The LT2940 measures a high side current and a differential voltage, multiplies them and outputs a current proportional to instantaneous power. Bidirectional high side currents and bipolar voltage differences are correctly handled by the four-quadrant multiplier and push-pull output stage, which allows the LT2940 to indicate forward and reverse power flow.
The LT2940 current sense inputs, I+ and I– are designed to operate over a range of 4V to 80V. Nevertheless, it is possible to translate bottom-side (ground) current sense information to I+ and I– using the circuit shown above. An LT1635 performs the necessary translation for both positive and negative current flow. The PMON and IMON outputs are biased with a Zener diode so that positive and negative power and current measurements are available.