LTC3891 PLLIN

Sorry, repeat question that I asked in the wrong forum.

Has anyone used the PLLIN/MODE for external synchronization?

I've tried all the options shown on the evaluation board schematic and had no luck.

Other circuits that I've designed recently using the LTC3895 and LTC3858-1, have both worked as described, locking on from a wide range of frequencies.

I just can't get this feature to work in the LTC3891.