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LMX2595: Recommended Initial Power-Up Sequence: 10ms necessary?

Dylan g
Prodigy 170 points

Part Number: LMX2595

I am planning on using the LMX2595 in Full Assist mode, and I noticed in 7.5.1 in the datasheet it recommends waiting 10ms after programming all the registers before setting FCAL_EN high.

How necessary is the waiting step, and do I even need to set FCAL_EN high if I'm going to be using it in Full Assist?

  • 0
    TI__Guru** 120270 points

    Hi Dylan,

    This 10ms applies to initial power up sequence, i.e. it is required after Vcc power up.

    After the initial power up sequence, you don't need this 10ms anymore. If you want to change frequency, program the necessary registers only and then R0 to trigger VCO calibration. If you use full assist mode, then program all the necessary VCO parameter registers, no need to program R0 as VCO calibration is not needed.

  • 0 in reply to Noel Fung
    Prodigy 170 points

    Yes I saw that it is only needed upon VCC power up, I plan to program all the registers upon power up then only do the necessary VCO registers for frequency changes (R0 not needed)

    I guess what I'm asking is if I only am using full assist mode, do I still need to wait 10ms upon power up and set the fcal_en bit high after? Or can I skip this bit and just get right into it once my power supply is stable and the registers are programmed?

  • +1 in reply to Dylan g
    TI__Guru** 120270 points

    Hi Dylan,

    You can go to full assist mode directly after Vcc is stable. 

  • 0 in reply to Noel Fung
    Prodigy 170 points

    Ok nice, so is the 10ms just to get the auto-calibration settled and ready to go then?

  • 0 in reply to Dylan g
    TI__Guru** 120270 points

    Hi Dylan,

    the 10ms is used to ensure the internal LDOs are settled down before we execute a VCO calibration, otherwise the calibration may not return the optimized result due to incorrect LDO voltage.