[Arm-netbook] hannstar LCDs LED backlight circuit

[joem]

On Tue, 2014-12-09 at 17:21 +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.
> 
>  there's a guy who has managed exactly that [reverse-engineering]...
> let me find his paaage.... ah here we go:
> http://tech.mattmillman.com/lcd/rm5251/
> 
>  he created a tool that modifies the binary so that you can select
> whatever you want.
> 
>  if you have one of those boards can i ask you the favour of locating
> the white LED circuit and sending me a macro-zoom photo (both sides)
> of that section of the board, highest resolution you can?

Will try

> 
> >> 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.
> 
>  pin 30 of the HSD101PWW datasheet is "LED PWM brightness".
> 
> > The example circuit is given in their PDF.
> 
> i found the application note:
> http://www.elecinfo.com/upload/community/2013/12/30/1388365231-20314.pdf
> 
>  the circuit is *radically* different from the [proven, working] one
> that has been in circulation for years.  it involves an RC circuit
> with a link into the FB: EN/PWM is used purely for "EN" and the FB
> line is given an extra "kick" periodically via a PWM to fool it into
> switching off.
> 
>  all that has to go, which leaves me in a position to need to
> understand how the circuit (that i copied) works, and adapt it.

From datasheet:

"SY7201:Enable and dimming control. When used as enable input, pull
high to turn on IC. When used as dimming input, the first pulse should
be longer than 200ns to turn on IC. And the recommend dimming
frequency range is 20kHz~1MHz.

SY7201A: Enable and dimming control. When used as enable input,
pull high to turn on IC. When used as dimming input, the first pulse
should be longer than 150us to turn on IC. And the recommend
dimming frequency range is 100Hz~10kHz."

So its saying if EN is set high, it will turn on the LED
to full brightness.
:
If instead you were to send a PWM signal say 1kHz
for SY7201A then LED can be dimmed depending on
MARK:SPACE ratio of the signal.
:
RC circuit not needed.
:
RC circuit can be set up to pull EN line high
to turn LCD on to max brightness if the PWM 
source (the CPU) failed to boot.