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PMP8740: What is the Purpose of R102 in PMP8740

Part Number: PMP8740

Dear TI Experts,

What is the Purpose of the resistor R102(27.4K) current loop of PMP8740 Schematics?. Is it to adding an offset to current sense output Iout?

Regards

Aneesh

  • Hi Aneesh,

    You are correct. This resistor adds a small offset ot the current sense output Iout. The reason why I added it, is because this converter can deliver charging currents down to zero. Therefore, if the microcontroller sets a zero reference on pin 5 of U1B, then this operational amplifers has to compare two voltages that are really close to zero, making the output unpredictable. With a small offset, when the converter starts to ramp up the output charging current, it starts always from zero with a clean output current.

    Best regards,

    Roberto

  • Hi Robert,

    The offset voltage is generated by R102(27.4K) and R39(49.9Ohm) upon 5V. That is 9.089mV. 

    Actually the R39(49.9ohm) is using for Two purpose

    1. loop measurement purpose

    2.  resistor divider for giving a offset for current sense

    Am i right? or correct me if i am wrong. 

    Regards

    Aneesh

  • Hi Aneesh,

    Correct: R39 has two purposes, loop measurement load resistor and load impedance for the current injection, through R102.

    By making few calculation, it´s like when the curret reference from the microcontroller is zero, the small bias "shows" the error amplifier like the actual output current is 275.4mA. This number comes from the 9.089mV offset, divided by the gain of U11 (165) and divided by the current sense resistor R92 (0.2 milli-Ohm).

    Best regards,

    Roberto