INA186A1IDDFT datasheet(19/46 Pages) TI

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8.1.2 RSENSE and Device Gain Selection

The accuracy of any current-sense amplifier is maximized by choosing the current-sense resistor to be as large

as possible. A large sense resistor maximizes the differential input signal for a given amount of current flow

and reduces the error contribution of the offset voltage. However, there are practical limits as to how large the

current-sense resistor can be in a given application because of the resistor size and maximum allowable power

dissipation. Equation 2 gives the maximum value for the current-sense resistor for a given power dissipation

budget:

MAX

SENSE

MAX

(2)

where:

• PDMAX is the maximum allowable power dissipation in RSENSE.

• IMAX is the maximum current that will flow through RSENSE.

An additional limitation on the size of the current-sense resistor and device gain is due to the power-supply

voltage, VS, and device swing-to-rail limitations. In order to make sure that the current-sense signal is properly

passed to the output, both positive and negative output swing limitations must be examined. Equation 3 provides

the maximum values of RSENSE and GAIN to keep the device from exceeding the positive swing limitation.

MAX

SENSE

REF

GAIN < V

(3)

where:

• IMAX is the maximum current that will flow through RSENSE.

• GAIN is the gain of the current-sense amplifier.

• VSP is the positive output swing as specified in the data sheet.

• VREF is the externally applied voltage on the REF pin. This voltage is zero for devices without a REF pin.

To avoid positive output swing limitations when selecting the value of RSENSE, there is always a trade-off

between the value of the sense resistor and the gain of the device under consideration. If the sense resistor

selected for the maximum power dissipation is too large, then it is possible to select a lower-gain device in order

to avoid positive swing limitations.

The negative swing limitation places a limit on how small the sense resistor value can be for a given application.

Equation 4 provides the limit on the minimum value of the sense resistor.

MIN

SENSE

REF

GAIN > V

(4)

where:

• IMIN is the minimum current that will flow through RSENSE.

• GAIN is the gain of the current-sense amplifier.

• VSN is the negative output swing of the device (see Rail-to-Rail Output Swing).

• VREF is the externally applied voltage on the REF pin. This voltage is zero for devices without a REF pin.

In addition to adjusting RSENSE and the device gain, the voltage applied to the REF pin can be slightly increased

above GND to avoid negative swing limitations.

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INA186

SBOS318B – APRIL 2019 – REVISED JULY 2021

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