LP8556: Current limit minimum and maximum value for QFN package

Hi Peter,

The current limit for QFN package is as same as DSBGA package.

BR

Patrick

Hi Patrick,

Thanks for your answer. Now I got another question. I need the typical value of headroom voltage. First I set the brightness to 100%, and I see that the headroom voltage rises from 540mV to 720mV. Then I set the brightness to 75%, the headroom voltage falls to 624mV. But when I continue to set the brightness to 50%, the headroom voltage rises to 650mV. Finally, I set the brightness to 25%, and the headroom voltage is 637mV. In my understanding, headroom voltage and output current are positively correlated. Why the 75% headroom voltage is smaller than 50% headroom voltage? And in what circumstance you get the typical value of headroom voltage? 

My test circumstance: Vin=3.3V, VDDIO=3.3V, Imax=23mA, adaptive control.

Best Regards,

Peter

Hi Peter,

What's the register settings related to headroom?

BR

Patrick

Hi Patrick,

The picture is below. Besides, the headroom voltage always rises after a while. Maybe it's because the MOS used to make the current sink have positive temperature coefficient.  Is that right?

Hi Peter,

The headroom voltage you mentioned above, are they all from the minimum VFB pin?

BR

Patrick

Hi Patrick,

I used LP8556SQ-EVM to test and its schematic diagram is below. I tested the voltage between LED1 and GND because it's common cathode circuit so the voltage between LEDs and GND is headroom voltage.

Best Regards,

Peter

Hi Peter,

I know what is headroom voltage. I mean you should measure the LED channel which has the minimum headroom voltage.

BR

Patrick

Hi Patrick,

Hope I fully understand your meaning. I just tested the Vhr for all six channels and got a list below. 

I still tested at the point of 25%, 50%, 75% and 100% brightness. It seems that channel 5 has the minimum headroom voltage. What factor causes the difference in headroom voltage? And all the channels have the feature that 50% brightness Vhr is larger than 25% Vhr.

Best Regards,

Peter

Hi Peter,

Because parameter differences between LEDs, not every channel has the same Vhr.

But why the Vhr at 75% brightness is lower than 50% and 25%, I need to further analyze.

BR

Patrick 

Hi Patrick,

I notice that the datasheet only gives typical value of Isw_lim when I_BOOST_2X = 1, I wonder the minimum and maximum value of it.

Hi Peter,

Several days ago I also tired to find this data, but couldn't find any material related to the QFN package. If I could find this data in the future, I'll let you know.

BR

Patrick