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LM3881: Controlling rail sequencing when the primary supply is removed

vic zap
Intellectual 740 points
Part Number: LM3881
Other Parts Discussed in Thread: LM3880

Hi!

As I've understood, the power supply sequencers control the power-up/power-down sequence by means of some sort of main EN signal. Rails are sequenced in needed order when the EN signal is activated and in reverse order when the EN signal is deactivated. But what if the primary supply is just removed? How can the shutdown sequence be controlled in such circumstances?

Best regards, Vic.

  • 0
    TI__Expert 5965 points
    Hi Vic,

    Here is the answer. You can refer to the sequencer FAQ below.

    e2e.ti.com/.../665464


    5. If EN signal stays assertion, what’re the behaviors of FLAG outputs when VCC
    ramp down below its minimum operating voltage?
    A: The behaviors of the device are unpredictable if EN is asserted before VCC ramp
    down its minimum operating voltage. .

    Regards,
    Weidong
  • 0
    TI__Guru 58060 points

    From design point view, LM3881 shall be the first one on and the last off to ensure the proper power up/down sequencing.

    The power down sequencing is not completed  and the result is not controlled if LM3881 loses its power first. you may need change the design to make sure LM3881 is the last one off, for example add some hold up capacitor 

    Regards

    Yihe

  • 0 in reply to yihe
    Intellectual 740 points

    Hi.

    So, the common approach (maybe the only) to handle abrupt primary supply removing is to have some hold up supply which is used to shut down all the circuitry in the right way, isn't it? Maybe there are other practices which are widely used? Thanks in advance.

    Best regards, Vic.

  • 0 in reply to vic zap
    TI__Guru 58060 points

    Hi

    the LM3880 must have power before it completely shutdown the output flags.

    hold-up capacitor/supply is the most common way unless you want to add a battery.

    What's the issue to having a capacitor? 

    Regards

    Yihe

  • 0 in reply to yihe
    Intellectual 740 points

    Hi, Yihe!

    It seems I need a eFuse like TPS2594x on the input followed by hold-up capacitance. The Fault output of the TPS2594x will initiate the power-off sequence upon the loss of input power. I'll consider this concept.

    Thank you for your replies.

    Best regards, Vic.