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LMX2572: Possible to swap with LMX2595

Dennis Brown
Prodigy 220 points
Part Number: LMX2572
Other Parts Discussed in Thread: LMX2595, , LMX2594

Hello,

Not sure this is the right forum for this question.  

We are currently using the LMX2572 in a design. Due to limited part availability, would the LMX2595 be a direct drop in for the LMX2572? The charge pump LPF would need to be changed so I am not suggesting that it is a direct drop in with zero prep work.

The pkg outline shows this to be the case:


I am concerned that the electrical aspect will not be correct since they probably have different VCOs (hence different tune constants). We currently are using the LMX2572 with a 4 component Vtune LPF ( I believe this is a 3rd order). The datasheet for the LMX2595 suggest a 7 component Vtune LPF (I belive this is a 5th order, maybe a little higher).  What is the downside of using a lower order charge pump filter, mostly spurious products  suppression?

Also the output RF signals are biased a bit differently.  Is this a mandatory requirement?

thanks

dennis brown

  • +1
    TI__Guru* 97840 points

    Hi Dennis,

    Package and pinout of these devices are compatible. 

    Please note, LMX2572 consumes much lower power supply current than LMX2595, if your power supply circuitry is design for LMX2572, it may not be suitable for LMX2595.

    In the application diagram, LMX2572 uses a 2nd order loop filter; LMX2595 uses a 3rd order loop filter. Because their VCO gain and max. charge pump current are different, 3rd order filter may limit the possible loop bandwidth in LMX2572. Of course, a 3rd order filter tends to have more suppression to spurs. You may use PLL Sim (www.ti.com/.../PLLATINUMSIM-SW) to design the loop filter and simulate loop performance. 

    LMX2572 RF output driver has internal pull-up, no external pull-up is required. With LMX2594, external pull-up is needed otherwise there will be no RF output.

  • 0 in reply to Noel Fung
    Prodigy 220 points

    Thank you for the reply Noel.  This is helpful.  We are trying to determine which direction to take with these devices.