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LM3488Q-Q1: How to shutdown LM3488-Q1 with less external components

NOBUO FUJIHARA
Genius 9395 points
Part Number: LM3488Q-Q1
Other Parts Discussed in Thread: LM3488

Hello Champs,

 

My customer wants to add shutdown circuitry in case MCU detects over boost failure.

At initial state, MCU is not powered, I’m recommending attached 2 circuitry.

But due to wide +B range and upper limit of SD port, external components increases.

 

Q.1) Is there any concern to apply attached shutdown circuitry?

Q.2) Pulling down Vin and getting it into UVLO status seems more simple way with single NMOS + resister.

Is there any concern with this method?

 

Best regards,

Nobuo

LM3841_shutdown.xlsx

  • 0
    TI__Intellectual 2655 points
    Hi Nobuo
    There are some things I am concerned.
    1 What is the LM3488 Vin? Is it +B or not?
    2 I know you want to shutdown LM3488 by MCU. But why do you use the 1k resistor? Do you want to control SD pin by +B together?
    3 If you just want to control by MCU, why don’t you use MCU I/O to control SD?
    You could explain the application in detail, which will be helpful for me give you some advice.
    Another thing, it is not good to connect +B to MOS gate, because the voltage is too high, you could add a Zener.
    Any question please tell me.
  • 0 in reply to Jing Ji
    TI__Genius 9395 points

    Hi Ji

    Thanks for your comment, I updated schematics.

    - Removed supply from +B divider

    - Added zennar to M1 gate

    - Withdrow Q2 in previous question

    - Added simpler shutdown idea, case 4

    Can you check case 3 and especially case 4 does work?

    Regard,

    LM3841_shutdown2.xlsx

  • 0 in reply to NOBUO FUJIHARA
    TI__Intellectual 2655 points

    Hi NOBUO

      The CASE3 is good way.

      For CASE 4, because the GPIO source capability is limited, it will be slowly when MCU shutdown, which will cause frequency shift.

      Any question please tell me.

  • 0 in reply to Jing Ji
    TI__Genius 9395 points
    Hi Ji,
    Thanks for your answer, and let me ask further related question about FA/SYNC/SD port.

    (1) Competitor device has EN port, so Case 4 is preferable with less external components.
    So let me confrim that, it may cause frequency shift during shtudown, but it will surely make device shutdown state.

    (2) Customer concerns about leakage path from FA/SYNC/SD port to Vin port when Vin is smaller than the voltage applied to FA/SYNC/SD during shutdown.
    Could you confirm there is no internal diode connection from FA/SYNC/SD to Vin ?

    Regards,
  • 0 in reply to NOBUO FUJIHARA
    TI__Intellectual 2655 points

    Hi NOBUO

    (1) case4 will be surely shutdown, but I DON'T recommend to do it. because diode's reverse current will influence FA pin current to cause frequency shift during working.

    for example this is 1SS367 diode datasheet. we can see the reverse current change a lot by temperature.

    I think case 3 is a good way.