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PMP21112: Function of the Z diodes D1 and D3 ?

Mirko Gläßer
Prodigy 60 points
Part Number: PMP21112

The Z diodes D1 and D3 and their circuitry give the voltage at CSN/CSP an offset of approx. 27mV
which would correspond to a current of 27A via the measuring shunt R2 or R32.
I wonder why this was done? I would be pleased to receive an answer.

  • 0
    TI__Mastermind 41770 points

    Hi Mirko, this offset is a function of the input voltage. For this boost converter, the peak input currents can be large. This means you have to set the sense resistor for the worst case peak current (minimum input voltage and max output current). At higher input voltages, the peak currents will be lower. This means the current limit threshold will be higher at higher input voltages than at lower input voltages. Therefore, to have a consistent current limit, you need to inject some voltage into the sense resistor (higher signal at higher input voltages). This is what this circuit achieves.