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LM3881: How to turn on devices sequentially, but disable them all at once

user4725373
Expert 1645 points
Part Number: LM3881
Other Parts Discussed in Thread: UCD9090A, LM3880

Hi,

I'd like to use the LM3881 to power 3 devices (U2, U3, U4) sequentially when a certain device (U1) is powered. However, once U1 loses its power, I'd like the other ICs to be shut down at once instead of sequentially. I thought about the following circuit:

That way, the pull-ups would be pulled to zero once U1 loses its power. Is this the way to do it, or is there a better way. Alternatively, is there another chip than the LM3881 that only does the power-up sequencing and powers down all devices at once?

Thank you,

Fred

  • 0
    TI__Expert 5965 points
    Hi Fred, if you have external control of EN, then you can pull FLAG1, FLAG2, FLAG3 to EN then when EN goes low FLAG1/2/3 will go low at the same time. If you do't have control (only pull EN to Vcc) then it will be difficult to power down at the same time.

    They are other digital sequencers (UCD9090A as example) can do this, but with bigger size.

    Regards,
    Weidong
  • 0 in reply to Weidong Zhu
    TI__Guru 58020 points

    We do not  recommend to connect the EN directly to VIN. Please refer the recommended method listed in the data sheet .

    You can AND the flag output with the POWER_GOOD signal of the VIN so all flags can be off instantly

    Regards

    Yihe

  • 0 in reply to yihe
    Expert 1645 points

    Hi Yihe,

    Sorry for the delay, I'm just back from vacation.

    So I'm testing the circuit, and I have a problem when the sequence is interrupted quickly. When EN goes high, the sequence starts normally (flag1, flag2, flag3). If EN goes low before the sequence has finished, then the reverse sequence starts. However, is EN goes high again before the reverse sequence has finished, then the normal sequence just starts from the point where it was before.

    I think it's better to remove the power from the IC to completely cancel the sequence, don't you think? Would it be acceptable if I added a delay capacitor on EN?

    Thanks,

    Fred

  • 0 in reply to yihe
    Expert 1645 points
    Hi Yihe,

    I just tested the original circuit that I sent (EN connected directly to VIN) and it looks like it gives me exactly what I need. Why do you not recommend connecting EN to VIN?
  • 0 in reply to user4725373
    TI__Guru 58020 points
    EN must stay low before LM3880 is power up. having EN directly connected with VIN will break this requirement and in the LM3880 data sheet, we do not recommend this.
    you can also refer below post
    e2e.ti.com/.../700103
    Regards
    Yihe
  • 0 in reply to yihe
    Expert 1645 points

    What do you think of this circuit, then? I added a low pass filter between VIN and EN.

  • 0 in reply to user4725373
    TI__Guru 58020 points
    If you want to connect the EN to V_U1, we would recommend a voltage divider as shown Figure 15 of
    www.ti.com/.../lm3881.pdf
    Regards
    Yihe