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LP3986 RF 900MHz immunity ?

Wojciech Noworyta
Prodigy 10 points
Other Parts Discussed in Thread: LP3986

My GSM modem 900MHz causes LP3986 voltage drops more than 0.3V on output.

What pin of LP3986 is the most susceptible to 900MHz (input / output)

Is LP3986 more immyne to GSM than average LDO (datasheet sugest it for GSM)?

Will LP3986 tolerate ferrite SMD on input/output ?

What is the best strategy for BYPASS pin

- left unconnected

- 10nF to GND

- 33pF to GND

What can improve RF immunity LP3986 except 33pF to GND put at IN and OUT?

 

  • 0
    TI__Mastermind 22610 points

    In general the LP3986 is no more sensitive than most regulators. A 900MHz issue is likely to be beyond the usable bandwidth of the control loop, so that the external passive components, and possibly physical layout, will have most influence over the behavior.

    The BYPASS pin is Hi-Z and goes into the band-gap circuit. I never recommend leaving the BYPASS pin open. Use ceramic MLCC capacitor connected directly back to the device common GND pins.

    Ferrite beads are OK but must be OUTSIDE the Cin and Cout connections to the device Vin and Vout pins.

    Cin and Cout must be ceramic, or predominantly ceramic, X5R or X7R. Cout may need to be an array of ceramics, with each having different a different (staggered) self-resonance frequency.