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.
An example of a linear servo loop is shown in the circuit above. The LT2940 forms the basis of a 0W to 10W load box that is used to test power supplies of 10V to 40V. An adjustable programming current of 0μA to 200μA is generated by an LT1635 op amp and reference, and controlled by a 10-turn potentiometer. This current balances against the measured power at PMON and regulates Q1 to a predictable power.
The LT2940’s integrated comparator is used to shut down the circuit in case of undervoltage or overtemperature conditions. At reduced input voltage, a constant power servo attempts to draw ever more current, leading to a theoretical result of infinite current at zero input voltage. Undervoltage lockout prevents the servo loop from shorting the supply at low input voltages. The input voltage is monitored by the comparator and if less than 10V, CMPOUT shorts the MOSFET gate to ground.
Overtemperature is sensed by Q2, which pulls down on CMP+ and shuts the MOSFET off in case of excessive temperature. Although the comparator includes its own small hysteresis, generous additional thermal hysteresis is provided by R13 and the complementary comparator output, CMPOUT. One last feature of the load box circuit is reverse polarity protection, courtesy of diode D1.