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Looking at the datasheet of the IR2136 it suggests this little resistor network as input for the ITRIP pin.

Part of IR2136 suggested circuit

https://www.infineon.com/dgdl/Infineon-IR213-DS-v01_00-EN.pdf?fileId=5546d462533600a4015355c8a02116a5

I understand the basic principle of current sensing resistors. But I don't get why the voltage divider is necessary.

ITRIP has a threshold of 0.5V. Let's say I want to trigger the overcurrent protection at 5A. Why can't I use a single 100mOhm resistor? Like this:

Adapted circuit

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I expect it might have something to do with the fact that the ITRIP input has an internal Zener diode clamping it to 5.2 volts above Vss. As with any Zener diode clamp, you need a resistor in series with the Zener to prevent excessive current flow and destruction. The way in which the two resistors are configured provides that needed resistance.

There is also the absolute maximum rating for ITRIP that suggests it shouldn't fall below Vss - 0.3 volts and the added resistor will help out here.

But, if you need to play tunes with the exact trip value and don't want to alter your series current sense resistor, the potential divider option helps you achieve that.

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