[Arm-netbook] hannstar LCDs LED backlight circuit

[joem]

On Tue, 2014-12-09 at 14:33 +0000, Luke Kenneth Casson Leighton wrote:
> On Tue, Dec 9, 2014 at 8:49 AM, joem <joem at martindale-electric.co.uk> wrote:
> > On Mon, 2014-12-08 at 15:45 +0000, Luke Kenneth Casson Leighton wrote:
> >
> >>  ... *deep breath*... but.... hold everything, i've never been a great
> >> fan of the 1024x600 LCD panels
> >
> > hmmm... thinks odroid-vu
> > http://www.hardkernel.com/main/products/prdt_info.php?g_code=G140383714860
> >
> > Miniature HDMI monitor for $120.
> > I bought one. If there were more of these things around
> > with variety then lessens need for fixed screen.
> 
> 
>  which reminds me: i need to work out a circuit for driving the LCD
> backlight.  does anyone have e.g. one of these:
> http://www.adafruit.com/products/1931

I have similar but 5" from Aliexpress (used in reversing
cars). When I try to replicate the board, came across
this idea that there is some 'firmware' like parameters
that have to be set up to make the chip and LCD work correctly.
There was no further information about how one might go about doing
that.

> i need an example circuit to work from: the one that i have uses the
> SY7201 and it is designed to pulse the LEDs, whereas the Hannstar LCDs
> have their own dedicated LED backlight PWM input.

Not sure what that means.
The example circuit is given in their PDF.
(The inductor is worked out by swapping
out some reasonable values until its just
right which you can get by looking at the scope
signals, or do some calculations to estimate best value.)
The inductance changes with changes to LED chain.
The LED chain is electrically isolated from the
rest of the display - so nothing to worry about
except burnt out LEDs. Net current down LED
chain normally not exceed 20mA.
That is how the limiting resistor is set.

(I normally use a 30V supply and limit the
current to 20mA by having a serial
resistor and upping the voltage until good
brightness is reached. Obviously not clever
if it has to work from batteries - but it
is a starting point to work out the net
LED voltage that has to be subtracted from
calculations to get limiting resistor, currents,
and inductance.)