Backlight Power Measurement
xaaronc at gmail.com
Tue Oct 13 08:48:27 CEST 2009
2009/10/10 Paul Fertser <fercerpav at gmail.com>:
> Aaron Carroll <xaaronc at gmail.com> writes:
>> 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.
> You've got quite puzzling and unexpected results indeed. Could you
> please provide us with the actual measurements data?
Here's some data. Each point is the average of 1000 samples @ 100 kHz,
alternating between voltage and current samples.
The percentages are backlight brightness according to the position of
the brightness slider (the phone runs Android).
BL is the backlight, with I measured across R1763, and V at C1764.
IN is measured at the battery with no other external connections,
with the current measured via a 100mohm resistor in series with (+).
The power is just the product of the quoted V/I.
To be completely accurate, the backlight figures are not raw measurements,
because they are scaled via an opamp-based attentuator before feeding
into the DAQ. The values are scaled back up in software, and I have verified
this all happens correctly with an accurate meter.
Let me know if I can provide more details or measurements.
BL = 14.38 V, 28.53 mA, 410.26 mW
IN = 4.01 V, 252.89 mA, 1014.09 mW
BL = 13.10 V, 15.91 mA, 208.42 mW
IN = 4.04 V, 178.86 mA, 722.60 mW
BL = 11.97 V, 6.45 mA, 77.21 mW
IN = 4.05 V, 129.57 mA, 520.71 mW
BL = 11.13 V, 1.82 mA, 20.26 mW
IN = 4.06 V, 110.96 mA, 450.50 mW
BL = 10.70 V, 0.49 mA, 5.24 mW
IN = 4.06 V, 104.06 mA, 422.49 mW
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