[Aaron Carroll Di 13. Oktober 2009]: > 2009/10/10 Paul Fertser <fercer...@gmail.com>: > > Aaron Carroll <xaar...@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. > > > Thanks, > -- Aaron > > > > > ===========x8============ > > > 100%: > BL = 14.38 V, 28.53 mA, 410.26 mW > IN = 4.01 V, 252.89 mA, 1014.09 mW > > 75%: > BL = 13.10 V, 15.91 mA, 208.42 mW > IN = 4.04 V, 178.86 mA, 722.60 mW > > 50%: > BL = 11.97 V, 6.45 mA, 77.21 mW > IN = 4.05 V, 129.57 mA, 520.71 mW > > 25%: > BL = 11.13 V, 1.82 mA, 20.26 mW > IN = 4.06 V, 110.96 mA, 450.50 mW > > 0%: > BL = 10.70 V, 0.49 mA, 5.24 mW > IN = 4.06 V, 104.06 mA, 422.49 mW
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 cheers jOERG
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