In this paper we present an autonomous Power Management Integrated Circuit PMIC for an electret-biased electrostatic kinetic energy harvester with a Bennet's doubler conditioning circuit. The circuit is designed in high voltage $0.35\ \mu\mathrm{m}$ standard bulk CMOS technology. It supplies a low voltage load from the energy extracted from a high voltage transducer. The circuit is provided with a “cold start” mechanism and with a “safe mode” (Recovery) feature that permits to the system to stay active (awake) for a longer time after external vibrations stop temporarily. An ultra-low static power hysteresis comparator is designed for a specific control of the capacitive transducer's conditioning circuit. The autonomy of the system is ensured by a voltage regulator that supplies the internal blocks of the circuit with 1.1 V. The proposed design consumes less than 400 nW making it adequate for use with existing state-of-the art capacitive harvesting MEMS devices.