An accurate sense-fet current limiter with embedded softstart for linear dc/dc converters

A fast sense-FET current limiter that protects linear regulators against overcurrent is proposed. By action of a secondary feedback loop (SFL), the current I_PF delivered by the converter is regulated to a threshold I_TH upon overloading. Since accuracy on current clamping dominantly relies on transistor geometry scaling, the approach is robust against process and temperature (PT) spread. Based on a temporary threshold I_THS during power-up, the limiter also incorporates a soft-start mechanism, at no increase of complexity. As part of a p-MOSFET LDO with 65mA-rate, the limiter design complies with 3.3V-input supply, I_TH=75mA and parameters of a standard 0.35μm CMOS process. A set of PSPICE simulations attests the good electrical performance. Upon ideal matching, I_PF is clamped to I_TH within 2%-deviation. Typically, current clamping occurs within 500ns after the overcurrent onset.