The Sense Resistor

Sense resistors in series with the load are central to regulating current in switch-mode power supplies. It is possible to optimise the power loss in these resistors, as we show with an example converter circuit that drives eight white LEDs from a 4.8 V battery.Switch-mode regulator ICs generally include a reference voltage generator and a differential amplifier. This combination is used to compare the voltage dropped across an external sense resistor with the reference and then suitably drive the output stage. The voltage drop across the resistor must be of the same order of magnitude as the reference, which in modern DC-DC converters is around 1.2 V. However, particularly when high currents are involved, this can mean an unreasonably large amount of power is wasted. With the following elegant circuit we can reduce the voltage drop across the sense resistor and hence reduce the power wastage.