LED7708 Random LED switch off issue with driver

Question

Our requirement was to create a LED driver circuit to power 12 strings of 7 LEDs from a 12V DC supply with a nominal current of 20mA per string.
Using the STEVAL-ILL035V1 Development board data for PCB layout guidance and the LED7708 design tool spreadsheet for component selection we successfully built a prototype driver PCB and produced firmware to setup the on-board registers and control the LEDs for our application.

Initial results were good and all appeared to be working as expected until we noticed that after a random period (5 to 10 minutes) the LEDs would go off completely and wouldn't recover normal operation untill the device was disabled (DEVCFG0.DEN = 0) and then re-enabled.

When the LEDs go off the boost converter goes into a skipping mode of operation but no errors are indicated either from the XFLT pin or the status registers.

After disabling/enabling the device, the LEDs would come back on normally but would then go off again some time later. Interestingly this would be a much shorter period than the initial illuminated period. I'm unsure if this indicates a heat issue but temperature measurements (taken at 25DegC ambient) of the LED7708 device and PCB don't show anything outside expected running temperatures.

If the LED brightness is lowered from 100% to below 75% or the number of enabled strings is reduced this appears to have an effect on how long they will stay illuminated. Sometimes they will stay on indefinitely at 50% dimming.

If anyone has had issues like this in the past, maybe you could tell us about your experiences or possible solutions you can suggest.

Many Thanks,
Shapeshifter

Shapeshifter · Accepted Answer

Hi Peter,Thanks for looking into the shutting down issue we were experiencing with the LED 7708 device.I believe we have now identified the cause of the problem which was partially temperature related but not quite what we originally suspected.I think its useful to let you know how the problem originated so others trying to use the device may proceed more cautiously and not run into similar issues.As I noted in my initial post, we used the LED7708 design tool spreadsheet for component selection which offers up suggested values for many parameters to get you started and lets you optimise them for your application. Unfortunately the spreadsheet allows you to change parameters without giving warnings that you may be moving outside of working range.We fell foul of this while setting up the internal dimming oscillator's external setting resistor value. The resistor value range is buried in the datasheet and at first it wasn't entirely clear whether the values sited were examples or the working range. This meant that the calculated value offered by the spreadsheet, that we used, was too large and outside the working range.We now believe that the dimming oscillator was starting up when the device was at room temperature but then failed when the temperature rose to the operating temp of around 40degC and caused the LED outputs to shut down.Hopefully this solution will prove helpful to any interested parties.Thanks again,Shapeshifter.

Peter BENSCH · Answer

Welcome @Shapeshifter, to the community!

I am quite sure that you have a heat problem. As you describe, the tests (including long-term tests?) with the STEVAL-ILL035V1 were successful. What are the differences between the EVAL and your board?

Which components do you use for the boost stage (L, T, D, C output)?

What does your layout look like, especially under the LED7708?

Regards
/Peter