Backlight Power Measurement

[Werner Almesberger]

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.

> 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.

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%.

- Werner