TSZ121 datasheet(24/38 Pages) STMICROELECTRONICS

24 / 38 page

Application information

TSZ121, TSZ122, TSZ124

24/38

DocID023563 Rev 5

5.11.3

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 TSZ121, TSZ122, and TSZ124 devices (see Figure 49: "Low-side current

sensing schematic").

Figure 49: Low-side current sensing schematic

Vout can be expressed as follows:

Equation 4

Assuming that Rf2 = Rf1 = Rf and Rg2 = Rg1 = Rg, Equation 4 can be simplified as follows:

Equation 5

The main advantage of using the chopper of the TSZ121, TSZ122, and TSZ124, 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.

ou t

shun t

I 1

–

io 1

–

out

shunt

io 1

–

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