1. An apparatus, comprising: a voltage source to generate a first voltage reference and a second voltage reference in response to a supply voltage; a charge transfer switch coupled to an integrating capacitor having a first plate and a second plate coupled to a reference voltage rail to distribute charge between a sensing capacitor and the integrating capacitor, the charge accumulated in response to the first voltage reference; a control switch coupled to the first plate of the integrating capacitor to selectively disconnect the first plate of the integrating capacitor from a reference voltage node to maintain an integrated charge on the integrating capacitor; and a comparator coupled to the second voltage reference and the integrating capacitor to compare a voltage proportional to the integrated charge on the integrating capacitor against the second voltage reference, wherein the first voltage reference changes proportionally to the second voltage reference in response to a change in the supply voltage.