TSX711 datasheet(23/31 Pages) STMICROELECTRONICS

23 / 31 page

TSX711, TSX711A, TSX712

Application information

DocID025959 Rev 5

23/31

5.11.2

Low-side current sensing

Power management mechanisms are found in most electronic systems. Current sensing is

useful for protecting applications. The low-side current sensing method consists of placing

a sense resistor between the load and the circuit ground. The resulting voltage drop is

amplified using the TSX711, TSX711A, or TSX712 (see Figure 37: "Low-side current

sensing schematic").

Figure 37: Low-side current sensing schematic

Vout can be expressed as follows:

Equation 9

Assuming that Rf2 = Rf1 = Rf and Rg2 = Rg1 = Rg, Equation 9 can be simplified as follows:

Equation 10

The main advantage of using a precision amplifier like the TSX711, TSX711A, or TSX712,

for a low-side current sensing, is that the errors due to Vio and Iio are extremely low and

may be neglected.

Therefore, for the same accuracy, the shunt resistor can be chosen with a lower value,

resulting in lower power dissipation, lower drop in the ground path, and lower cost.

Particular attention must be paid on the matching and precision of Rg1, Rg2, Rf1, and Rf2, to

maximize the accuracy of the measurement.

Taking into consideration the resistor inaccuracies, the maximum and minimum output

voltage of the operational amplifier can be calculated respectively using Equation 11 and

Equation 12.

Rshunt

Rg1

Rg2

Rf1

5 V

Vout

Rf2

ou t

shun t

I 1

–

io 1

–

out

shunt

io 1

–

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