Varactor-Based Passive Ripple Filter With Automatic Tuning for Low-Noise Power-Supply Systems

Use of parallel resonance to notch filter the fundamental ripple frequency on a switching regulator output offers advantages in the combined metric of size and power compared to traditional low-pass solutions. Availability of barium strontium titanate varactors with capacitance of several nanofarads now makes automatic tuning of parallel resonance alignment with the ripple fundamental a feasible alternative. A phase-lock automatic-tuning circuit is demonstrated. The effective use of this technique as the power-supply filter used to power a 16 bit ADC is shown. The suitability of the resulting filter and regulator to full surface mount encapsulation is demonstrated.