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LMX2595: Unused pins, open or pull down

Taketo Sato
Genius 10585 points

Part Number: LMX2595

Hello team,

I saw in other e2e thread that recommended connection of RampDIR, RampCLK and SYNC is pull down to GND when we don't use them.

1) That being said, can RampDIR and RampCLK be open when RAMP_EN=0 because ramping function is disabled?

2) When we use Sync mode with VCO_PHASE_SYNC bit without SYNC pin and when we use also SysRefReq pin, I think INPIN_IGNORE bit has to be 0, correct?

I understand pull down the SYNC pin is recommended in above case, however could you please let me know what unexpected behavior can be happened if we left open the SYNC pin in above case?

Best regards,

  • 0
    TI__Guru** 122400 points

    Hi Sato-san,

    1) Keeping those pins open or GND is acceptable.

    2) in SYNC mode, INPIN_IGNORE must be 0 otherwise you cannot do sync with the SYNC pin. Similarly, the same requirement applies in SYSREF mode where the SYSREFREQ pin cannot be ignored.

    If you are not doing SYNC, set VCO_PHASE_SYNC=0, the SYNC pin can be left open.

  • 0 in reply to Noel Fung
    TI__Genius 10585 points

    Hello Noel-san,

    I understand 1), open or GND is acceptable.

    Regarding 2), They do Sync by VCO_PHASE_SYNC bit toggling, without using SYNC pin toggling, however I understand we need INPIN_IGNORE must be 0 because we use SYSREFREQ pin input. We cannot leave the SYNC pin open in the situation? if so, could you please let me know why?

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru** 122400 points

    Hi Sato-san,

    Put it this way, if we can ensure the SYNC pin is always quiet, free from crosstalk by other signal traces, then in any case, we can leave SYNC pin floating. otherwise, to play safe, we recommend keep this pin GND if it is unused.