Back light design issue

Discussion in 'The Projects Forum' started by Jaytronic, Mar 2, 2016.

  1. Jaytronic

    Thread Starter New Member

    Mar 2, 2016
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    Hello Everyone,

    I have a bit of a design issue that is preventing me from using a basic switching white LED driver the way it was intended to be used. I am using the MIC2297 as well it's development board to design a back light circuit which should be pretty simple. The problem is that we first intended to use a constant current source PWM'd to control dimming ( I should mention that this project is for an LCD Touch Screen ) which we designed in but caused the screen to flicker because of voltage droop and had to resort using a huge cap as a band-aid solution. We are now at the point were the band-aid needs to come off and now we are in a pickle.

    The development board works great at regulating using it's own LEDs while being powered using our product but the instant the we disable the on-board LEDs and apply it to our application all regulation stops and nothing works. We traced the problem back and found out that due to a design restriction we were bypassing the feedback pin so no regulation was occurring.

    This project consists of a mother and daughter board, one is the CPU board and the other is the Interface board. The CPU board controls the PWM and can not be respun to fix the issue, but the IO board can. The only access I have to the CPU from the Interface board is back light power (after the schottky ) and ground, please see the provided pdf for clarity.

    This is also my first post, so if i did anything wrong let me know and I will not repeat it. I did take a look to see if there was a solution already available but no one had one.

    Thank you for taking the time to very my post

    Jason
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    Looks doable, if I've understod your request correctly.
    What is the PWM output voltage and frequency?
    What will power your grey box-of-tricks?
    Why aren't you PWM-ing the BRT input of the dev board IC?
    So where is the PWM output?
     
    Last edited: Mar 2, 2016
  3. Jaytronic

    Thread Starter New Member

    Mar 2, 2016
    6
    1
    Thanks for the welcome!

    The grey box-of-tricks is really just a low-side N-FET circuit and is PWM'd by the uC by a 3.3V signal from the CPU board and switches to ground so there would be no output voltage. I am not to sure what frequency it is being switched at not that fast.

    The reason that I am not using the BRT pin is because of previous design restrictions and I am stuck using the low-side driver for dimming, but even if we did use the BRT pin the FB pin is still by passed.

    I really appreciate the help, thank you very much
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    :confused: If you're not using the BRT pin or the FB pin of the IC then I fail to see any purpose in having the dev board in the system.
    How have you 'disabled' the LEDs on the dev board? If the dev board LEDs are removed/unpowered it's my understanding that the IC will try to put out its maximum voltage which, from its spec, seems to be ~40V. So that would be the voltage after the Schottky. That seems excessive for a back-light?
    What is the present supply (input) voltage for the dev board?
    What voltage does the back-light require? Can we assume 20mA (the current that would be taken by the dev board LEDs) through the back-light gives the maximum brightness you require? A link to the datasheet would be helpful.
     
  5. Jaytronic

    Thread Starter New Member

    Mar 2, 2016
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    I got the Dev board because I had problems bread boarding a prototype and also to eliminate any human error. It was suppose to be an easy test, because it's just a switcher but reality likes to sucker punch you:mad: lol :).

    There is a jumper right after the schottky that will disable the on board LEDs ( I attached the datasheet for the IC and dev board). The dev board is supplied by a 5VDC switcher from the Interface board, I have also used a bench supply for testing. The back light requires a minimum of 24V, and consists of 8 white LEDs which drop ~3.2-3.5V each, and will draw 20mA for maximum brightness.

    It was not my intention not to use FB pin, but instead I inadvertently by passed it and I need to figure out a way to get the feed back to its pin.

    It might be a little more clear if you were to look at the dev board datasheet page 4. My CPU board houses the back-light but the Interface board will house the back light driver. This causes a problem because the only interaction the Interface board and the CPU board has regarding the back-light is right after the schottky and ground, which then by-passes the FB pin causing the output to swing to ~40V.

    I thought about moving R3 after the schottky and some how get FB from there but I am not to sure how to go about it.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Hmm, I see your dilemma. Unless you can gain access to the FB pin of the IC, the IC will be running continuously in its self-protect mode with a 40V output (as per P9 of the datasheet). I doubt that's good for its long-term reliability. Whether you have the current-sense resistor R3 low-side or high-side obviously wouldn't make any difference to the voltage on the FB pin.
    Won't trying to PWM the ground line of the dev board cause problems by effectively turning the IC on/off rapidly?
    I take it the dev board is now well and truly integrated into the CPU board? If so, my suggestion would be to disable the dev board entirely and instead incorporate a suitable alternative back-light driver on the Interface board :(.
     
  7. Jaytronic

    Thread Starter New Member

    Mar 2, 2016
    6
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    Yes! it's a bit of a dilemma that I was not expecting :eek: but that is what keeps electronics interesting :), Right!

    Talking to a college of mine he stated that we are going to by pass the switched ground circuit and use the BRT pin so that will no longer be an issue, but it still does not solve the FB issue :(. Grrrrr

    I never intended to leave the dev board in the design it was there just to prove concept and then I was just going to copy the layout, tweak it, and be done with it.

    I might have to find another option, but from what I have seen LED back light drivers seem to follow the same IC topology.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I don't see how that solves the problem. The BRT voltage effectively tells the IC what the demanded output current should be, but without the FB signal the IC will think the demand hasn't been met and will pump out 40V.
    Then can't you just use another MIC2297 on the Interface board?
    It's a pity the dev board jumper JP2 wasn't placed between LED10 and R3 instead of next to the Schottky. That would have allowed disabling the on-board LEDs but still give access to the FB pin :(.
     
  9. CraigC

    New Member

    Jul 23, 2012
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    I'm the colleague that Jaytronic mentioned, I'm just going to quickly pop in and explain some more of what we have done so far.

    As Jaytronic mentioned, we will be bypassing the low-side circuit entirely so that it is a direct connection. Instead we have an unused PWM line that we can connect to the BRT pin. This should prevent the power supply from spiking to 40VDC, which resolves that issue.
    We have also tried creating an instrumentation amplifier circuit that would sense the voltage across the feedback resistor and the output of which would be fed back into the FB pin. The obvious benefit of this solution is that the resistor can be placed anywhere in the string and still give the appropriate feedback. Unfortunately, we were unable to create one that didn't need to be tweaked once it was in production, and buying one was either too expensive, and/or had a gain of more than 1x.
     
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