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Supply-current monitoring is a necessary feature in high-reliability systems for which excessive current can cause damage or compromise safety. Such systems avoid overload faults by monitoring their power supply and shutting it down before the fault occurs. However, most current-monitoring ICs are designed for positive-voltage supplies. For negative supplies, the circuit of Fig. 1 monitors load current and provides a proportional output voltage.
Voltages at the inverting and noninverting terminals of op amp IC1A are forced to be equal by an active-feedback current mirror. VR1 = VSENSE, and therefore:
Three alternatives are now possible. You can convert the output current (IR1) to voltage by connecting resistor RO to ground, to VCC or to an inverting amplifier. Connecting RO to ground (GND) eliminates the need for a positive supply. In that case, the output voltage is negative and proportional to load current:
(RO connected to GND).
You can connect RO to VCC for applications that require a positive output voltage, but the output will be referenced to VCC:
(RO connected to VCC).
To reference the positive voltage output to ground, you must use an inverting amplifier (IC1B) as shown in Fig. 1:
(RO connected to an inverting amplifier).
Note that RO does not affect output voltage for the inverting-amplifier case, but that resistor is usually needed for stability. RG can be optional, but it also provides stability by isolating the op amp from the capacitive load of the MOSFET gate. Finally, resistor RC provides compensation for the op amp's input bias current.
Fig. 2 shows measurement error versus load current for the Fig. 1 circuit. To ensure accurate current measurements, the resistors (except for RG and RC) should have a tolerance of 1% or better. RSENSE must be rated to dissipate the power associated with high load currents.