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TPS548A29: PGOOD Pin driving EN Pins

d_zero
Genius 4670 points
Part Number: TPS548A29
Other Parts Discussed in Thread: TPS62825,

Dear *,

can you tell how many EN pins of TPS62825ADMQ can i drive with TPS548A29RWW ? 

we would like to enable 3x TPS62825ADMQ, can i direct drive 3 EN inputs or do i need a buffer or mosfet between? PGOOD and EN pins?

The PGOOD has a pull up on VIN =5V of TPS548A29RWW.

How to proper size the pull-up resistor in this configuration ( any app note ) ?

Best Regards,

David.

  • 0
    TI__Expert 8130 points

    Hello David,

    I would not recommend to directly drive the 3 EN inputs pin. I would recommend Power-Supply Sequencing, which can be accomplished in various ways:

    1. Cascading PGOOD pin into enable pin (recommended)
    2. Sequencing using a reset IC
    3. Analog up/down sequencers

    The pull-up resistor value can be between 1kΩ and 100kΩ. Attached below are two App Notes in which you might find helpful.

    Regards,
    Alejandro

    slyt730.pdf slyt598.pdf

  • 0 in reply to iAlejandro
    Genius 4670 points

    Hi Alejandro,

    thank you for the suggestions, but i need that the PG from TPS548A29RWW enables in the same time all three EN pins from different TPS62825ADMQ.

    So i can't cascading them. And because we are ok with either Ratiometric or Simultaneous start up, we would like to avoid the reset or sequencer ic because they add cost.

    is it possible to use a non inverting buffer ( SN74LVC1G17DCK)  between PG pin one one side and 3xEN pin connected on other side ?

    Best Regards,

    David.

  • +1 in reply to d_zero
    TI__Mastermind 20375 points

    The abs max voltage rating of TPS62825 device is 6V.

    Use a single pullup resistor on PGOOD to VCC on TPS548A29

    and directly connect the TPS62825 enable pins to PGOOD.   

    The value of the pullup resistor should be able source more current

    than the max leakage of the 3 TPS62825 pins which is 0.1uA per device and the enable high threshold is a minimum of 1V and the leakage of the PGOOD which is a 5uA.

    The TPS548A29 recommends 1kOhm to 100kOhm. 

    Use the lower value of (VCC min voltage - 1V (enable threshold))/(5uA + 3x0.1uA)    or 100kOhm.