Predictive ripple-cancelling signal into error amplifier of switch mode power supply
A predicted ripple in the feedback voltage of a switching converter is generated, based on the ripple over a certain number of recent switching cycles. The DC portion of the feedback voltage is filtered out. This predicted feedback voltage ripple is then added to a fixed reference voltage to create a compensated reference voltage. The compensated reference voltage is applied to the non-inverting input of an error amplifier, and the feedback voltage (having a DC component and ripple) is applied to the inverting input of the error amplifier. Thus, substantially the same ripple component is applied to both inputs and cancels out. Therefore, the output of the error amplifier is not affected by the ripple in the feedback voltage, and a non-rippling control voltage is generated by the error amplifier. As a result, the gain-bandwidth product of the converter can be increased for faster response to transients.
1. A switching converter comprising:
a transistor switch;
a voltage feedback circuit configured to generate a feedback voltage, the feedback voltage having a ripple component at a switching frequency of the transistor switch;
an error amplifier configured to generate a control signal for use in determining when to turn off the transistor switch during a switching cycle, the error amplifier having inputs comprising an inverting input and a non-inverting input;
a filter configured to pass the ripple component of the feedback voltage;
a processing circuit configured to process the ripple component of the feedback voltage over a number of switching cycles to derive a predicted ripple component of the feedback voltage;
a reference voltage generator configured to generate a reference voltage; and
a combiner circuit configured to combine the predicted ripple component of the feedback voltage with one of the reference voltage or the feedback voltage before being applied to the respective inputs of the error amplifier to at least partially offset effects of the ripple component of the feedback voltage.
2. The converter of claim 1 wherein the combiner circuit is configured to combine the predicted ripple component of the feedback voltage with the reference voltage to obtain a compensated reference voltage, such that the compensated reference voltage has a ripple similar to the feedback voltage ripple component, wherein the compensated reference voltage is applied to a respective one of the inputs into the error amplifier, and the feedback voltage is applied to another respective one of the inputs into the error amplifier.
3. The converter of claim 1 wherein the combiner circuit is configured to combine the predicted ripple component of the feedback voltage with the feedback voltage to obtain a compensated feedback voltage, such that the compensated feedback voltage has a reduced ripple compared with the feedback voltage, wherein the reference voltage is applied to a respective one of the inputs into the error amplifier, and the compensated feedback voltage is applied to another respective one of the inputs into the error amplifier.
4. The converter of claim 1 wherein the processing circuit comprises an averaging circuit for averaging the ripple component of the feedback voltage over a number of switching cycles.
5. The converter of claim 1 wherein the processing circuit comprises:
a plurality of delay circuits configured to delay the ripple component of the feedback voltage by various multiples of a transistor switch switching period;
a plurality of current sources, the current sources being connected in parallel and generating currents corresponding to outputs of associated ones of the delay circuits; and
a resistance connected in series with the reference voltage, a sum of the currents generated by the current sources being coupled through the resistance to generate a voltage drop substantially equal to the predicted ripple component of the feedback voltage.
6. The converter of claim 1 wherein the processing circuit comprises a plurality of digital delay circuits configured to delay the ripple component of the feedback voltage by various multiples of a transistor switch switching period.
7. The converter of claim 6 wherein the plurality of digital delay circuits are clocked by a clock signal, wherein the clock signal tracks a frequency of the ripple component.
8. The converter of claim 1 wherein the filter configured to pass the ripple component of the feedback voltage comprises a capacitor.
9. The converter of claim 1 wherein the converter is a current mode converter, the converter further comprising a slope compensation circuit generating a first ramping voltage, wherein the control voltage is combined with the first ramping voltage to create a compensated control voltage for comparison with a ramping inductor current sense signal.
10. The converter of claim 1 wherein the converter is a buck converter generating a regulated output voltage.
11. The converter of claim 1 wherein the converter is a boost converter generating a regulated output voltage.
12. The converter of claim 1 wherein the processing circuit is configured to process the ripple component of the feedback voltage over at least ten switching cycles to derive the predicted ripple component of the feedback voltage.
13. The converter of claim 1 wherein the error amplifier comprises a transconductance amplifier, the converter further comprising a capacitor coupled to an output of the error amplifier, wherein a voltage at a terminal of the capacitor is used to determine when to turn off the transistor switch during a switching cycle.
14. The converter of claim 1 wherein the converter is a current mode converter, the converter further comprising:
an inductor configured to conduct a ramping inductor current as the transistor switch is turned on and off at a particular duty cycle and switching frequency;
a current sensor configured to convert the ramping inductor current to a corresponding current sense signal, the current sense signal having ripple; and
a comparator configured to receive at least the current sense signal, wherein tripping of the comparator turns off the transistor switch during a switching cycle.
15. A method performed by a switching converter comprising:
turning on and off a transistor switch at a particular duty cycle and switching frequency;
generating a feedback voltage, the feedback voltage having a ripple component;
generating a control signal by an error amplifier, the error amplifier having inputs comprising an inverting input and a non-inverting input;
turning off the transistor switch during a switching cycle when a first ramping voltage exceeds a threshold;
filtering the feedback voltage to obtain the ripple component of the feedback voltage;
processing the ripple component of the feedback voltage over a number of switching cycles to derive a predicted ripple component of the feedback voltage;
generating a reference voltage; and
combining the predicted ripple component of the feedback voltage with one of the reference voltage or the feedback voltage before being applied to respective inputs of the error amplifier to at least partially offset effects of the ripple component of the feedback voltage.
16. The method of claim 15 wherein the step of combining comprises combining the predicted ripple component of the feedback voltage with the reference voltage to obtain a compensated reference voltage, such that the compensated reference voltage has a ripple similar to the feedback voltage ripple component, wherein the compensated reference voltage is applied to a respective one of the inputs into the error amplifier, and the feedback voltage is applied to another respective one of the inputs into the error amplifier.
17. The method of claim 15 wherein the step of combining comprises combining the predicted ripple component of the feedback voltage with the feedback voltage to obtain a compensated feedback voltage, such that the compensated feedback voltage has a reduced ripple compared with the feedback voltage, wherein the reference voltage is applied to a respective one of the inputs into the error amplifier, and the compensated feedback voltage is applied to another respective one of the inputs into the error amplifier.
18. The method of claim 15 wherein the control voltage is used to set the threshold for turning off the transistor switch.
19. The method of claim 15 wherein the processing comprises averaging the ripple component of the feedback voltage over a number of switching cycles.
20. The method of claim 15 wherein the processing comprises:
delaying the ripple component of the feedback voltage by various multiples of a transistor switch switching period;
generating currents corresponding to an output of associated ones of the delay circuits, and summing the currents to create a summed current; and
coupling the summed current across a resistance connected in series with the reference voltage to generate a voltage drop substantially equal to the predicted ripple component of the feedback voltage, wherein a sum of the reference voltage and the predicted ripple component of the feedback voltage forms the compensated reference voltage.
21. The method of claim 15 wherein filtering the feedback voltage to obtain the ripple component of the feedback voltage comprises filtering the feedback voltage by a capacitor to pass the ripple component of the feedback voltage.
22. The method of claim 15 wherein the converter is a current mode converter and wherein the first ramping voltage corresponds to a current through an inductor.