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LP8860-Q1: LP8860, True Output Dimming resolution

Martin Stein
Intellectual 1365 points
Part Number: LP8860-Q1

Hello,

I like to know what dimming resolution is actually achievable at output.

The input offers 16-bit. PWM is 13-bit best. Meaning, we would activate hybrid-dimming to achieve > 13 bit resolution. The current seems to be resolved with 12-bit according to datasheet.

It is not obvious, whether combination of 12-bit and 13-bit will achieve more than 13-bit. Is there something to consider like an optimal switch-point ?

I think with 12.5 % switch-point the ratio of maximum current vs. minimum average current is like a 16-bit LSB.

Thanks for your advice,

Martin

  • 0
    TI__Expert 5475 points

    Hi Martin,

    Today is Chinese public holiday and I'll come back to you next Monday. 

  • 0 in reply to Shirley He
    TI__Expert 5475 points

    Hi Martin,

    The dimming resolution is the number of brightness steps between 100% and 0% current. The dimming resolution limited by the PWM dimming frequency and oscillator frequency is 13bits. With hybrid dimming transition threshold set at 12.5%, the current amplitude can be reduced to 1/8, which adds another 3bits to the dimming resolution, meaning 16bits in total.

  • 0 in reply to Shirley He
    TI__Intellectual 1365 points

    Hello Shirley,

    can you double check your statement ? I understand that PWM is only activated below transition threshold. Meaning that 13-bit apply only for 1/8 of whole current range. But, the other 7/8 of current range are only resolved with 12-bit.

    Meaning that the number of brightness steps is actually 13-bit (PWM) + 12-bit (current dimming). Which is less than 14 bit dimming resolution overall.

    What do you think ?

    Regards

    Martin

  • 0 in reply to Martin Stein
    TI__Expert 5475 points

    Hi Martin,

    Your understanding is right. During the high 7/8 brightness range, the current is resolved with 12-bit. During the low 1/8 brightness range, the brightness is resolved with 13+3=16 bit, because the brightness step between each brightness is 1/(2^13)*(1/8) of the maximum brightness.

    But I don't think it makes any sense to think about the "overall resolution", because the step is different in the two range.