A differential-Mode Current Sensor And Its Application To A LDO regulator

A compact and accurate differential-mode sense-FET current sensor for linear regulators is presented. A NMOS pass transistor M_PF delivers a current I_PF to the load. The sensor comprises a couple of matched sense transistors M_SNS ⁺ and M_SNS ^-, in series with asymmetrical sense resistors R_SNS+DeltaR and R_SNS-DeltaR. A differential voltage V_SNS across these resistors is converted into a current at the sensor output I_OUT by means of a transconductor with heavily-degenerated differential-pair. By proper design, the sensor transfer collapses to I_OUT/I_PF = 2DeltaR/(MR_E), where M is the scaling factor between MPF and sense transistors and R_E is the differential-pair emitter resistor. Since I_OUT/I_PF relies only on a resistor-ratio and a geometrical scaling factor, its variation with fabrication process and temperature is minimized.