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LM2664: How much time does LM2664 need for stable output

Part Number: LM2664
Other Parts Discussed in Thread: LM2663

LM2664 seems to need about 3.48ms until output voltage becomes stable as a picture I attached. Why these time is required for LM2664? Also this waveform is correct?

Ch1: Input, Ch2: output, Ch3: SD (enable)

  • Hi User,

    Thank you for using the LM2663.   What is your downstream circuit connected at the LM2663 output rail?    By the way, the SD pin should be at 2V min to guarantee the operation of the IC, but your scope picture looks like ~1V.  Could you raise SD pin voltage to 2V?

    Thanks,

    Youhao Xi, Applications Engineering

  • Hi 

    Thank you for your comments.

    LM2664 output is connected with LDO.

    And I change SD voltage and I attached the waveform. There is unstable output still. What is this cause?

  • I am afraid it is related to the turn-on of your LDO and there could be some over loading at the charge pump output when your LDO is charging its output.   Can you test by replacing the LDO with an equivalent resistive load first to see if the Vout can "rise" smoothly?

    Thanks,

    Youhao

  • Hi Youhao-san

    I removed 3x22uF which is placed on LDO output, then LM2664 output becomes stable. So root-cause is inrush current... But I can not remove these caps with some reasons. 

    Is it no problem for LM2664 except for that unstable output when power-up timing.

    B.R, Yamada

  • Hi Yamada-san,

    Thank you for the updates.  I don;t think it is a problem for the LM2664.   If you want to make the LM2664 Vout look smoother for whatever reason, you may consider to insert a current limit resistor between the LM2664 output and the LDO input.  But I don't think it is needed. 

    Thanks,

    Youhao

  • Hi Yamada-san,

    May I assume the issue is resolved?  I am closing the thread  here but you can re-open it by adding a new post. 

    Thanks,

    Youhao