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Ferrite/resistor combo on LM3434 eval boards

Michael Doyle
Intellectual 710 points

Hi all

I'm reviewing a colleague's LED driver design. He's basically copying the LM3434 eval module design. I'm trying to understand how part of the circuit is supposed to work.  Basically, a bunch of ferrites and resistors are placed in parallel on the drains of the low-side MOSFETs. 

What is the purpose of this arrangement? It seems to me that the resistors short past the ferrites, limiting whatever high-frequency filtering the ferrites offer.

Can anyone enlighten me, please?

Cheers!

MADman

  • 0
    TI__Mastermind 25500 points
    I'm not sure how the values were calculated exactly, but the person who added these to the circuit explained what they are there for. Basically with some FETs at higher currents the switch node can have large spikes both positive and negative. That could lead to damage of the HS pin. There is a footnote describing HS to VEE transients from the abs max ratings (I think it's 6V below VEE for xxns and 22V above GND for xxns). The ferrite beads reduce the spikes and the resistors act as dampers. I'm not sure if the ringing is present with the FETs on the EVM or not, but they were added so that any FET could be evaluated without damaging the IC. In any end application you can check the HS pin without them and if it does not exceed abs max they are not needed. However, as with any buck regulator, they also help with EMI to some degree.