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LMX2485E: Power Consumption For the RF PLL is higher than expected.

Part Number: LMX2485E
Other Parts Discussed in Thread: LMX2485

To Whom it May Concern,

I am using the RF PLL only and operating at an RF frequency of 1.6GHz and a reference frequency of 19.2MHz. The device is setup in 22 bit fractional mode. The measured current with a 3V supply is 8mA. I expected it to be closer to 3.3mA based on the data sheet. I turned off the charge pump in order to match the data sheet, but the current still remained around 8mA. The IF pll current measured near 0 since it is powered down. Is this current what you would expect? I used the same PLL circuit with a different vco and an RF output frequency of approximately 300 MHz and the current was 3.5mA. Any suggestions for lowering the output current? I did disable the optional test outputs and clock output.

Thanks,

Shawn

  • I meant to write : Any suggestions for lowering the supply current?
  • Shawn,

    As for setup

    1.  Ensure IF PLL is powered down

    2.  Ensure that power is still supplied to all Pins of IF PLL

    As for the current, realize that this is taken in a very favorable condition for current consumption with a low phase detector frequency.   Also the high frequency input pin had no input and was held at a DC bias level to prevent oscillation.  This was the way this current was measured in the past by both us AND our competitors.  As a result, you might see current about 2 mA higher when you hook up your 1600 MHz input.  Even accounting for this, your 8 mA seems high compared to the 3.3 in the datasheet.  ENsure that you have a low phase detector frequency as this might explain the other part of the difference.

    Since the LMX2485 family, we have changed our method of current measurement to have the device running and PLL locked.

    Regards,

    Dean

  • Hi Dean,

    Thanks for your quick response! Power was applied to the 2 VddIF pins; the measured current to them was 5.9uA. The FinIF an CPoutIF pins are open (with do not populate components to ground). Looking at my data it looks like the current was actually 7mA instead of the 8mA I stated previously. If set the reference divider to divide by 2, for a 9.6MHz comparison frequency the current drops to 5.3mA.

    Thanks,

    Shawn