For sensing current of a negative power supply, the designers potential first considers a current sensing amplifier since these parts were widely used for current sensing with excellent performance, compact package, low cost, and sourcing easy. However, there almost has no current sensing amplifier that can endure common input voltage less than -22V in market for current sensing amplifier. How an the designer make a normal current sensing amplifier work for a negative power supply? Figure 3-4 shows how a general current sensing amplifier was used for monitor current for a negative power supply.

Figure 3-4 Current Sensing for Negative Power Supply Based Current Sensing Amplifier

With connect GND pin of current sensing amplifier to negative power supply, the designer makes the common input voltage meet specification of current sense amplifier. the output voltage is positive since the chip was supply by a positive power supply and output is a current source. Any current sense amplifier can be used for monitoring current of negative power supply if the amplifier meets the following conditions.

1. The output is a current source. This condition keeps the output voltage is positive.
2. The chip’s power supply range needs to be larger than difference between positive power supply for chip and negative power supply for load.

Assume sensing resistor is Rs, load resistor of current sensing amplifier is R_L, Output voltage is Vo and input resistor of internal resistor is R, Then the output voltage for monitor current as shown in Equation 4.