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LMX2615-SP: Vcc Noise sensitivity

Steve Widener
Genius 11010 points
Part Number: LMX2615-SP

We are seeing a phase discontinuity on the RF output of the LMX2615-SP when there is a transient on the 3.3V Vcc supply.

We'd like to know what typical currents are drawn by each of the VCC supplies (there are 6 inputs but only the total Icc is specified) - is there any guidance for that (rough % of total, or rough estimates are fine). This would help in tracking down the problem & possible mitigations.

There are internal LDOs mentioned that provide some PSRR, are there any other circuits that are not on internal LDOs that might be sensitive to PS disturbances?

  • +1
    TI__Mastermind 33790 points

    Hi Steve,

    LMX2615-SP uses external pull-ups, so this would be my first guess of where the transients are coupling in. This would especially be true for cases where LMX2615-SP output is used single-ended instead of differential, with one leg of the differential pair terminated at the pins and unused.

    The current consumption of the device will change depending on register configuration, VCO frequency, charge pump output, etc. For an arbitrary typical configuration with 2050MHz out and 100MHz in, I find the following:

    • VccDIG: 63mA
    • VccCP: 26mA (charge pump set to 15mA effective gain)
    • VccBUF: 193mA (one output active - increase by about 92mA for two outputs active. Includes current through external pull-ups)
    • VccMASH: 18mA (3rd order, N=61.5)
    • VccVCO + VccVCO2: 130mA (I can split these up if required, pulling raw data from a previous test that grouped them).
    • Total: ~430mA

    Only the external pull-up components are on a supply that sits outside of an internal LDO.

    Regards,

    Derek Payne