Backlight Power Measurement

Aaron Carroll xaaronc at
Wed Oct 14 08:33:52 CEST 2009

2009/10/13 Joerg Reisenweber <joerg at>:
> Seems our converter is really operating with 66% efficiency :-/
> I suspect L1704 and C1764 see some loss due to ESR.
> You might want to test what happens if you connect a couple of low-ESR
> capacitors (e.g. 5 pcs 4u7) parallel to C1764.
> If bat current drops significantly on doing this, then this component really
> wasn't a good choice for the purpose.
> OTOH 200mW loss should be detectable anyway, just check where the temperature
> rises ;-) Then you know which component is the hog


I wired up L1704, C1764, and the PCF50633 with a thermocouple.
Here are the results for max and min backlight, expressed as
degrees K above ambient.

  L1704 = 18.3
  C1764 = 6.6
  PCF  =    9.0

  L1704 = 6.0
  C1764 = 6.5
  PCF  =    7.2

I think this rules out C1764 as the problem.  However since the
PCF550633 has a large surface area compared with L1704, I'm
not sure this helps work out where most of the power is going...

The original question has been answered though... the power is
lost :(   What I still don't understand is why the efficiency so linear
over the entire backlight power range.

            -- Aaron

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