[Arm-netbook] hannstar LCDs LED backlight circuit

[Luke Kenneth Casson Leighton]

On Tue, Dec 9, 2014 at 4:05 PM, joem <joem at martindale-electric.co.uk> wrote:
> 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.

 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?


>> 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.

 ... or find an alternative.

> (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.

 unfortunately i don't have the luxury of being able to experiment
like that.  remember that i am endeavouring to *establish* this
business, i am not operating from the position of having an R&D budget
from established business funds and established sales in order to make
these kinds of experiments.

 so my preferred path is to use a pre-existing proven circuit.  this
is proving hard to track down.

 last resort i will make something [conservative] and test it.  my
primary goal here is to get a working prototype that can begin a 2nd
crowdfunding campaign.


>  Obviously not clever
> if it has to work from batteries

 yes, this does.

> - 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.)

 ack.  thx joe.