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TPS51200: About question the output of the sequence IC LM3881 for the EN signal of TPS51200 doing.

Bob Lee62837
Mastermind 7230 points
Part Number: TPS51200
Other Parts Discussed in Thread: LM3881, LM3481

Dear support member,

My customer used TPS51200 and LM3881.

My customer consider now using the output of the sequence IC LM3881 for the EN signal of TPS51200 doing.


VENIL max of EN terminal of TPS51200 is 0.3 V,
The VOL max of the LM3881 is 0.4 V.

In some cases, you may not be able to drop it to Low.

Question 1
Is there a technique to reliably lower the TPS51200 EN to the Low with the above combination?


Question 2
What happens when EN of TPS51200 reaches 0.4 V?

Best regard.
Bob Lee.

  • 0
    TI__Mastermind 24075 points
    Hi Bob,

    EN pin current is very small. Can you use a resistor divier to make the voltage lower than 0.3V?
    When EN voltage is 0.4V, the turn-off can not be garanteed.

    Thanks
    Qian
  • 0 in reply to Qian Chen
    Mastermind 7230 points

    Dear Qian Chen,

    Thank you very much for reply.
    I confirm my customer your idea.


    May I ask a question?

    Incidentally, how many is the divider resistance value?

    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Mastermind 24075 points

    Hi Bod,

    I just checked LM3881 datasheet. FLAGx pin is a open drain pin. The VOL spec is tested at 1.2mA condition. That means the internal  pull down resistance is 0.4V/1.2mA=0.333 kohm.

    Assuming pull-up voltage is 5V and pull-up resistor is 10kohm, the FLAG pin voltage is 5V*0.333/(10+0.333)=0.161V when the pin is pulled low internally.

    Also, the TPS51200 test data shows the actual low threshold of EN pin is around 0.9V, so I think it is fine to connect LM3881 FLAG pin to TPS51200 EN pin directly.

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Mastermind 7230 points

    Dear Qian Chen,

    Thank you very much for reply.
    May I ask a add question?


    My customers are experimenting with direct connection,
    and it has happened that TPS51200 can not drop to Low.


    Q1.
    Your answer reply 『Also, the TPS51200 test data shows the actual low threshold of EN pin is around 0.9V』.

    Where is there description in datasheet?


    Q2.
    Your answer reply 『it is fine to connect LM3881 FLAG pin to TPS51200 EN pin directly.』

    Is don't this direct connection problem ?

    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Mastermind 24075 points

    Hi Bob,

    Do you mean TPS51200 EN pin can not be pulled low by LM3881 FLAG pin?

    What's the pull-up voltage and resistor on LM3881 FLAG pin? Can you show the circuit?

    The test data on EN pin threshold is internal data. It is not available on datasheet.

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Mastermind 7230 points

    Dear Qian Chen,

    Thank you very much for reply.


    I heard that
    My customers are experimenting with direct connection, and it has happened that TPS51200 can not drop to Low by LM3481 FLAG pin.


    I confirm my customer informaiton again.
    I will feedback it.


    Thank you very much for EN pin threshold internal data.

    Best regard.
    Bob Lee.