7.3.2 Power Calculation

The current and power are calculated after a shunt voltage and bus voltage measurement as shown in Figure

7-2. Power is calculated based on the previous current calculation and the latest bus voltage measurement. If

the value loaded into the SHUNT_CAL register is zero, the power value reported is also zero. The current and

power values are considered intermediate results (unless the averaging is set to 1) and are stored in an internal

accumulation register. Following every measured sample, the newly-calculated values for current and power are

appended to this accumulation register until all of the samples have been measured and averaged. After all of

the samples have been measured and the corresponding current and power calculations have been made, the

accumulated average for each of these parameters is then loaded to the corresponding output registers where

they can then be read. These calculations are performed in the background and do not add to the overall

conversion time.

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Current Limit Detect Following

Every Shunt Voltage Conversion

Bus and Power Limit Detect

Following Every Bus Voltage Conversion

Power Average

Bus Voltage Average

Shunt Voltage Average

Figure 7-2. Power Calculation Scheme

7.3.3 Low Bias Current

The INA239 features very low input bias current which provides several benefits. The low input bias current of

the INA239 reduces the current consumed by the device in both active and shutdown state. Another benefit of

low bias current is that it allows the use of input filters to reject high-frequency noise before the signal is

converted to digital data. In traditional digital current-sense amplifiers, the addition of input filters comes at the

cost of reduced accuracy. However, as a result of the low bias current, the reduction in accuracy due to input

filters is minimized. An additional benefit of low bias current is the ability to use a larger shunt resistor to

accurately sense smaller currents. Use of a larger value for the shunt resistor allows the device to accurately

monitor currents in the sub-mA range.

The bias current in the INA239 is the smallest when the sensed current is zero. As the current starts to increase,

the differential voltage drop across the shunt resistor increases which results in an increase in the bias current

as shown in Figure 6-14.

7.3.4 High-Precision Delta-Sigma ADC

The integrated ADC is a high-performance, low-offset, low-drift, delta-sigma ADC designed to support

bidirectional current flow at the shunt voltage measurement channel. The measured inputs are selected through

the high-voltage input multiplexer to the ADC inputs as shown in Figure 7-1. The ADC architecture enables lower

drift measurement across temperature and consistent offset measurements across the common-mode voltage,

temperature, and power supply variations. A low-offset ADC is preferred in current sensing applications to

provide a near 0-V offset voltage that maximizes the useful dynamic range of the system.

The INA239 can measure the shunt voltage, bus voltage, and die temperature, or a combination of any based on

the selected MODE bits setting in the ADC_CONFIG register. This permits selecting modes to convert only the

shunt voltage or bus voltage to further allow the user to configure the monitoring function to fit the specific

application requirements. When no averaging is selected, once an ADC conversion is completed, the converted

values are independently updated in their corresponding registers where they can be read through the digital

interface at the time of conversion end. The conversion time for shunt voltage, bus voltage, and temperature

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INA239

SLYS027 – JANUARY 2021

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