An apparatus includes a voltage divider circuit including a plurality of series-connected capacitors and including an input terminal of one of the capacitors configured to receive a first voltage from a switch, and a ring node comprising the connection between at least two of the series-connected capacitors. The apparatus further includes a negative clamp circuit coupled to the ring node of the voltage divider circuit and a bias voltage node. The bias voltage node is configured to receive a bias voltage and responsive to a ring voltage on the ring node being less than the bias voltage, the negative clamp circuit is configured to clamp the ring voltage at a first threshold voltage. The apparatus also includes a peak detector circuit coupled to the ring node of the voltage divider circuit and configured to detect a peak amplitude of the ring voltage. The apparatus further includes a switch driver coupled to the peak detector circuit and configured to adjust a control signal to the switch responsive to the detected peak amplitude.