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TPS2662: FLT voltage level connected to PCF8575

Andreas Lanz
Prodigy 20 points
Part Number: TPS2662
Other Parts Discussed in Thread: PCF8575

Hello

I have a custom board on which the FLT pin of the TPS26625 is directly connected to an IO-Expander PCF8575 input. There is no additional resistor in line or a pull-up or pull-down resistor. The PCF8575 is connected to 5.25V supply. The problem is that if there is no error reported (The fault pin doesn't pull to low.) The measured voltage is only 3.6V on the iSUP1_nFLT signal and not the expected 5.25V.

The PCF8575 has an pull-up current between 30..300uA. The leakage of the FLT pin according to the datasheet is around 100nA. So this shouldn't be a problem. Does anyone have an idea why the voltage is so low?

Best regards

Andreas

  • 0
    TI__Guru* 95891 points

    Hi Andreas,

    Welcome to E2E 

    Please use pull-up resistor between iSUP1_nFLT and 5.25V supply and check

    Best regards 

    Rakesh 

  • 0 in reply to Rakesh
    Prodigy 20 points

    Hi Rakesh

    Yes we have other inputs like the iSUP3_nFLT which has a 10kΩ pull-up resistor. On the iSUP3_nFLT input we have a voltage of 5.25V. Of course we can add a pull-up resistor on every input but we don't want to place resistors if they are not really necessary. Do you have an explanation why the direct connection of the TPS26625 and the PCF8575 doesn't have the right voltage levels? Is the leakage current higher as written in the datasheet?


    Best regards

    Andreas

  • 0 in reply to Andreas Lanz
    TI__Guru* 95891 points

    Hi Andreas,

    Direct connection is sourcing more current into the nFLT pin of the eFuse that is the reason for the voltage at nFLT pin to be high 3.6V during error state (nFLT = LOW)

    Does it clarify ?

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 20 points

    Hi Rakesh

    Thank you for the informations. The problem is not present when nFLT pin is LOW. The lower voltage of 3.6V only occurs in the nFLT = HIGH state.

    nFLT = LOW = 0.1V

    nFLT = HIGH = 3.6V

    I don't understand why the voltage is not 5.25V if the nFLT pin doesn't pull the voltage to Ground.

    Best regards

    Andreas

  • 0 in reply to Andreas Lanz
    TI__Guru* 95891 points

    Hi Andreas,

    Ok understood. then it looks strange to me as well because the nFLT pin has very low leakage of just 100nA (max)

    How are you measuring the nFLT pin voltage ? multimeter ? Is nFLT pin voltage toggling between 5.25V to 0V and we are measuring avg value by any change.

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 20 points

    Hi Rakesh

    I have measured the voltage with a True-RMS Multimeter. But now i measure the voltage additionally with an oscilloscope and the voltage is steady. No switching between 5.25V and 0V. Just a stable line on 3.6V.

    Best regards

    Andreas

  • 0 in reply to Andreas Lanz
    TI__Guru* 95891 points

    Hi Andreas,

    Thanks for checking!

    It looks like some dependency on the o/p structure and impedances of PCF8575. Can you please post a separate E2E thread to check with PCF8575 team.

    Sorry for the inconvenience. 

    Best Regards,

    Rakesh