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TPS7A78: PFD top resistor divider

Koji Ikeda
Expert 5605 points
Part Number: TPS7A78

Hello Support team,

Datasheet recommend PFD top resistor divider up to 200k ohm, but, the value on EVM is 2.21Mohm. Does EVM have wrong design? Please clarify it.

Thanks,

Koji Ikeda

  • 0
    TI__Genius 12511 points

    Hi Koji Ikeda,

    Generally we recommend you select resistors such that your resistor divider network's total current is >100x the leakage current from the pin, however we don't spec the leakage current into the high impedance terminal on the comparator. For the EVM a principal concern was power dissipation, and so a resistor an order of magnitude larger was selected for the PFD resistive divider. I have contacted the designer to see if there were additional concerns with the PFB pin resistor selection, I will get back to you on whether the datasheet or the EVM has the error in resistor selection size. 

    Regards,

    John

  • 0 in reply to John Schneider
    TI__Expert 5605 points

    John-san,

    Thank you for your support. Please let me know your feedback soon, customer need to finalize their design. Conflict between datasheet and EVM makes customer confused. 

    Thanks,

    Koji Ikeda

  • +1 in reply to Koji Ikeda
    TI__Genius 12511 points

    HI Koji Ikeda,

    I've heard back from the Design team and they have come to the conclusion that it is a datasheet error, the EVM is in a perfectly acceptable configuration. The designers and definers aim in limiting the resistance is to have the resistors large enough to have single digit uA of current consumed at the trip point. From the designer themselves:

    "Lower resistors are wasting power, and too high a resistor will make that PFD node more sensitive to AC shunt coupling in from nearby."

    Let me know if there are further questions from the customer.

    Regards,

    John