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

(6)

8.2.2.2 Configure the Device

The first step to program the INA2227 is to properly set the device configuration registers, CONFIG1 and

CONFIG2. On initial power up, the configuration registers are set to the reset values (see Table 7-3 and

Table 7-4). In the default power on state the device is set to measured on the ±81.92mV range with the ADC

continuously converting the shunt and bus voltages for all channels. If the default power up conditions do not

meet the design requirements, these registers need to be set properly after each disable or VS power cycle

event.

8.2.2.3 Program the Shunt Calibration Registers

There are two shunt calibration registers for each channel that need to be correctly programmed after each

power up for the device to properly report any result based on current. The first step to calculate the value

for the calibration register is to calculate the minimum LSB value for the current by using Equation 2. Applying

this equation with the maximum expected current of 10A results in an minimum LSB size of 305.17578μA. The

INA2227 allows selection of the CURRENT_LSB to be up to 8 times larger than the minimum LSB size. For

this example a value of 500μA 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 registers results in a zero value for any result based on current for that channel. Programming these

registers is not required for reading shunt voltage, bus voltage or setting corresponding alert limits.

8.2.2.4 Set Desired Fault Thresholds

The INA2227 has the ability to assert the alert pin on several different fault conditions as described in

Alert Configuration Registers. The desired fault condition to assert the alert pin needs to be selected by

appropriately programming the ALERT MASK field in the Alert Configuration Register. Fault thresholds are set

by programming the desired trip threshold into the Alert Limit Registers.

For example, channel 1 can be configured to alert on an over current condition by setting the ALERT1 register

CHANNEL field to channel 1(00b) with the ALERT MASK field set to shunt over voltage (001b). The desired

threshold for the over current condition has to be programmed in the Limit1 Register. In this example, the over

current threshold is 9.0A and the value of the current sense resistor is 8.0mΩ, which give a shunt voltage limit of

72mV. Once the shunt voltage limit is known, the value for the shunt over voltage limit register is calculated by

dividing the shunt voltage limit by the shunt voltage LSB size.

For this case, the calculated value of the alert limit register is 72mV / 2.5μV = 28800d (7080h).

Values stored in the LIMIT1 and LIMIT2 registers are set to the default values when the device is disabled or VS

is power cycled.

Fault limits programmed into the LIMIT registers can be applied to a single channel or distributed to each of

the 2 measurement channels. For example, if monitoring of the bus voltage is also required on channel 1, the

CHANNEL field of the ALERT2 register can be also set to channel 1(00b) with the ALERT MASK field set to

monitor over bus conditions (011b). The value for the over voltage fault can be set as desired in the LIMIT2

register.

8.2.2.5 Calculate Returned Values

Parametric values are calculated by multiplying the returned value by the LSB value. Table 8-4 shows the

returned values for this application example, assuming the design requirements shown in Table 8-3.

Table 8-4. Register Values

Register

Contents

LSB Value

Calculated Value

Shunt_Voltage_CH1

(00h)

19200d (4B00h)

2.5µV

19200 × 2.5μV = 0.048V

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