LMX2594: Settling time speedup options for Fvco=12G/Fout=3G

Jay Kubicky

Part Number: LMX2594

We need to generate a 3G output from the LMX2594 which appears to be possible only with Fvco=12G. In the datasheet there is a note that says "VCO assistance cannot be used" for Fvco from 11.9 to 12.1G. Does this mean none of the three assistance modes are useable and only the no-assist mode (with a 50us typ VCOCAL time) can be used for Fvco=12G?

over 4 years ago

0 Noel Fung over 4 years ago

TI__Guru** 108110 points

Hi Jay,

At this frequency range, the calibration must be started with VCO_SEL = 4, VCO_DACISET_STRT = 300, and VCO_CAPCTRL_STRT = 1. As a result, there is no difference between auto-calibration and partial assist. Close frequency assist may not be possible as the last calibrated parameters may be different from the requirement. Full assist is possible as we re-apply the pre-calibrated parameters to the PLL, no calibration will be performed.

0 Jay Kubicky over 4 years ago

Prodigy 50 points

Noel,

Just to be clear... we can do full assist, but not partial or close frequency assist for Fvco=12GHz - is that correct?

I've implemented this based on the guidance in SNAA336 and it seems to work fine, even for Fout=3GHz/Fvco=12G. One question though - in SNAA336 section 3.3 they say registers R36,20,19 and 16 should be re-programmed to change frequencies. Should I also re-program the PFD_DYL_SEL in R37 if that changes? All of my frequencies are integer mode, but even with MASH_ORDER 0 the PFD_DLY_SEL is supposed to vary with Fvco (per table 2 of the datasheet).

Thanks,

Jay

0 Noel Fung over 4 years ago in reply to Jay Kubicky

TI__Guru** 108110 points

Hi Jay,

Correct, full assist and close frequency assist should not be used at this particular frequency range.

Yes, program all the registers you need to change as well, the example in the application note is simplified to make it easy to read and demo.

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LMX2594: Settling time speedup options for Fvco=12G/Fout=3G