Figure 7-1 Current Monitor Example Circuit

As seen in Figure 7-1, the current flowing through the shunt resistor ( R_S) develops a voltage drop equal to V_SENSE across R_S. The resulting voltage at the –IN pin will now be less than +IN pin proportional to the V_SENSE voltage.

The sense amplifier senses this indifference and increases the gate drive to the MOSFET to increase I_S′ current flowing through the R_IN+ string until the amplifer inputs are equal. In this way, the voltage drop across R_IN+ now matches the votlage drop across V_SENSE.

The R_IN resistors are trimmed to a nominal value of 5 kΩ each. The current I_S′ flows through R_IN+ , the MOSFET, and R_GAIN to ground. The I_S′ current generates the voltage V_G across R_GAIN. The gain is created bythe ratio of R_GAIN and R_IN.

A current proportional to I_S is generated according to the following relation:

where

• R_IN = 1 / Gm

This current flows entirely in the external gain resistor developing a voltage drop equal to:

This voltage is buffered and presented at the output with a very low output impedance allowing a very easy interface to other devices (ADC, μC…).

where

• G = R_GAIN / R_IN