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LMX25312820EVAL: EVAl board schematics - Single resistor instead of R2,R3,R4,R5

Sachin NP
Prodigy 150 points
Part Number: LMX25312820EVAL


Hi,

In the Eval board schematics, If VccVCO, VccBUF, VccPLL, VccDIG are of same voltage, can i connect all these via single resistor and capacitor instead of multiple resistors & capacitors of same value?

  • 0
    TI__Genius 16845 points
    Yes, it is fine to use the same input filter (just R2 and C2) for all of the device rails if you would like to power from a single supply. I recommend to keep the local bypass capacitors at the device pins (C9. C12, C17, C18).

    Kind regards,
    Lane
  • 0 in reply to Lane Boyd
    Prodigy 150 points

    Hi Lane,

    We have below doubts:

    1. If we are using single supply, what shall be the values of R2 & C2? Will there be any change in specifications or working if we go with the single supply configuration?

    2. In eval board BOM, C2,C3,C4,C5 are made open. If so how does the resistor R2,R3,R4,R5 helps in filtering?

    3. Also bypass capacitors C9,C17,C18 are also made open.

  • 0 in reply to Sachin NP
    TI__Mastermind 28021 points
    Sachin,

    1. This board is designed for a single supply already. For the build, the bill of material is what we build to. From this we see that: R1 = 3.3 ohm, C1 = 10 uF. IN general, C1 should be a large capacitor and R1 should be so that there is some filtering, but not a huge voltage drop.
    2. They don't. However, by having them, you could try and do experiments with each power supply.
    3. The bypass capacacitors are not there because they did not make a huge difference.

    In general, this device has integrated LDOs and this does a good job of power supply filtering, but you could. For evaluation purposes, if we add a bunch of filtering that really doesn't do anything, then it deemphasizes the fact that we have a VCO. However, we want to keep the board flexible for experiments.

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 150 points
    Hi Dean,
    I understand the circuit is made for evaluation board but in our design, we have space constraints & want to remove unwanted components.
    In our design, we will use R1 & C1, remove R2,R3,R4,R5,C2,C3,C4,C5. We will place C9,C12,C17,C18 close to respective pads. Is this OK?
  • 0 in reply to Sachin NP
    TI__Mastermind 28021 points

    Sachin,

    Yes, this is a good approach.

    For testing purposes with our EVM, put 0 ohm resistors for the ones that you plan to leave out in your design.


    Regards,l
    Dean