UM1556 datasheet(31/87 Pages) STMICROELECTRONICS

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UM1556

Analogue current sense

Doc ID 023520 Rev 2

31/87

Figure 30.

VSENSE measurement

The trend is almost linear in the application range and so we can approximate the VSENSE

trend with the following equation:

Equation 6

Where m [ohm] is the rectangular coefficient and a is a constant.

The output current can be calculated by inverting this equation:

Equation 7

Instead of IOUT =ISENSE × K, once M[S] and b are known, it is possible to evaluate the IOUT

with a high accuracy, leaving only the spread due to temperature variation.

The current sense fluctuation due temperature variation is expressed in the datasheet with

the parameter dK/K.

How to calculate M and b

To calculate M and b two simple measurements performed at the end of the production line

are required. Chose two reference output currents (Iref1 and Iref2) and measure the

respective VSENSE1 and VSENSE2. These four values must thenbe stored in an EEPROM in

order to let the µC use this information to calculate K and b using the simple formulas

reported below.

Since we defined:

it is also true that:

sense

m*I

out

M*V

sense+b

out

M*V

sense+b

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