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TPS3838 current leakage problem

David Landfried
Prodigy 30 points
Other Parts Discussed in Thread: TPS3838

I am using the TPS3838E18QDBVRQ1. It is sensing the 1.8V core voltage for a CPLD. However, the reset goes to a 3.3V processor. Therefore, I have a pullup resistor on the RESET# to 3.3V. With nothing else connected on this part the 1.8V supply is sinking current (-185uA). As soon as I remove the TPS3838, the current returns to a positive current.

I thought that since this was an open drain output, the voltage on the RESET# pin was good to 7V. Is there an internal diode? If so, then that should be stated in the datasheet.

Please confirm if this is the problem I am seeing with this part.

  • 0
    TI__Mastermind 41790 points

    Hi David,

    Please attach a schematic of your application with the TPS3838. Some comments on the description above. The maximum recommended RESET pullup voltage is VDD+0.3V. 3.3V with 1.8V VDD is violating this condition.

    We can dedug this issue by first getting this device in the recommended operating conditions for a known working IC shown in page 3 of the datasheet.

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Prodigy 30 points

    No need for a schematic. CT = GND, VDD = 1.8V, MR# = no connection, RESET# = 65k pullup to 3.3V and goes to input of CPLD.

    RESET# is an output pin, not an input pin. Datasheet indicates maximum voltage is 7VDC on all other pins. Block diagram on page 3 does not show anything between RESET# and Vdd. An open drain output implies a true open drain.

    So is it truly an open drain or is it open drain with either an R or Diode to Vdd? I just need to know if it is truly an open drain or is there something else there.

  • 0 in reply to David Landfried
    TI__Mastermind 41790 points

    Hi David,

    Yes this is an open drain output which allows the use of ORing outputs together. The 7V is an abs max rating so as to not damage the device. The recommended operating conditions VDD+0.3V on RESET is the parameters that allows proper operation shown in the electrical characteristics table of the datasheet.

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Prodigy 30 points

    Let's make sure we are looking at the same datasheet. I am looking at the one I just downloaded earlier this week:

    SGLS141A − DECEMBER 2002 − REVISED JANUARY 2007

    There is nothing for the TPS3838 RESET# in the Electrical Characteristics table on page 6. If it is limited to Vdd+0.3V then it is not truly open drain - it has some tie back to Vdd. I am just trying to figure out what that is since there is nothing in the datasheet indicating that.

    It also sounds like this cannot reset a CPLD or any IC that has a core voltage lower than the I/O voltage. Is that true?

    -Dave