I don't know if this would apply to your situation or not, but I controlled
an LED backlight with a CdS photoresistor on the base of an NPN transistor.
I think.  The long and short of it is that the sensor responds to light in
the same way (about) that a human eye does, and so it turned up the
brightness in response to lower ambient light.  It was a very subtle effect,
you couldn't tell it was working at all unless you knew.  The LCD was just
always readable.

That's the one real benefit of the LED backlight.  It can be dimmed.  If I
remember right it was a generic run of the mill BJT which handled the
current quite handily.  No need to involve the PWM or digital pot at all.

>What would you think is the best method to control an LED LCD back light?
>
>The back light can pull up to > 300ma (at 5V) if I let it, but the working
>range is about 60 to 150ma
>
>I have a my disposal a HPWM output and an spare wiper on a digital pot.
>
>I would love just to put it through the digital pot (AD4800) but I'm sure
>it
>would far to much current for it to handle.
>
>I have thought about gating it on and off with the PWM driving a Mosfet but
>am not sure if it would generate to much noise and am unsure what freq I
>should be driving it at.
>
>Another idea is to use the Pot to control the gate of  a power transistor
>but am unsure of the values etc.
>
>So what do you guys think would work best? And any tips on what values etc
>would be appreciated too.
>
>Many thanks
>
>Tim
>
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