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LMR62014: Feed forward capacitor value

Harmeet Singh
Intellectual 1460 points
Part Number: LMR62014

Hi Team,

One of my customers is using LMR62014 to make 16V  from 5V in energy meter application.The load is around 100mA for 20-50msec.

He is presently using 220pf as feedforward capacitor but is failing in radiated emission test .

If the value of Cff is increased from 220pf to 440pf the radiated emission result passes.

As the PCB is freezed The layout can not be changed.

There are two queries:

1. How feedforward capacitor is resulting in less Radiated emissions?

2. Is it ok to use 440pF in the design?What implications it can have?

Regards,

Harmeet

  • 0
    TI__Mastermind 34231 points
    Hi Harmeet,

    Can you share the schematic and layout? The recommended frequency for the zero fz should be approximately 6 kHz. The Cff cap is related to the upper resistor. If the calculating zero now is around 6kHz when Cff=440pF, then it's find to choose it.
    Do you have the load transient waveform? The reason 440opF capacitor helps to pass radiated emission test may related to the voltage ringing during load transient. Too big voltage drop and ringing is bad for radiated emission.
  • 0 in reply to Zack Liu
    TI__Intellectual 1460 points

    Hi Zack,

    I have shared the schematic and layout on your email .

    Let us further discuss on email.

    Regards,

    Harmeet