Programmable substrate bias generator with current-mirrored differential

Abstract

A substrate bias generator for an integrated circuit has a charge pump driven by an oscillator. The oscillator is enabled and disabled to save power and control the voltage-level itself for the substrate bias. An enabling circuit senses the substrate voltage and enables the oscillator when the substrate voltage rises above a bias set by a programmable reference voltage. The enabling circuit which senses the voltage on the substrate draws no active current from the substrate. The sensing circuit includes a transistor with only its bulk terminal connected to the substrate; the source, gate, and drain of this sensing transistor are not connected to the substrate. A differential comparator compares the output of the sensing transistor to the programmable reference voltage and enables the oscillator when the sensing transistor output is lower than the reference voltage. The sensing transistor attenuates large swings in the substrate voltage to provide the differential comparator with a small voltage swing which keeps the differential comparator operating near its optimum design point. Since no active current is drawn from the substrate when sensing the substrate voltage, no IR voltage drops can develop from the enabling and sensing circuit. Thus latch-up immunity is improved and substrate noise is reduced.