b. step-up converter as LED driver
Here some words, about how to safely test and obtain LED backlight parameters information in most easy way.
I suppose that reader already saw website about LED driver theory, if not please read first here a. LED backlight
As mentioned before, there is possible to buy universal LED drivers but they are not so comfortable to testing purposes and not so cheap.
For most safe way of LED testing, let use very popular on many internet shops and places, universal regulated step-up converter module.
There are very popular step-up DC/DC modules made on LM2577 or on XL6009 chips.
I strongly recommend, to buy and use XL6009 only, as the best solution for LED purposes. It is also very cheap, about $2 .
Well, lets look on schema of typical basic design
As on schema, it is design of constant voltage step-up converter, regulated output by R2/R1 voltage divider at feedback threshold.
As we know, in most cases, LEDs are driven by constant current design, so there are some cases that LED are driven by constant voltage :) .
Theoretically is possible to made it as constant current, by exchange R2 by LED strings and count R1 as (Vreference of XL6009=1.25V and LED current 200mA) R1= 1.25 / 0.2 = 6.25 OHm , what Wattage ? W = U * I = 1.25 * 0.2 = 0.25W , so R1 should be 6.25 OHm at least 0.5W for LED current 200mA .
However, I am too lazy for such modification and will use the module as is, as constant voltage module :) .
It is because XL6009 is very good and keep stable output Voltage independently of changing input Voltage, and for small Current like around 200mA to 400mA , there are very low Voltage ripple at output.
The XL6009 step-up converter module, looks like on picture below
Such step-up converter is available IN THIS SHOP , however, on picture there is different type but they ship the same as on picture below
As we see, module have regulated precise potentiometer, so is possible to regulate very precise output voltage.
XL6009 have ability to work up to 60V and 3A , the module on picture have maximum voltage regulation to 47V - what I checked personally.
When we need more Volts, is easy by adding 20kOHm additional resistor in series to potentiometer, such modification by SMD resistor is simple. When made more voltage ex.60V , BE SURE to exchange output capacitor for bigger Voltage like 80V or 100V .
For our work, the voltage 47V is absolutely enough for most use, when modified for 60V, even some MacBook screens can be driven, as they need 50-60V .
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Ok, now time to test and use it in practice.
Lastly I bought on Allegro (it is kind of Polish Ebay) broken monitor LG E2242 , seller wrote that electronic is completely destroyed by thunder and possible that screen also. Specification say that is monitor HD 1920x1080 LED screen.
OK, let open it. Indeed, electronic board is destroyed, no sign of life, so I decided to test screen and use as donor for universal LCD controller.
The screen type is LGM215AC01, so after little googling, there are not any sign of datasheet for this screen.
So I analyze LVDS cable and made measurement of GND pins in connector of screen, so I knew it is standard monitor LVDS input 2channel 8bit like that one
Now, the only problem was LED backlight.
As we know already, LED backlight works since 20V to 60V, and current from 160mA to 440mA for big screens.
It is good, as we can check the LED using XL6009 module at least to 47V , I do not believe that 22" will have more.
The module have precise potentiometer, so we can increase voltage slowly without risk of burn LED strings.
When measured LED cable, it was connection CCAACC when in fact CC was short and AA also,like that C-C A-A C-C , so real connection was as two LED strings CAC .
So now, is only to connect step-up module to 12V and check by Voltmeter, that output Voltage is as low as possible (12V)
Now, we can connect one string, A to positive and C to negative.
Slowly revolving potentiometer, increase Voltage until moment when LED string start to light, not much but light, it was 32V.
At this moment, we can connect second string, so both Cathodes are connected to step-up converter.
As connector, I used two thin cables with soldered wires.
After power up, I was sure that both LED strings are working well, I checked Current and it was about 10mA, so lot too little for nominal, what I predict for around 200mA for 22" screen.
At this time, I connected Voltmeter and Current meter the same time (most for nice picture) to check Voltage at nominal Current. With connected Ammeter and Voltmeter, I increased Voltage till current was 170mA, what I expect as save current for this screen of unknown parameters.
As on picture below, the Current is on multimeter left and Voltage at small "China" voltage module (BTW very useful)
As conclusion, finally I knew that this screen is nice light at 39.2V , so Voltage in practice can be used 38.5 to 40V as safe voltage for this screen.
From my experiences, some screens for example 19", working at range 23V to 30V at nominal around 170mA , and as I assumed, each factory LED driver is configured for particular screen type, delivering Current exactly for this screen.
I do not advice to exchange other types/models of LED screens between monitors, as this will case on burning LEDs or not working backlight.
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And finally, let resolve why this XL6009 is so useful for driving LED screens with universal controller.
It is because it have ENable input, state low - module is not working, when high (logic high) - it start to work.
When is state "not working", at output is Vcc voltage - it is normal (look at schema) , when starts to work it give at output regulated voltage.
To made such modification, first of all, check by multimeter if ENable input (pin no. 2 ) is shorted with positive of input voltage, if so, we must cut off this connection and leave EN pin unconnected.
To do this, in particular module on picture, please cut this trace marked red.
After cut, we can solder BL (on/off) cable from controller to pin no.2 of XL6009 chip.
Since now, this cheap step-up converter module, can serve as LED driver, working with universal LCD controller.
