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LMX2582: PLLatinum Simulator does not have the correct charge pump currents

David Hawkins
Prodigy 70 points
Part Number: LMX2582
Other Parts Discussed in Thread: ADC12DJ3200EVM, , LMX2592

I am currently using the ADC12DJ3200EVM which uses the LMX2582 to generate the ADC clock. I want to use PLLatinum Sim to estimate the ADC clock jitter, however, the charge current settings in PLLatinum do not match those in the ADC GUI or in the data sheet. The LMX2582 has a 5-bit charge pump current, with each bit having a step size of about (5/32)mA = 0.15625mA, with a full-scale of 31x(5/32)mA = 4.844mA. There are charge pump multiplier values of 1.0, 1.5, 2.0 and 2.5, so the maximum charge pump current could be 2.5x31x(5/32)mA = 12.109mA. PLLatinum Sim has settings that go up to 24.219mA. While I realize I can just select a charge pump current that the device actually supports, my confidence in the jitter estimate is reduced, due to the error in the charge-pump current menu.

The LMX2582 and LMX2592 data sheets have a strange definition of the charge-pump current bits, as it appears the 2 MSBs of the charge-pump current register have been swapped. This could be an error in the data sheets, or it could be an error in the chip, and so the data sheet reflects reality. Could someone at TI please review the CP_IUP/IDN bit to current mappings and confirm that the data sheet is either correct or in error. I do not currently have access to a spectrum analyzer or phase noise analyzer, otherwise I would just look at the loop bandwidth.

Thanks!

Regards,
Dave Hawkins.

  • 0
    Prodigy 70 points

    Hi All,

    I was comparing the ADC GUI vs TICS Pro vs PLLatinum Sim, and I think I see the discrepancy. TICS Pro shows "Effective Charge Pump Gain" on the GUI equal to 2 x CP_IUP/CP_IDN x CP_ICOARSE, or since CP_IUP=CP_IDN, this could also be (CP_IUP+CP_IDN)xCP_ICOARSE.This extra factor of 2 means the maximum current is 2x2.5x31x(5/32)mA = 24.219mA, and that value is consistent with the PLLatinum Sim pull-down menu.

    I also see that TICS Pro has the MSB bit mappings as shown in the data sheet. It still looks weird that the bit weightings from 0 to 4, are 0.156mA, 0.312mA, 0.625mA, 2.5mA, 1.25mA, i.e., the top two weights seem backwards.

    Regards,
    Dave

  • +1
    TI__Guru* 81380 points

    Hi Dave,

    LMX2282/92 use dual charge pump, the total charge pump current is equal to IUP+IDN. Their max. setting is 4.844mA, so the total is 9.6875mA. If you enable the multiplier, then the max. is 2.5 x 9.6875 = 24.21875mA.

    Lock detect accuracy degrades when multiplier is enabled, we recommend not use the multiplier whenever possible.

  • 0 in reply to Noel Fung
    Prodigy 70 points

    Thanks Noel! I appreciate the help! Regards, Dave.