On-period setting circuit, power control IC, and switching power supply device

1. An ON-period setting circuit provided in a fixed-ON-period switching power supply device that produces a desired output voltage from an input voltage by driving a coil by turning ON and OFF an output transistor and a synchronous rectification transistor, wherein

the ON-period setting circuit is configured to shorten an ON period the more the lighter a load is in a discontinuous-current mode, wherein

the ON-period setting circuit comprises:

a first voltage generator configured to generate a first voltage having a ramp waveform that keeps rising over the ON period;

a second voltage generator configured to generate a predetermined second voltage; and

a comparator that compares the first and second voltages with each other to end the ON period, wherein

the second voltage generator pulls down the second voltage the more the lighter the load is in the discontinuous-current mode.

2. The ON-period setting circuit of claim 1 , wherein the second voltage generator includes:

an RC filter that generates the second voltage by integrating a switching voltage appearing at a connection node between the output transistor and the synchronous rectification transistor, or by integrating a divided voltage of the switching voltage;

a current path breaker that cuts off a current path from an input terminal of the switching voltage to a ground terminal when switching stops; and

a pull-up circuit configured to instantaneously pull up the second voltage to a predetermined initial value by capacitively dividing the output voltage when switching restarts.

3. The ON-period setting circuit of claim 2 , wherein the pull-up circuit is configured to make the initial value the lower the lighter the load is.

4. The ON-period setting circuit of claim 3 , wherein the pull-up circuit is configured to make the initial value higher than an integral value obtained from the RC filter in a load region immediately before a transition from the discontinuous-current mode to a continuous-current mode.

5. The ON-period setting circuit of claim 1 , wherein the second voltage generator includes a pull-down circuit configured to transiently pull down the second voltage on completion of a transition from the discontinuous-current mode to a continuous-current mode.

6. The ON-period setting circuit of claim 2 , wherein the second voltage generator includes a pull-down circuit configured to transiently pull down the second voltage on completion of a transition from the discontinuous-current mode to a continuous-current mode.

7. The ON-period setting circuit of claim 3 , wherein the second voltage generator includes a pull-down circuit configured to transiently pull down the second voltage on completion of a transition from the discontinuous-current mode to a continuous-current mode.

8. The ON-period setting circuit of claim 4 , wherein the second voltage generator includes a pull-down circuit configured to transiently pull down the second voltage on completion of the transition from the discontinuous-current mode to a continuous-current mode.

9. An ON-period setting circuit provided in a fixed-ON-period switching power supply device that produces a desired output voltage from an input voltage by driving a coil by turning ON and OFF an output transistor and a synchronous rectification transistor, wherein

the ON-period setting circuit is configured to transiently shorten an ON period on completion of a transition from a discontinuous-current mode to a continuous-current mode, wherein

the ON-period setting circuit comprises:

a first voltage generator configured to generate a first voltage having a ramp waveform that keeps rising over the ON period;

a second voltage generator configured to generate a predetermined second voltage; and

a comparator that compares the first and second voltages with each other to end the ON period, wherein

the second voltage generator transiently pulls down the second voltage on completion of the transition from the discontinuous-current mode to the continuous-current mode.

10. A power control IC comprising, integrated therein:

a ripple injection circuit configured to generate a feedback voltage by superposing a ripple voltage, which imitates a coil current, on an output voltage or on a divided voltage of the output voltage;

a reference voltage generation circuit configured to generate a predetermined reference voltage;

a main comparator that compares the feedback voltage and the reference voltage with each other to generate a comparison signal;

a one-shot pulse generation circuit configured to generate a one-shot pulse in a set signal according to the comparison signal;

an RS flip-flop that sets an output signal at a first logic level according to the set signal and that resets the output signal at a second logic level according to a reset signal;

an ON-period setting circuit configured to generate a one-shot pulse in the reset signal at a lapse of an ON period after the output signal is set at the first logic level;

a gate driver circuit configured to generate a drive signal for an output transistor and a synchronous rectification transistor according to the output signal; and

a detection circuit configured to detect reversal of the coil current to forcibly turn OFF the synchronous rectification transistor, wherein

the ON-period setting circuit is provided in a fixed-ON-period switching power supply device that produces the output voltage from an input voltage by driving a coil by turning ON and OFF the output transistor and the synchronous rectification transistor, wherein

the ON-period setting circuit is configured to shorten the ON period the more the lighter a load is in a discontinuous-current mode.

11. A power control IC comprising, integrated therein:

a ripple injection circuit configured to generate a feedback voltage by superposing a ripple voltage, which imitates a coil current, on an output voltage or on a divided voltage of the output voltage;

a reference voltage generation circuit configured to generate a predetermined reference voltage;

a main comparator that compares the feedback voltage and the reference voltage with each other to generate a comparison signal;

a one-shot pulse generation circuit configured to generate a one-shot pulse in a set signal according to the comparison signal;

an RS flip-flop that sets an output signal at a first logic level according to the set signal and that resets the output signal at a second logic level according to a reset signal;

an ON-period setting circuit configured to generate a one-shot pulse in the reset signal at a lapse of an ON period after the output signal is set at the first logic level;

a gate driver circuit configured to generate a drive signal for an output transistor and a synchronous rectification transistor according to the output signal; and

a detection circuit configured to detect reversal of the coil current to forcibly turn OFF the synchronous rectification transistor, wherein

the ON-period setting circuit is provided in a fixed-ON-period switching power supply device that produces the output voltage from an input voltage by driving a coil by turning ON and OFF the output transistor and the synchronous rectification transistor, wherein

the ON-period setting circuit is configured to transiently shorten the ON period on completion of a transition from a discontinuous-current mode to a continuous-current mode.

12. A switching power supply device, comprising:

the power control IC of claim 10 ; and

a switching output stage of which a part or a whole is externally fitted to the power control IC, the switching output stage being configured to generate the output voltage from the input voltage.

13. A switching power supply device, comprising:

the power control IC of claim 11 ; and

a switching output stage of which a part or a whole is externally fitted to the power control IC, the switching output stage being configured to generate the output voltage from the input voltage.