Methods and circuits for constant on-time control of switching power supplies

1 . A power supply comprising:

a switch having a switch control terminal, a first current-handling terminal connected to an input-voltage supply node, and a second current-handling terminal connected to a switching node, the switch to generate a switching signal on the switching node responsive to a switch-control signal on the switch control terminal;

an inductor having a first inductor terminal connected to the switching node and a second inductor terminal connected to an output-voltage supply node to issue a power-supply output signal having a DC component and a ripple component;

an error amplifier having a first feedback node coupled to the output-voltage supply node to receive the power-supply output signal and an error node to issue an error signal responsive to the power-supply signal;

a ripple injector having a capacitor coupled to the first feedback node to receive the power-supply output signal, and a second feedback node to receive at least one of the switching signal and a replica of the switching signal, and a ripple-injector output node, the ripple injector to integrate the DC component of the power-supply output signal and the at least one of the switching signal and the replica of the switching signal to develop a feedback signal on the ripple-injector output node; and

a constant on-time control circuit coupled to the error node to receive the error signal, the ripple-injector output node to receive the feedback signal, and to the switch control terminal, the constant on-time control circuit to selectively assert the switch-control signal for a fixed on-time and deassert the switch-control signal for an off-time that varies responsive to the error signal and the feedback signal.

2 . The power supply of claim 1 , the ripple injector further comprising a resistor in series with the capacitor between the second feedback node and the capacitor.

3 . The power supply of claim 2 , the ripple injector further comprising a ripple-injector output node between the resistor and the capacitor, the ripple-injector output node to issue a ripple-feedback signal to the constant on-time control circuit.

4 . The power supply of claim 1 , the error amplifier including a second capacitor connected between the error node and a supply terminal.

5 . The power supply of claim 3 , the constant on-time control circuit including a comparator having a first comparator input coupled to the ripple-injector output to receive the ripple-feedback signal, a second comparator input coupled to the error node to receive the error signal, and a comparator output node.

6 . The power supply of claim 5 , the constant on-time control circuit further including a timer circuit coupled to the comparator output to issue the switch control signal responsive to a comparison between the ripple-feedback signal and the error signal.

7 . The power supply of claim 1 , further comprising a replica circuit to develop the replica switching signal.

8 . The power supply of claim 7 , the replica circuit to develop the replica of the switching signal responsive to the switch-control signal.

9 . An integrated circuit for controlling a switching power supply having a switching node to develop a switch signal and an output-voltage node to develop an output-voltage signal having a ripple component and a DC component, the integrated circuit comprising:

an error amplifier having a first feedback node coupled to the output-voltage node to receive the DC component of the output-voltage signal and an error node to issue an error signal responsive to the DC component;

a ripple injector to integrate the DC component of the output-voltage signal with at least one of the ripple component and a replica ripple component to issue an integrated feedback signal; and

a constant on-time control circuit coupled to the error node to receive the error signal and the ripple injector to receive the integrated feedback signal, the constant on-time control circuit to selectively assert a switch-control signal for a fixed on-time and deassert the switch-control signal for an off-time that varies responsive to the error signal and the feedback signal.

10 . The integrated circuit of claim 9 , wherein the ripple injector includes a second feedback node coupled to the switching node to receive the ripple component.

11 . The integrated circuit of claim 9 , the ripple injector including a replica switch to develop the ripple component.

12 . The integrated circuit of claim 9 , the ripple injector including an AC-coupling capacitor to integrate the DC component with the ripple component.

13 . The integrated circuit of claim 9 , further comprising a replica circuit to generate the replica switching signal.

14 . The integrated circuit of claim 13 , the replica circuit to generate the replica of the switching signal responsive to the switch-control signal.

15 . A method of generating an output voltage between terminals of a capacitor from an input voltage between input-voltage nodes, the method comprising:

drawing current from the input-voltage nodes to the capacitor over a sequence of on-times separated by off times to develop the output voltage across the capacitor, the output voltage having a DC component and a ripple component;

integrating the DC component of the output voltage with at least one of the ripple component and a replica of the ripple component to produce a feedback signal;

producing an error signal proportional to a difference between the DC component of the output voltage and a reference voltage; and

adjusting the off times responsive to the feedback signal and the error signal.

16 . The method of claim 15 , further comprising replicating the ripple component to produce the replica of the ripple component.

17 . The method of claim 16 , wherein replicating the ripple component comprises switching a replica current responsive to the error signal.

18 . The method of claim 15 , wherein producing the error signal comprises integrating the difference between the DC component of the output voltage and the reference voltage.