Simple, efficient constant power driver IC solution

Summary: A combined current-sense amplifier and multiplier (MAX4211F) measures the voltage across the load and provides a voltage at one of its outputs (the power output) that is proportional to the instantaneous power in the load. An external op amp generates a corresponding PWM (pulse-width modulated) output signal that controls a P-channel MOSFET in series with the load.

Actuator and sensor systems sometimes include resistive loads that require a controlled, constant power drive regardless of the load's resistance value. If this value varies with operating conditions, then a simple control and regulation of voltage or current is not sufficient to ensure constant power delivery. The circuit in Figure 1 provides a way to exploit these resistive characteristics for constant power and provide a DC drive with variable duty cycle that has a simple, low cost, and high efficiency implementation.

Figure 1. This circuit provides constant power to the load, subject to the limits stated in the text.

A combination current-sense amplifier and multiplier (MAX4211F) measures both the current through and the voltage across the load and produces a voltage at one of its outputs (the power output) that is proportional to these variables, proportional to the load's instantaneous supplies.

One half of a dual op amp (MAX4163) generates a constant frequency (approximately 300Hz) pseudo-sawtooth signal that is connected to a non-inverting comparator at the MAX4211F auxiliary input. The other op amp (bottom half) acts as an error amplifier to average this power signal and then compare it with a reference while amplifying the difference. The output of this error amplifier is connected to the inverting input of the auxiliary comparator, which generates a PWM (pulse-width modulated) output signal. This PWM signal drives a P-channel MOSFET in series with the load.

The circuit application is simplified because there is a ground on the load side and because the control input is a low-level DC signal (or a microcontroller generates a PWM control signal). About 1mA is required by the +5V power supply of the control circuit.

The power limits of this circuit are defined as the supply operating voltage range, the maximum allowable peak load current at which it can operate, and the load resistor value. The supply voltage range is 8V to 24V, set by the characteristics of the MOSFET and the voltage range of the MAX4211F input (IN). (This range covers the maximum DC power supplies for industrial and instrumentation systems.) The peak load current is 4A, the voltage dynamic range is affected by the combination of the fixed current sense input of the MAX4211F, and the value of the current sense resistor chosen (25mΩ in this case).

Given the required power level, the voltage and peak current limits are set within the limits of the load resistance in each case. The minimum allowable load resistance gives the maximum power expected as a function of the voltage to peak current limit (4A). The maximum load power the circuit can regulate is approximately the square of the voltage expected to divide the maximum load resistance expected.

In any case, the maximum power loss in the circuit is 0.4W in the sense resistor of the power MOSFET plus 0.8W, for a total of 1.2W. With the control voltage input (VCONT) at 0.5V, the circuit as shown delivers a regulated 10W into a 6Ω load and is stable at ±1% as the supply voltage sweeps from 8V to 24V. It can deliver a regulated 60W at ±0.2% to the same load (with VCONT = 3V), but only over a supply voltage range of 19V to 24V. From 24V to 22V with VCONT = 4V, it can deliver up to 80W at ±0.2%.

With a supply voltage of 16V and 0.5V = VCONT, the output power still varies from 10W into a 4Ω load to 12Ω ±1%. The largest variation in supply regulation values, caused by supply voltage or load variations, is due to nonlinearity in the MAX4211F multiplier and is within the specified error range for this IC.