Backlight Power Measurement

[Aaron Carroll]

2009/10/14 Werner Almesberger <werner at openmoko.org>:
> Aaron Carroll wrote:
>> BL is the backlight, with I measured across R1763, and V at C1764.
> [...]
>>   BL = 14.38 V, 28.53 mA,  410.26 mW
>
> Is the BL voltage what you measure at LED+ relative to GND or have
> you already subtracted LED- ? In the former case, your result would
> be about 10% high with maximum backlight.

LED+ to GND, because I want to include the power lost in R1763.

>> 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.
> [...]
>>   IN = 4.01 V,  252.89 mA, 1014.09 mW
>
> Which side is IN - battery side of the shunt resistor or Neo side ?
> In the former case, you'd be another 6% high.

I can't see which shunt resistor you are refering to, but the
measurement is taken at the battery connector (i.e. CON1701).

> Regarding expected efficiency, the 50633 manual is silent about it,
> the 50606 manual has the following data:
>
> Current         Conversion
> (mA)            3.6 V -> 1.8 V  3.6 -> 3.3 V
>  1             75%             92%
>  10             85%             95%
> 100             85%             96%
>
> Up conversion seems to have similar characteristics as down
> conversion, but the voltages given don't make sense (Vi = 5.0 V
> and Vp = 1.8 V. That's a down conversion !)
>
> So we should expect an overall conversion loss of not much more
> than 25%.

The 50606 converter has a much lower peak voltage/higher peak
current... can we make a meaningful extrapolation here?


Cheers,
          -- Aaron