port has reached the requested valley value; and

exit the burst mode in response to the output voltage reaching the requested valley value.

2. The circuit of claim 1 , wherein the controller circuitry is configured to provide the second bursts in response to the supply voltage reaching the lower bound value.

3. The circuit of claim 2 , wherein the controller circuitry is configured to discontinue providing the second bursts in response to the supply voltage reaching the upper bound value.

4. The circuit of claim 2 , wherein the lower bound value is higher than an under voltage lock out threshold.

5. The circuit of claim 1 , wherein the timeout limit is short enough to avoid reaching an under voltage lock out threshold.

6. The circuit of claim 1 , wherein the auxiliary winding is arranged at a side of the transformer corresponding to the primary winding.

7. A device comprising:

the circuit of claim 1 ; and

an electrical load coupled to the output port.

8. The circuit of claim 1 , wherein the controller circuitry comprises a comparator circuit configured to compare the supply voltage to a threshold voltage.

9. The circuit of claim 1 , wherein the supply circuitry further comprises a diode coupled between a low-dropout (LDO) regulator and the auxiliary winding.

10. A method for operating a circuit, wherein the circuit comprises a transformer having a primary winding coupled to an input port for receiving an input voltage and a secondary winding for providing an output voltage at an output port, controller circuitry for switching on and off a current through the primary winding so that energy is transferred to the secondary winding while switching on and off, and supply circuitry connected to the controller circuitry, and wherein the supply circuitry is coupled to an auxiliary winding in the transformer, the method comprising:

receiving, at the controller circuitry, a request to reduce the output voltage at the output port towards a requested valley value;

transitioning, by the controller circuitry, to a burst mode to switch on and off the current through the primary winding in first bursts during which the current through the primary winding is switched on and off, wherein the first bursts are separated by intervals during which switching on and off the current through the primary winding of the transformer by the first bursts is discontinued;

providing, by the controller circuitry, second bursts during the intervals to keep a supply voltage of the controller circuitry between a lower bound value and an upper bound value while the output voltage ramps down to the requested valley value;

providing, by the controller circuitry, the second bursts during the intervals after reaching a timeout limit to provide the supply voltage to the controller circuitry while the output voltage ramps down to the requested valley value, and wherein the timeout limit is short enough to avoid reaching an under voltage lock out threshold;

determining, by the controller circuitry, whether the output voltage at the output port has reached the requested valley value; and

exiting, by the controller circuitry, the burst mode in response to the output voltage reaching the requested valley value.

11. The method of claim 10 , wherein the lower bound value is higher than the under voltage lock out threshold.

12. The method of claim 10 , further comprising discontinuing providing the second bursts in response to the supply voltage reaching the upper bound value.

13. The method of claim 10 , wherein the auxiliary winding is arranged at a side of the transformer corresponding to the primary winding.

14. The method of claim 10 , further comprising providing the second bursts in response to the supply voltage reaching the lower bound value.

15. The method of claim 10 , wherein the controller circuitry comprises a comparator circuit to compare the supply voltage to a threshold voltage, and wherein the supply circuitry comprises a diode coupled between a low-dropout (LDO) regulator and the auxiliary winding.

16. A method for operating a circuit, wherein the circuit comprises a transformer having a primary winding coupled to an input port for receiving an input voltage and a secondary winding for providing an output voltage at an output port, controller circuitry for switching on and off a current through the primary winding so that energy is transferred to the secondary winding while switching on and off, and supply circuitry connected to the controller circuitry, and wherein the supply circuitry is coupled to an auxiliary winding in the transformer, the method comprising:

receiving, at the controller circuitry, a request to reduce the output voltage at the output port towards a requested valley value;

transitioning, by the controller circuitry, to a burst mode to switch on and off the current through the primary winding in first bursts during which the current through the primary winding is switched on and off so that the first bursts are separated by intervals during which switching on and off the current through the primary winding of the transformer by the first bursts is discontinued;

decreasing, by the controller circuitry, a slope of a supply voltage for the controller circuitry slower than a slope of the output voltage during the intervals;

receiving, at the controller circuitry, a request for an electrical load to reduce the output voltage at the output port towards the requested valley value;

transitioning, by the controller circuitry, to the burst mode, in response to the receiving the request for the electrical load to reduce the output voltage;

determining, by the controller circuitry, whether the output voltage at the output port has reached the requested valley value; and

exiting, by the controller circuitry, the burst mode in response to the output voltage reaching the requested valley value.

17. The method of claim 16 , wherein the auxiliary winding is arranged at a side of the transformer corresponding to the primary winding.

18. The method of claim 16 , further comprising:

providing, by the controller circuitry, second bursts during the intervals to keep the supply voltage of the controller circuitry between a lower bound value and an upper bound value while the output voltage ramps down to the requested valley value; or

providing, by the controller circuitry, the second bursts during the intervals after reaching a timeout limit to provide the supply voltage to the controller circuitry while the output voltage ramps down to the requested valley value, and wherein the timeout limit is short enough to avoid reaching an under voltage lock out threshold.

19. The method of claim 18 , wherein the lower bound value is higher than the under voltage lock out threshold.

20. The method of claim 16 , wherein the controller circuitry comprises a comparator circuit to compare the supply voltage to a threshold voltage, and wherein the supply circuitry comprises a diode coupled between a low-dropout (LDO) regulator and the auxiliary winding.