Electronic Design

Battery-Charger IC Doubles As Current Sensor

It’s always fun to try and find applications for an IC which its designer never intended. The circuit shown here is one such design. Many applications require a circuit that provides a ground-referenced output voltage that’s proportional to a measured current. Frequently, the current must be measured with a shunt in the positive rail that may be well above ground and,worse yet, may vary considerably with time.

The LT1620 was originally intended as a controller for a synchronous buck regulator in battery-charger applications. In its normal operating mode, this IC mirrors a current signal down to a 5-V reference supply. By adding a single small-signal MOSFET and a few resistors around the LT1620, it’s possible to again mirror this signal to provide a ground-referenced output.

Circuit operation is as follows: The LT1620 produces a voltage between the VCC pin and the average (AVG) pin, which is 10 times the voltage across the sense resistor R5 (Fig. 1). C2 filters this voltage. An internal op amp is provided with its non-inverting input on the AVG pin (pin 8), its inverting input on the PROG pin (pin 7), and its output on the IOUT pin (pin 2).

With the circuit connected as shown, this amplifier will force enough current through R4 to make the voltage drop on R4 equal to the voltage across C2. This current is mirrored down through R3 and filtered by C3, producing a clean, ground-referenced, dc output voltage. Resistor R2 is intended to cancel a small built-in offset in the LT1620’s amplifiers. The output voltage obeys the following relationship: VO = IO(R5)(R3)(10)/R4. Changing the value of R3 allows different scale factors to be selected.

The circuit yields excellent linearity over a wide range of load and input voltages.The curve shown was measured with the sense resistor referenced to a 5-V input source (Fig. 2). The curve looks the same even at inputs over 25 V, so only one curve is presented. The maximum input voltage is 36 V. There’s a small offset at no load. However, in a typical microprocessor-based data-acquisition system, a simple two-point calibration is all that’s required to attain excellent absolute accuracy.

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