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LMX2595: Loop Filter Component Placement

David Hildebrand
Expert 4860 points

Part Number: LMX2595

I have a customer who is looking at the LMX2595.  Can you provide guidance as to where the External Loop Filter components should be placed?

Should they be placed near the Charge Pump Output (pin 12) or near the VCO input (pin 35)?

  • 0
    TI__Mastermind 28031 points
    David,
    Our EVM shows a good layout example.

    As the charge pump is on one side of the chip and the Vtune input is on the other, most likely you will end up going through a Via. The VCO likes to see a capacitance close to it, ideally 3.3 nF or higher, but you can compromise with lower values to sacrifice VCO phase noise. Every loop filter will have a "highest order cap", which is the one that directly connects the Vtune pin to ground. This component should be closest to the Vtune pin. The rest of the loop filter, we typically put on the other side of the chip. On our EVM, we put C4_LF on the Vtune pin and like to have this populated not only for the VCO, but also it can filter any spur noise that gets picked up going through the Via. Note that our EVM is for the 4th order loop filter, but you can make it lower by just setting R3_LF=R4_LF=0 and putting C1 capacitor in the C4_LF spot.

    Regards,
    Dean