INA226 Datasheet, PDF (16/30 Pages) Texas Instruments – High-or Low-Side Measurement, Bi-Directional CURRENT/POWER MONITOR with I2C Interface

INA226
SBOS547 – JUNE 2011
www.ti.com
The Power Register (03h) is then be calculated by multiplying the decimal value of the Current Register, 10000,
by the decimal value of the Bus Voltage Register, 9584, and then dividing by 20,000, as defined in Equation 4.
For this example, the result for the Power Register is 12B8h, or a decimal equivalent of 4792. Multiplying this
result by the power LSB (25 times the [1 × 10^–3 Current LSB]) results in a power calculation of (4792 ×
25mW/bit), or 119.82W. The power LSB has a fixed ratio to the current LSB of 25W/bit to 1A/bit. For this
example, a programmed 1mA/bit current LSB results in a power LSB of 25mW/bit. This ratio is internally
programmed to ensure that the scaling of the power calculation is within an acceptable range. A manual
calculation for the power being delivered to the load would use a bus voltage of 11.98V (12V_CM – 20mV shunt
drop) multiplied by the load current of 10A to give a result of 119.8W.
Power =
Current · BusVoltage
20,000
(4)
Table 1 shows the steps for configuring, measuring, and calculating the values for current and power for this
device.
Table 1. Configure/Measure/Calculate Example⁽¹⁾
STEP #
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
REGISTER NAME
Configuration
Shunt
Bus
Calibration
Current
Power
ADDRESS
00h
01h
02h
05h
04h
03h
CONTENTS
4127h
1F40h
2570h
A00h
2710
12B8h
DEC
—
8000
9584
2560
10000
4792
(1) Conditions: Load = 10A, V_CM = 12V, R_SHUNT = 2mΩ, and V_BUS = 12V.
LSB
—
2.5µV
1.25mV
—
1mA
25mW
VALUE
—
20mV
11.98V
—
10A
119.82W
PROGRAMMING THE INA226 POWER MEASUREMENT ENGINE
Calibration Register and Scaling
The Calibration Register makes it possible to set the scaling of the Current and Power Registers to whatever
values are most useful for a given application. One strategy may be to set the Calibration Register such that the
largest possible number is generated in the Current Register or Power Register at the expected full-scale point.
This approach would yield the highest resolution based using the previously calculated minimum current LSB in
the equation for the Calibration Register. The Calibration Register can also be selected to provide values in the
Current and Power Registers that either provide direct decimal equivalents of the values being measured, or
yield a round LSB value for each corresponding register. After these choices have been made, the Calibration
Register also offers possibilities for end user system-level calibration. By physically measuring the current with
an external ammeter, the exact current is known. The value of the Calibration Register can then be adjusted
based on the measured current result of the INA226 to cancel the total system error as shown in Equation 5.
Corrected_Full_Scale_Cal = trunc
Cal ´ MeasShuntCurrent
INA226_Current
(5)
16
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