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TPS54678: How to connect MCU IO to control EN PIN

Rocky Chen
Expert 8820 points
Part Number: TPS54678

Hi

When using tps54678, I hope to use MCU to control the enable timing of 54678. As said by d/s, open drain / collector type of IO are recommended. But if using OD IO to control, it was found during power up the device will be default at enable because MCU need initial time to start up to control IO. What I want is to make MCU power up firstly and once it is ready, then to make IO high to enable TPS54678 to start work. So it looks the OD/OC circuit could not work here. Would you please  verify this and advise some work around? Does enable pin can be driven by push pull type IO of MCU?

Thanks

-Rocky

  • 0
    TI__Guru**** 191445 points

    Rocky,

    The TPS54678 EN pin only sources current, 0.7 uA when disabled and 3.5 uA when enabled.  That is why we recommend open drain output as open drain only needs to sink current.  That being said, I often drive the EN pin wit a signal generator, which I assume has some sort of totem pole output.  As long as your output logic is compatible with the EN input not being able to sink any significant current when EN is driven high, you should be ok.  To address your second question, what is the state of the I/O at initial power up?  You need to actively pull EN low.  If left open, it will auto enable.

  • 0 in reply to JohnTucker
    TI__Expert 8820 points

    Hi John

    Thanks. I have two questions:

    1, For TPS54678, is there a calculator for its loop compensation?

    2, In Webench (old version) simulation, do we need to run worst case analysis? And tune RC compensation to make result passed worst case analysis finally? 

    - Rocky

  • 0 in reply to Rocky Chen
    TI__Guru**** 191445 points

    Rocky,

    I don't believe there is any calculator for TPS54678.  If there is, it would not accurately model or predict the loop response.  It would just have the datasheet equations.  I personally use and recommend the pspice average model for compensation design:

    You can use worst case analysis.  At least check it under various line and load conditions.  I usually make my loop BW fairly conservative with crossover around 30 kHz to 50 kHz.