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WEBENCH® Tools/LP2985: OCXO-Deterioration of phase noise

chen gavin
Prodigy 70 points
Part Number: LP2985

Tool/software: WEBENCH® Design Tools

Dear All,

The LP2985 has been used for more than 10 years. Recently, it has been found that some LDOs will cause the phase of the OCXO to deteriorate 10dbm.

Figure：OCXO-PHASE NOISE（FIRST-NG, SECOND-OK）

In swap test, the problems follow the chip.

Test conditions：
Vin = 5V，Power by adapter or battery.
When the LP2985 is working, the output current is about 100 mA in the first 3 minutes. At this time, the ambient temperature will rise from 30°c to about 80°c. After that, the output current is about 30 mA, and the ambient temperature is also 80°c.

Vout：

  • 0
    TI__Guru 55940 points
    Hi Chen,

    Thank you for providing the noise plots! Unfortunately since LDO noise is defined as the noise generated by the LDO itself, the only thing we can do in an application is filter the noise.

    The primary source of noise is the internal voltage reference. LP2985 has a Bypass pin that allows us to add a capacitor to filter this noise. Per the datasheet the effect of increasing the capacitor saturates around 10 nF.

    The only other place that we can filter the noise generated by the LDO is on the output rail. Increasing the output capacitors or adding other additional filtering on the output will further filter the output noise.

    The following article goes into further detail on how to reduce LDO noise:

    www.ti.com/.../slyt489.pdf

    Very Respectfully,
    Ryan