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LM3880-Q1: start-up sequence of EN and VCC

Part Number: LM3880-Q1
Other Parts Discussed in Thread: LM3880


I have a question about start up sequence.

Should we turn on the EN pin after VCC rise up?  If it so, how much delay should we set on EN pin after VCC rise up?

At our configuration of VCC and EN pin, the EN signal and VCC voltage are start up at same time. Because the EN signal is connected to the switch of VCC line.

So that,VCC rise up at same time when the EN signal turned on. We are considered to add the some RC delay to EN pin.

If we add the some delay, how can we add the delay between VCC and EN? I mean that the delay should be set between VCC and threshold of EN?

Or should we set the voltage of EN pin to zero until VCC rise up to min voltage of VCC?


Koji Hamamoto

  • Hi Koji-San
    Please refer the DS 8.3 Do's and Don'ts. It also list some recommends circuit.

    During power-up, the EN voltage should be kept below the EN threshold until VCC rises above the minimum operation voltage.

    Hope this helps.
    Here are the copy form DS.
    Connecting the EN pin to VCC is not recommended. During power up, the EN voltage should be kept below the
    EN threshold until VCC rises above the minimum operating voltage. This will be violated if EN is connected to
    VCC, and undefined operation at the flag outputs can occur, especially during slow VCC rising slew rates. For
    systems requiring only power-up sequencing, a capacitor at the EN pin can be used to create a delay or a
    resistor divider can be used to enable the LM3880 based on a certain voltage threshold. While these solutions
    will work for power-up, it will not power-down the flag outputs in sequential fashion since the flag outputs will
    simply follow the input supply. For systems requiring both power-up and power-down sequencing, an external
    enable signal should be used, such as a GPIO signal from a microcontroller, to properly control power-up and
    power-down of the flag outputs.
  • Hi Yihe-san

    Thank you. I found the explanation about my question on the datasheet.

    Koji Hamamoto