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LMX2486 question

Steven Tin
Prodigy 10 points
Other Parts Discussed in Thread: LMX2486

Hi, there

I am only using the RF PLL part  of LMX2486, so what I should do with VddIF1 and VddIF2? Should I ground them, connect them to Vdd, or leave them open? I

I am currently leaving them open. However, with ENOSC high and 10MHz input at OSCin, I am getting a 1.2V DC output at OSCout. Any idea what might be the reason?

Thanks

Steven

  • 0
    TI__Mastermind 28001 points

    Steven,

    You should  NOT ground or float unused pins supply pins on this device;  they should be connected to supply like the others, although there is no need for fancy bypassing.

    The IF PLL should be powered down in software and it will draw no current when done so.  For the CPoutIF and FinIF, you can float these since the IF PLL is powered down anyways.

    We call the pin "VddIF1" because the primary block it supplies is the IF PLL.  But there are other blocks, such as the microwire that could be powered by this.

    Also, in the recomended operating conditions we say that all the suppy voltages should be equal.  The reason for this is that there is an ESD protection ring that links them together.  If they are different by more than a diode drop, then these diodes will be forward biased.

    If you float the unused Vdd pins, the ESD protection ring will pull up the voltage on these pins to some value that is below spec and the part will limp along with intermittent problems.   This voltage drop may drift over temperature and cause undesired effects.   If you ground the Vdd pins, then you will forward bias the ESD diodes and draw a lot of current.