Datasheet

R
2mW
SHUNT
10A
Load
+12V
VCM
GND
V
I
V
IN+
V
IN-
PowerRegister
I C
Interface
2
CurrentRegister
VoltageRegister
SDA
SCK
A0
A1
0.1 Fm10 Fm
+3.3Vto+5V
V (SupplyVoltage)
S
´
INA220
SBOS459D –JUNE 2009–REVISED SEPTEMBER 2010 www.ti.com
CONFIGURE/MEASURE/CALCULATE shunt voltage. By knowing the value of the shunt
EXAMPLE resistor, the device can then calculate the amount of
current that created the measured shunt voltage drop.
In this example, the 10A load creates a differential
The first step when calculating the calibration value is
voltage of 20mV across a 2mΩ shunt resistor. The
setting the current LSB. The Calibration Register
voltage present at the V
IN–
pin is equal to the
value is based on a calculation that has its precision
common-mode voltage minus the differential drop
capability limited by the size of the register and the
across the resistor. The bus voltage for the INA220 is
Current Register LSB. The device can measure
measured at the external V
BUS
input pin, which in this
bidirectional current; thus, the MSB of the Current
example is connected to the V
IN–
pin to measure the
Register is a sign bit that allows for the rest of the 15
voltage level delivered to the load. For this example,
bits to be used for the Current Register value. It is
the voltage at the V
IN–
pin is 11.98V. For this
common when using the current value calculations to
particular range (40mV full-scale), this small
use a resolution between 12 bits and 15 bits.
difference is not a significant deviation from the 12V
Calculating the current LSB for each of these
common-mode voltage. However, at larger full-scale
resolutions provides minimum and maximum values.
ranges, this deviation can be much larger.
These values are calculated assuming the maximum
current that will be expected to flow through the
Note that the Bus Voltage Register bits are not
current shunt resistor, as shown in Equation 2 and
right-aligned. In order to compute the value of the
Equation 3. To simplify the mathematics, it is
Bus Voltage Register contents using the LSB of 4mV,
common to choose a round number located between
the register must be shifted right by three bits. This
these two points. For this example, the maximum
shift puts the BD0 bit in the LSB position so that the
current LSB is 3.66mA/bit and the minimum current
contents can be multiplied by the 4mV LSB value to
LSB would be 457.78µA/bit assuming a maximum
compute the bus voltage measured by the device.
expected current of 15A. For this example, a value of
The shifted value of the bus voltage register contents
1mA/bit was chosen for the current LSB. Setting the
is now equal to BB3h, a decimal equivalent of 2995.
current LSB to this value allows for sufficient
This value of 2995 multiplied by the 4mV LSB results
precision while serving to simplify the math as well.
in a value of 11.98V.
Using Equation 4 results in a Calibration Register
The Calibration Register (05h) is set in order to
value of 20480, or 5000h.
provide the device information about the current
shunt resistor that was used to create the measured
Figure 22. Example Circuit Configuration
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