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UCC256403: UCC256403 work unusual in a constant current LED driver

Part Number: UCC256403

Hi:

I use UCC256403  in a constant current LED driver.

When dim to light load,it looks like entered brust mode ,but output current has large ripple( Electronic load CV mode).

I tested IFB,source from 140uA to 215uA,so i think it not reallay entered brust mode,it just turned off because ifb>165uA

I don't know how can make it work in  brust mode.

Thanks!

  • Hi 5321944,

    For UCC256403, If you see the FB pin current periodically move between 140uA and 215uA, you are likely in burst mode. When in burst mode, you will see larger output ripple. 

    Could you share a copy of your current schematic and a waveform for VCR when you see burst mode? I will see if there are some ways to minimize the output ripple at light load.

    Best Regards,

    Ben Lough

  • Hi,Ben Lough:

    Thanks for you reply.This in my schematic. I'm not in the lab today, maybe I' ll give you VCR waveform next time.

    I set BMTH=1V,and BMTL=0.6V. If ifb=140uA,VFBReplica=2V. If  VFBReplica>BMTH,UCC256403 will exit burst mode.

    That is say ic enter and exit  burst mode in one cycle ,it is unusual.

    I think UCC256403 will always stay in  burst mode if load is not changed, VFBReplica is always below BMTH.

    Thanks!

  • Hi 5321944,

    Thanks for the schematic. It looks like the IC has all the right connections. If you are able to share a copy of the schematic with component values listed, I can review in greater detail.

    Regarding the burst mode, I would suggest using a smaller burst threshold on the LL/SS pin. For BMTH, the value can be as low as 0.2V. A smaller burst threshold should reduce the output ripple.

    Best Regards,

    Ben Lough