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LP8556: Issues with all six channels enabled, but only use three?

Part Number: LP8556

Hi guys,

My customer has the following question:

I have a question about open channels on the LP8556 backlight driver.  We are currently using the –E11 version (see page 3)

which enables 3 channels but has a max boost output voltage set to 21V.  We use the preconfigured version as-is and do not have a software

driver enabled to be able to change the register settings.

 

We need to raise the max boost voltage and I tried the -E02 version on a board – it worked and solved the issue I was having.  But this configuration

enables all 6 channels on the device.  We only use three, our backlight is 7S3P – three strings of 7 LEDs in series – so we only have three channels

hooked up, the other three are open. 

 

The part is obviously able to figure this out and still function properly since it is working on the board I modified, but are there any reliability or other

issues that I have to worry about if we switched to using the -E02 version in this configuration?  Does having three enabled open channels have any

negative consequences?

Thanks,

Brian 

  • One more thing, can the three channels that are active be tied together? One of the LCD panels we are looking at ties the cathode
    connections of the three strings together, so the backlight drivers are shorted together. Again, I see that it does work, but does it
    hurt the driver chip to do this? Or affect its performance? Seems like the PWM phase shifting feature would need to have these
    connections separate for each driver, is that right?

    Thanks,
    Brian
  • Hi Brian,
    there is a problem about your situation that the output voltage will trigger OVP(30V) and the efficiency is low. So I suggest you to use 6 channels , you can tie two channels together to driver one led string, in this way you have used all six channels.
    You'd better not tied channels together, because it will have influence on the current match.
    Best Regards
    Sean
  • Sean,

    In this situation, using all six channels would require a board re-spin, which is not under consideration at this point. So the question
    remains about leaving the three extra enabled channels open. It doesn’t appear to be triggering an OVP condition of 30V, the output
    boost voltage seems to properly adapt to the voltage the string requires. Also, I don’t understand the comment about low efficiency, can
    you explain more? But basically, I have it hooked up as I described and it seems to work, so it doesn’t seem like I am experiencing the
    conditions described. Is there any further guidance you can offer about leaving the unused channels open ?

    For the second part, when you refer to tying channels and a current mismatch, the mismatch would be due to the parallel LED strings all
    being driven the same with no way to set the current in each string, correct? I maybe could live with that, but I wanted to know what effect
    that might have on the driver chip.

    Thanks,
    Brian
  • Hi Brian,
    For your application and connection, if you leave that three unused channels open, when the device detact this , it should force the output to the maximum setting voltage. In this situation , because the output volatge is larger than what you need, so the efficiecny is low. But you have not find that the output volatge triger OVP, so this is a little strange. Could you check your circuit again?
    For current match, your understand is right, if you don't want to use that three channel, just leave it open. Thanks.
    Best Regards
    Sean
  • Sean,

    Please check your inbox, I have responded but I included a schematic so I wanted to keep it internal.

    Thanks,

    Brian