Backlight Power Measurement

Aaron Carroll xaaronc at
Sat Oct 10 04:40:46 CEST 2009


I'm setting up a GTA02v6 for fine-grained power
measurements, and I'm having trouble getting sense out of
the backlight readings.

To determine backlight power, I've got a DAQ sampling the
voltage drop across R1763 (Rsense for the PCF50663 LED
supply) which gives me the current, and I'm sampling LED+
at C1764 for the input voltage.  I was hoping the product
of these two would yield the power consumed by the
backlight and Rsense.

However, if I vary the backlight brightness (in software)
and measure the power difference, I'm seeing significant
disparity between the power measured at the B/L compared
with total input power measured at the battery connector.

Here's where it gets weird.  If I multiply the measured
B/L power by ~1.5, suddenly everything falls into place
and the measurements match up.  This value was determined
numerically; I have no idea if it has any physical
significance or if it's just an interesting coincidence.

Looking at the signals on a CRO, the input voltage is
nice and DC (though it varies with brightness).  However
the current though Rsense has interesting transients which
I'm assuming is inherent to the converter design.  Even so
the RMS of this signal is very close to the average so I
think it shouldn't affect the results appreciably.

Any ideas in solving this would be greatly appreciated.

             -- Aaron

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