INA3221

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SBOS576 – MAY 2012

BASIC ADC FUNCTIONS

The INA3221 performs two measurements on up to three power supplies of interest. The voltage developed from

the load current passing through a shunt resistor creates a shunt voltage that is measured between the IN+ and

IN– pins. The device also internally measures the power-supply bus voltage at the IN– pin for each channel. The

differential shunt voltage is measured with respect to the VIN– pin while the bus voltage is measured, with

respect to ground.

The INA3221 is typically powered by a separate power supply that ranges from 2.7 V to 5.5 V. The monitored

supply buses range from 0 V to 26 V. Note that, based on the fixed 8-mV bus voltage register LSB, a full-

scale register value results in 32.76 V. However, the actual voltage applied to the INA3221 input pins

should not exceed 26 V. There are no special power-supply sequencing considerations between the common-

mode input ranges and the device power-supply voltage because they are independent of each other; therefore,

the bus voltages can be present with the supply voltage off and vice-versa.

As noted, the INA3221 takes two measurements for each channel. Each measurement can be independently or

sequentially measured, based on the mode setting (bits[2:0] in the Configuration Register). When the INA3221 is

in normal operating mode (that is, the MODE bits of the Configuration Register are set to '111'), the device

continuously converts a shunt voltage reading followed by a bus voltage reading. This procedure converts one

channel and then continues to the shunt voltage reading of the next enabled channel, followed by the channel

bus voltage reading for that channel, and so on, until all enabled channels have been measured. The

programmed Configuration Register mode setting applies to all channels. Any channels not enabled are

bypassed in the measurement sequence, regardless of mode setting.

The INA3221 has two operating modes (continuous and single-shot) that determine the internal ADC operation

after these conversions complete. When the INA3221 is set to continuous mode, based on the MODE bit

settings, the device continues to cycle through all enabled channels until a new configuration setting is

programmed.

The Configuration Register MODE control bits also enable modes to be selected that convert only the shunt or

bus voltage. This feature further allows the monitoring function configuration to fit specific application

requirements.

In single-shot modes, writing any single-shot convert modes to the Configuration Register (that is, the

Configuration Register MODE bits set to ‘001’, ‘010’, or ‘011’) triggers a single-shot conversion. This action

produces a single set of measurements for all enabled channels. To trigger another single-shot conversion, the

Configuration Register must be written a second time, even if the mode does not change. When a single-shot

conversion is initiated, all enabled channels are measured one time and then the device enters a power-down

state. The INA3221 registers can be read at any time, even while in power-down. The data present in these

registers are from the last completed conversion results for the corresponding register. The Conversion Ready

Flag bit (Mask/Enable Register CVRF bit) can help coordinate single-shot conversions, which is especially helpful

during longer conversion time settings. The CVRF bit is set after all conversions are complete. The CVRF bit

clears under the following conditions:

1. Writing to the Configuration Register, except when configuring the MODE bits for power-down mode; or

2. Reading the Status Register.

In addition to the two operating modes (continuous and single-shot), the INA3221 also has a separate selectable

power-down mode that reduces the quiescent current and turns off current into the INA3221 inputs, thus

reducing the impact of supply drain when the device is not used. Full recovery from power-down mode requires

40 µs. The INA3221 registers can be written to and read from while the device is in power-down mode. The

device remains in power-down mode until one of the active MODE settings are written to the Configuration

Register.

Copyright © 2012, Texas Instruments Incorporated

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Product Folder Link(s): INA3221