AN4366 Datasheet, PDF (10/14 Pages) STMicroelectronics – Compensate the input offset of a high-side current sensing

Current source
AN4366
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Current source
The current necessary to compensate the Vos should be high enough not to be impacted
by the gain error and low enough to allow reasonable values of Rs1 and Rs2 to be chosen.
Note that the current is generated by the current source and that gain error may appear in
temperature due to the external resistors.
To achieve good accuracy and to maintain the initial gain, the external resistances are set
as follow:
• Rs1 = Rs2 = 10 Ω
• Rs3 = 250 Ω which maintains K1 = 25
• K2 = 4 by setting SEL = Vcc+
In this configuration the total gain is: AV = K1.K2 = 100.
If we consider the worst situation, i.e. when the TSC1031 has a positive Vos offset of
+1.1 mv, in this case the output of the TSC1031 could become saturated. To avoid this,
add a negative offset compensation of 2 mV. Then, we can deduce that the current which
must be drawn by the current source is:
2 mV
Rs2
10 Ω
The current delivered by the current source is mainly fixed thanks to the reference voltage
TL431 (2.5V) and the resistance Rc4. So,
200 μA
A possible op-amp to drive the NPN transistor is the automotive grade LMV821A (order
code LMV821AIYLT).
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