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LP8860-Q1 Fault detection in cluster mode - need more detail.

James Teske
Prodigy 50 points

I have 3 strings of 11 LEDs in series and I am running the driver in cluster mode.  I have been adjusting things such as initial boost voltage and current scale to try and operate without causing a short or open string fault.  I need to operate the strings from fairly bright down to a very very dim brightness but want to be able to not generate false faults with the on-board diagnostics.  Is there an app note of tech brief that can shed a lot more light on how to use the fault detection package in cluster mode?  Vsat + 50mV just doesn't tell me enough to understand.  Right now, all I can do is turn fault detection in cluster mode off.  Please help. 

  • 0
    TI__Expert 4565 points
    Hi,

    LED fault(open/short) detection can still work on cluster mode. If you're using current mode(analog) or hybrid mode(PWM+current) dimming, total Vf of LED string will change a lot due to LED current change which also causes big headroom voltage(LED input voltage) change, thus LED short or open fault may be triggered. But if you use PWM only mode dimming and set Vboost output voltage correctly, total Vf won't be changed as LED current value will be kept while PWM duty changes for brightness control.
    For example, if your total LED Vf is 35V, then you can set initial voltage to 36 or 36.5V(you will need high extra voltage more than Vsat+50mV in case LED current is high like 100mA or more), and Vf will be set to this level at all brightness. You can maintain headroom voltage at 1~1.5V which doesn't trigger open or short LED fault.
    You can use LED fault detection function in this case.
    Hope this helps,