Backlight Power Measurement

Werner Almesberger werner at
Thu Oct 15 13:53:41 CEST 2009

Aaron Carroll wrote:
> LED+ to GND, because I want to include the power lost in R1763.

Okay, this would explain about 10% of the loss.

>> 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).

I mean the 0.1 Ohm resistor you used to measure the current:

Battery -----/\/\/\----- CON1701
          ^          ^
          |          |
          A          B

Your "IN" voltage is taken at point B, so it doesn't add a (false)
extra loss.

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

It's all ballpark figures ;-) I was just looking for a reference for
the typical performance of the sort of DC-DC converters NXP use in
these chips. They all seem to have their peak efficiency of 85% in
the middle of their operating range and drop to 75% at the edges.

- Werner

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