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TPS7A85: Soft Start and Feed Forward Cap Values

Michael Holyoak
Prodigy 120 points
Part Number: TPS7A85

Hello,

I am using a TPS7A85 (output = 2.5V) and TPS7A83 (output = 1.5V) to supply power to two separate rails on an IC.  I need the TPS7A83 output to come up first, so I increased the soft start cap for the TPS7A85 from 10nF to 100nF; the TPS7A83 soft start cap is still set to 10nF.  The feed forward caps for each circuit are set to 10nF.

The TPS7A85 datasheet has opposing statements regarding the time constants for soft start and feed forward:

1. "The CFF time constant must be greater than the soft-start time constant to ensure proper operation of the PG during start-up", page 28

2. "To ensure proper PG functionality the time constant defined by CNR/SS must be greater than or equal to the time constant from the CFF", page 21

Which statement is correct?  Table 1 (row 3) in the TPS7A85 datasheet shows that my soft start and feed forward values are OK .. but are they?  Once I made the change, the TPS7A85 output appeared unstable and was always lower than 2.5V.  Prior to the change, it was working OK.  To make matters worse, I changed the soft start cap back to 10nF, and the problem remains.

Additionally, is it OK to tie the PG pins of each LDO together with a pull-up value specified by the range in the data sheet?  The pull-up is set to 3.3V, as it is driving a CPLD input with 0/3.3 logic.

Thanks,

Mike 

  • 0
    TI__Mastermind 23361 points
    Hi Mike,

    1. "To ensure proper PG functionality the time constant defined by CNR/SS must be greater than or equal to the time constant from the CFF". This is the correct statement. The other one is wrong, we will fix it in our next datasheet revision. Thanks for pointing this out for us.

    2. The values you picked seem to be okay according to the datasheet. I will need to see your schematic to comment further.

    3. It's okay to tie two PG pins with one pull-up resistor, but your PG pin could go down even only one of the output drops below the monitored level.

    Regards,
    Jason Song
  • 0 in reply to Jason Song
    Prodigy 120 points

    Jason,

    Here is my schematic for bother the TPS7A85 and TPS7A83 LDO regulators.  Please advise if OK.

    What pull-up value do you recommend for multiple PG pins tied together? 

    Thanks,

    Mike

  • 0 in reply to Michael Holyoak
    TI__Mastermind 23361 points
    Hi Mike,

    "TPS7A85 output appeared unstable and was always lower than 2.5V. Prior to the change, it was working OK. To make matters worse, I changed the soft start cap back to 10nF, and the problem remains."

    Since after you changed the cap back to 10nF, the problem remains. The problem is not directly related to the soft start cap. Will you provide more details on the problem? Is the output still stable?

    It's okay to use the datasheet recommended pull up value even you have two PG ties together.

    Regards,
    Jason Song