ShuntVoltage

CalibrationRegister

2048

·

Current =

´

Power Register

Current Register

I C

2

Interface

Voltage Register

GND

BUS

ADC

V

I

A0

ALERT

A1

SDA

SCL

C

0.1 F

BYPASS

m

10A

Load

Alert Register

V

(Supply Voltage)

S

+12-V Supply

IN-

IN+

R

2 m

SHUNT

W

INA231

SBOS644 – FEBRUARY 2013

www.ti.com

CONFIGURE/MEASURE/CALCULATE EXAMPLE

In this example, shown in Figure 23, a nominal 10-A load creates a differential voltage of 20 mV across a 2-m

Ω

shunt resistor. The bus voltage for the INA231 is measured at the external BUS input pin; in this example, BUS

is connected to the IN– pin to measure the voltage level delivered to the load. For this example, the BUS pin

measures less than 12 V because the voltage at the IN– pin is 11.98 V as a result of the voltage drop across the

shunt resistor.

Figure 23. Example Circuit Configuration

For this example, assuming a maximum expected current of 15 A, the Current_LSB is calculated to be 457.7

μA/bit using Equation 2. Using a value of 500 μA/bit or 1 mA/bit for the Current_LSB significantly simplifies the

conversion from the Current register and Power register to amps and watts, respectively. For this example, a

value of 1 mA/bit was chosen for the Current register LSB. Using this value for the Current_LSB trades a small

amount of resolution for a simpler conversion process on the processor side. Using Equation 1 in this example

with a current LSB of 1 mA/bit and a shunt resistor of 2 m

Ω results in a Calibration register value of 2560, or

A00h.

The Current register (04h) is then calculated by multiplying the decimal value of the Shunt Voltage register

contents by the decimal value of the Calibration register and then dividing by 2048, as shown in Equation 3. For

this example, the Shunt Voltage register value of 8000 is multiplied by the Calibration register value of 2560 and

then divided by 2048 to yield a decimal value for the Current register of 10000, or 2710h. Multiplying this value

by 1 mA/bit results in the original 10-A level stated in the example.

(3)

The LSB for the Bus Voltage register (02h) is a fixed 1.25 mV/bit. This fixed value means that the 11.98 V

present at the BUS pin results in a register value of 2570h, or a decimal equivalent of 9584. Note that the MSB of

the Bus Voltage register is always zero because the BUS pin is only able to measure positive voltages.

16

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