High-side current sensor monitors negative rail
Edited by Bill Travis
By Ken Yang, Maxim Integrated Products, Sunnyvale, CA -- EDN, April 17, 2003
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, VEE. 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 ROUT 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≥VEE≥–(32–V+), and ((V+)–VEE))≥3V. Figure 2 shows the variation of current measurement accuracy with load current.
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