High-side current sensor monitors negative rail

All dedicated current-sense amplifiers provide high-side sensing on a positive supply, but you can adapt such circuits for monitoring a negative supply (Figure 1). The positive-supply pin, V+, connects to the system's positive supply, and the ground pin, GND, connects to the negative supply, V_EE. That arrangement monitors the negative supply and provides a positive output voltage for the external interface—typically, an A/D converter. The RS+ pin of the current-sense amplifier, IC₁, connects to the load, and the RS– pin connects to the negative supply. IC₁'s current-source output drives a current that is proportional to load current flowing to ground, not to the GND pin. Output resistor R_OUT converts the current to a voltage, which an optional ADC then digitizes.

Saturation in the internal transistors, which occurs at approximately ((V+)–1.2V), limits the maximum output voltage. Thus, V+ must exceed the full-scale output by at least 1.2V. If, for instance, the full-scale output is 1V, then V+≥2.2V. To meet the device's maximum and minimum operating voltages, 0≥V_EE≥–(32–V+), and ((V+)–V_EE))≥3V. Figure 2 shows the variation of current measurement accuracy with load current.

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