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 _______________________________________________ hardware mailing list hardware@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/hardware