9.2.2 Detailed Design Procedure

The transfer function of the circuit in Figure 33 is shown in Equation 1.

Equation 1.

The load current (I_LOAD) produces a voltage drop across the shunt resistor (R_SHUNT). The load current is set from 0 A to 1 A. To keep the shunt voltage below 100 mV at maximum load current, the largest shunt resistor is defined using Equation 2.

Equation 2.

Using Equation 2, R_SHUNT is 100 mΩ. The voltage drop produced by I_LOAD and R_SHUNT is amplified by the MCP629x to produce an output voltage of roughly 0 V to 4.95 V. The gain needed by the MCP629x to produce the necessary output voltage is calculated using Equation 3:

Equation 3.

Using Equation 3, the required gain is calculated to be 49.5 V/V, which is set with resistors R_F and R_G. Equation 4 is used to size the resistors, R_F and R_G, to set the gain of the MCP629x to 49.5 V/V.

Equation 4.

Choosing R_F as 165 kΩ and R_G as 3.4 kΩ provides a combination that equals roughly 49.5 V/V. Figure 34 shows the measured transfer function of the circuit shown in Figure 33.