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LMX2594: Unlock

arun verma
Prodigy 30 points
Part Number: LMX2594
Other Parts Discussed in Thread: LM2594

We have designed our board with LM2594 but what we have noticed that it unlocks after some time and if we send 0x2514 then it locks again. It seems it VCO is drifting slightly may be because of heat or some other factor and VTune voltage goes out of band. we are using single regulator 3.3V for all supply. Any suggestion ?

  • 0
    TI__Mastermind 27581 points

    Arun,

    Are you programming with FCAL_EN=1, I think this would be 0x251C?

    When you program the device and calibrate the VCO, the tuning voltage should be about 1.5 volts or so.  But if it drifts too close to grould or the charge pump rail (about 2.5V), then this could be the issue.  You might want to check this charge pump voltage when you are out of lock and ensure it has not drifted too close to one of these extremes.

    When you calibrate the VCO by programming register R0 with FCAL_EN=1, it should center the tuning voltage and this should be able to handle temperature drift of 125C.  IN other words, you could program at -40 C and the temperature could drift to 85C.  Now if you push it much higher like 125C, then at some time you will have to re-calibrate.

    In other words, this device should be able to handle this temperature drift unless you are really overstressing it or you are not calibrating the VCO when you first lock.

    Another consideration is that if you lock the VCO at frequency F1 and calibrate, but then change the frequency to something close like  F1+30 MHz, likely the VCO will stay in lock.  But then you might find that the temperature drift is much less.

    Regards,
    Dean

    Regards,

    Dean