Backlight Power Measurement

Aaron Carroll xaaronc at
Tue Oct 13 08:48:27 CEST 2009

2009/10/10 Paul Fertser <fercerpav at>:
> Aaron Carroll <xaaronc at> 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.

   -- Aaron


  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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