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LM3880: Intermittent very long delay on EN3

Other Parts Discussed in Thread: LM3880

Hi,

I have a design where LM3880 is used to sequence a switching power supply. On a small percentage of units occasionally (about 5-10% of power-ups) EN1 and EN2 work as expected, but EN3 is asserted much later than expected. The delay is in the order of 10 seconds, up to 60 sec or so. Power is 5V, the EN pin is connected to a divider (10K to power, 5K to ground), pull-ups on ENx are 10K.

Has anyone seen anything like that?

  • Hello,

    I am not sure I understand. The LM3880 has only one EN pin per device and three FLAG pins. Are you referring to the FLAG pin that is not being asserted? Also note, when using the EN pin connected to the VCC pin with a voltage divider, this configuration is only for Power-up sequencing. For power up and power down sequencing, you need a separate external EN signal.

    Please provide the configuration you are using so we can better assess the issue. Thanks!

  • Michael,

    thank you for your response. Sorry, what I meant by EN1,2,3 were actually FLAG1,2,3.

    The problem seems to be fixed by placing a 0.1uF decoupling cap close to the LM3880. I haven't confirmed it yet, but I suspect a glitch on the input supply after FLAG2 assertion.