Switched-capacitor filter
A filter is implemented as cascaded stages, and in at least one stage all resistances are implemented as double-sampled switched-capacitor circuits. In a variation, at least one resistance is implemented as a double-sampled switched-capacitor T-network. In a variation, in an integrator stage, a resistance is implemented as a transconductance, and the cutoff frequency of the integrator stage scales with a switching frequency of a DC-DC voltage converter.
1. A filter, comprising:
a plurality of cascaded active filter stages each comprising an amplifier;
at least one active filter stage having a resistance in parallel with a capacitance at an input; and,
the resistance being implemented as a double-sampled switched-capacitor circuit; and
a sample-and-hold circuit located between a last one of the plurality of active filter stages and an active filter stage of the plurality of active filter stages preceding the last stage.
2. The filter of claim 1 where the resistance is implemented as a double-sampled T-network switched-capacitor circuit.
3. The filter of claim 2 , wherein the double-sampled switched capacitor circuit operates on complementary phases and wherein for a first clock phase, a first T-network having three capacitors is coupled to an input of the amplifier of one of the active filter stages, and for a second clock phase, a second T-network having three capacitors is coupled to the input of the amplifier of that active filter stage.
4. The filter of claim 1 , where the filter is a compensation filter suitable for use in a feedback loop of a DC-DC voltage converter.
5. The filter of claim 4 , where the active filter stage is a Type-III compensation filter.
6. The filter of claim 4 , where the active filter stage is a Type-II compensation filter.
7. The filter of claim 4 , where all filter time constants all of the compensation filter track a switching-frequency of the DC-DC voltage converter.
8. A filter, comprising:
a plurality of cascaded active filter stages;
at least one active filter stage having a resistance in parallel with a capacitance at an input;
the resistance being implemented as a double-sampled switched-capacitor circuit;
the plurality of active filter stages further comprising an integration stage, the integration stage including an integration resistance, and the integration resistance implemented as a transconductance amplifier which comprises a bipolar junction transistor differential input stage where a differential input voltage is divided by at least two additional series bipolar junction transistor differential input stages.
9. The filter of claim 8 , wherein the filter is a compensation filter suitable for use in a feedback loop of a DC-DC voltage converter and wherein a cutoff frequency of the integration stage tracks a switching frequency of a DC-DC voltage converter.
10. The filter of claim 9 , where the transconductance amplifier includes a current source, and where the current source is controlled by a digital-to-analog converter.
11. The filter of claim 10 , where a digital input signal for the digital-to-analog converter also controls the switching frequency of DC-DC voltage converter.
12. The filter of claim 10 , where the current generated by the current source is proportional to absolute temperature.