A constant-on-time power-supply controller includes an adder and a control circuit. The adder generates a sum of a sense voltage and a regulated output voltage generated by a filter inductor. The sense voltage is generated by a sense circuit that sources a current to the filter inductor while the inductor is uncoupled from an input voltage, and the sense voltage is related to the current. The control circuit couples the filter inductor to the input voltage for a predetermined time in response to the sum having a predetermined relationship to a reference voltage. Such a power-supply controller may yield a relatively tight regulation of the output voltage even with a power supply having with a low-ESR filter capacitor, and may do so with little or no additional compensation circuitry as compared to prior controllers and with no additional pin on the power-supply-controller chip.