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.
By using pulse width modulation, heat can be dissipated efficiently in one or more resistors. The figure above illustrates a 10W pulse-width modulated heat source that operates over a wide, telecom-like supply range. To distribute the heat across a circuit board, around an enclosure, or at various key points on a heat sink (to create a physical model of actual thermal conditions), multiple resistors connected in series and/or parallel may be substituted for the single 50Ω unit shown.
The integrated comparator is used as a PWM engine. When the CMP+ pin is low, Q1 is turned on, which connects the load resistor to the input. The PMON output sources current into C4 and R4, charging the CMP+ pin to its threshold of 1.24V. Q1 turns off, the power (and PMON’s output current) falls to zero, and R4 discharges C4 slightly until the comparator trips and again drives Q1 on. The typical 35mV hysteresis in the comparator assures oscillation. Constant average power is maintained, with CMP+ maintained around 1.24V, and R4 sinking a roughly constant current away from the CMP+ node, balanced by an equivalent average from PMON.