Table 8-3 lists the design requirements for the circuit shown in Figure 8-2.

Table 8-3. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Power-supply voltage (VS)

3.3 V

Bus supply rail (VCM)

12 V

Average Current

6 A

Overcurrent fault threshold

9 A

Maximum current monitored (IMAX)

10 A

ADC Range Selection (VSENSE_MAX)

±81.92 mV

8.2.2 Detailed Design Procedure

This design example walks through the process of selecting the shunt resistor, programming the calibration

register, setting the correct fault thresholds, and how to properly scale returned values from the device.

8.2.2.1 Select the Shunt Resistor

Using values from Table 8-3, the maximum value of the shunt resistor is calculated based on the value of

the maximum current to be sensed (IMAX) and the maximum allowable sense voltage (VSENSE_MAX) for the

chosen ADC range. When operating at the maximum current, the differential input voltage must not exceed the

maximum full scale range of the device, VSENSE_MAX. Using Equation 5 for the given design parameters, the

maximum value for RSHUNT is calculated to be 8.192 mΩ. The closest standard resistor value that is smaller than

the maximum calculated value is 8.0 mΩ. Smaller resistors can be used to minimize power loss at the expense

of reduced accuracy. The shunt resistor selected must have sufficient wattage to handle the power dissipation at

maximum load at the desired operating temperature.

RSHUNT <

VSENSE_MAX

IMAX

(5)

8.2.2.2 Configure the Device

The first step to program the INA232 is to properly set the device Configuration register (0h). On initial power up,

the configuration register is set to the reset values (see Table 7-4). In the default power on state the device is set

to measured on the ±81.92 mV range with the ADC continuously converting the shunt and bus (voltage at IN–)

voltages. If the default power up conditions do not meet the design requirements, these registers will need to be

set properly after each VS power cycle event.

8.2.2.3 Program the Shunt Calibration Register

The shunt calibration register needs to be correctly programmed at each VS power up in order for the device to

properly report any result based on current. The first step is to calculate the minimum LSB value for the current

by using Equation 2. Applying this equation with the maximum expected current of 10 A results in an minimum

LSB size of 305.17578 μA. The INA232 allows selection of the CURRENT_LSB to be up to 8 x larger than the

minimum LSB size. For this example a value of 500 μA is used. Applying Equation 1 to the Current_LSB and

selected value for the shunt resistor results in a shunt calibration register setting of 1280d (500h). Failure to set

the value of the shunt calibration register will result in a zero value for any result based on current. Programming

this register is not required for reading shunt voltage, bus voltage, or setting alert limits.

8.2.2.4 Set Desired Fault Thresholds

The INA232 has the ability to assert the Alert pin on several different fault conditions as described in ALERT

Pin. The desired fault condition to assert the Alert pin needs to be selected by appropriately programming the

Mask/Enable Register (6h). Fault thresholds are set by programming the desired trip threshold into the Alert

Limit Register (7h).

For example, an overcurrent fault condition would be selected by setting the SOL bit in the Mask/Enable

Register to 1. The desired threshold for the over current condition would have to be programmed in the Alert

Limit Register. In this example, the over current threshold is 9.0 A and the value of the current sense resistor is

INA232

SBOSAA2 – DECEMBER 2022

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