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TPS65381-Q1: maximum Reset Extension delay time

Shinji Tsuji
Genius 13399 points
Part Number: TPS65381-Q1

Hi team,

I would like to know the maximum allowable Reset Extension time of TPS65381-Q1. 

It is described in the datasheet that if Rrsext is OPEN, the device is stucked in a RESET state. 

And the equation of this delay is Trstext=0.148*(R+8.5).

How much is the maxim allowable Trstext? 

Since my customer having trouble in the field, your quick response is very much appreciated. 

regards,

  • 0
    TI__Genius 17380 points
    Hi Shinji,

    I've moved your post to the automotive forum to ensure the proper experts can find it.
  • 0
    TI__Mastermind 23810 points
    The engineers who support this are out of the office for the holidays. They will be back after the new year, and will respond to your question then.

    Thanks,

    -Clancy
  • 0
    TI__Mastermind 20590 points
    Hi Tsuji-san,

    The resistor on RSTEXT should be chose so that the RESET generated by the TPS65381-Q1 matches the reset time needed by the attached MCU or other circuitry. If the RSTEXT pin is shorted to GND (0 ohms) the nRES pin will be held in RESET (low) for the system for 0.98 (minimum) to 1.89 (maximum) ms, at 22k ohms RESET will be asserted for between 4.05 (minimum) to 4.95 (maximum) ms. If this pin opens up it will cause continual RESET, so that in a fault case on the pin (open) it will be a safe state (RESET). The typical expected value range for the resistor on RSTEXT pin is between 0 and 22k ohms for most MCUs and systems using the TPS65381-Q1, as noted in datasheet line POS 9.3, RRSTEXT. This line also notes "in case of open-connect, device stays in RESET state".

    Is your customer having an issue with a single system where the resistor or PCB may have failed open, or did they choose a resistance value much higher than 22 k ohms?

    Scott
  • 0 in reply to Scott Monroe
    TI__Genius 13399 points
    Hi Scott,

    Yes, the customer has an issue in their system and need to extend the Reset extention time.
    I suggested them to follow the equation and the simulation result I received from the product line before upto 110kohm.

    The customer most likely select the value below 110kohm, but I would like to know if there is any issue connecting the larger
    resistor. for example the tolerance of reset extention time get worse at high resistance...
    Could you advise the maximum allowable reset extention value?

    regards,
  • 0 in reply to Shinji Tsuji
    TI__Mastermind 20590 points
    Hi Tsuji-san,

    To close this topic out on E2E, external resistors up to 110k ohm could be used, but the spread of min vs max for the reset extension time will get larger and larger as the external resistance goes higher. As long as the customer's system can accept that it should be fine. We would recommend staying at or below 110 k ohm though.

    Scott