A dimmable ballast circuit for a compact fluorescent lamp controls the intensity of a lamp tube in response to a phase-control voltage received from a dimmer switch. The ballast circuit comprises a phase-control-to-DC converter circuit that receives the phase control voltage, which is characterized by a duty cycle defining a target intensity of the lamp tube, and generates a DC voltage representative of the duty cycle of the phase-control voltage. Changes in the duty cycle of the phase control voltage that are below a threshold amount are filtered out by the converter circuit, while intentional changes in the duty cycle of the phase control voltage are reflected in changes in the target intensity level and thereby the intensity level of the lamp tube.