OPA186DBVT datasheet(26/48 Pages) TI1

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8.2.1.2 Detailed Design Procedure

Designing a high-side current monitor circuit is straightforward, provided that the amplifier electrical

characteristics are carefully considered so that linear operation is maintained. Other additional characteristics,

such as the input voltage range of the analog to digital converter (ADC) that follows the current monitor stage,

must also be considered when configuring the system.

For example, consider the design of a OPAx186 high-side current monitor with an output voltage range set to be

compatible with the input of an ADC with an input range of 3.3 V, such as one integrated in a microcontroller.

The full-scale input range of this converter is 0 V to 3.3 V. Although the OPAx186 is specified as a rail-to-rail

input/output (RRIO) amplifier, the linear output operating range (like all amplifiers) does not quite extend all the

way to the supply rails. This linear operating range must be considered.

In this design example, the OPAx186 is powered by 24 V; therefore, the device is easily capable of providing the

3.3-V positive level; or even more, if the ADC has a wider input range. However, because the OPAx186 output

does not swing completely to 0 V, the specified lower swing limit must be observed in the design.

The best measure of an op-amp linear output voltage range comes from the open-loop voltage gain (AOL)

specification listed in the Electrical Characteristics table. The AOL test conditions specify a linear swing range

300 mV from each supply rail (RL = 10 kΩ). Therefore, the linear swing limit on the low end (VoMIN) is 300 mV,

and 3.3 V is the VoMAX limit, thus yielding an 11:1 VoMAX to VoMIN ratio. This ratio proves important in determining

the difference amplifier operating parameters.

A nominal load current (IL) of 10 A is used in this example. In most applications, however, the ability to monitor

current levels far less than 10 A is useful. This situation is where the 11:1 VoMAX to VoMIN ratio is crucial. If 11 A

is set as the maximum current, this current must correspond to a 3.3-V output. Using the 11:1 ratio, the minimum

current of 1 A corresponds to 300 mV.

Selection of current sense resistor RS comes down to how much voltage drop can be tolerated at maximum

current and the permissible power loss or dissipation. A good compromise for a 10-A sense application is an RS

of 10 mΩ. That value results in a power dissipation of 1 W, and a 0.1-V drop at 10 A.

Next, determine the gain of the OPAx186 difference amplifier circuit. The maximum current of 11 A flowing

through a 10-mΩ sense resistor results in 110 mV across the resistor. That voltage appears as a differential

voltage, VR, that is applied across the OPAx186 difference amplifier circuit inputs:

I * R

11 A * 10 m

110 mV

(1)

The OPAx186 required voltage gain is determined from:

OMAX

3.3 V

0.11 V

(2)

Now, checking the VoMIN using IL = 1 A:

OMIN

SMIN

OMIN

* I

* R

* 1 A * 10 m

300 mV

(3)

OPA186, OPA2186, OPA4186

SBOS968C – JUNE 2022 – REVISED JULY 2023

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