a. LED backlight

LED backlight is more complicated subject than CCFL lamps.

Basically we can divide LED screens on two types :

- those with integrated LED driver

- those without LED driver, there are only raw LED strings/chains wires and this LEDs needs external driver (proper supply power) to power them up.

IMPORTANT: LED current varies exponentially with the voltage, i.e., a small change in voltage can cause a large

change in current. That’s why, in most cases, LEDs are driven with constant-current sources.

Also if there are matrix of M LED by N strings MxN, each N string can have different Vf (forward Voltage), so at each Series LED String must be Current Sink Control, to keep current equal at each String.

Sample of typical step-up converter, LED backlight driver

That theory, now some practice

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How to recognize what type of screen we have, the only way is to look at screen and at datasheet of screen.

If we see information about LED BL (on/off) pin and about LED ADJ (dimming), that means the screen have on-board LED driver, so we need to deliver only LED Vcc voltage, BL and possibly ADJ signals from controller to made LED backlight work.

Many times we need to reverse phase of PWM, in "service menu" of ADJ signal, to made LED backlight work, if ADJ is used.

Below is sample page from screen datasheet, with connector description informing about LED BL and ADJ pins - that means screen have onboard LED driver already, so it need only voltage between 7-21V as LED Vcc supply and BL, ADJ pins to control it.

In this case, controlling LED backlight, is as easy as using inverter with CCFL lamp.

In second case, we simply connect BL pin from controller to ADJ in screen,

that will light up LED backlight at maximum, because PWM = 100% .

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The other type of screens, have only raw LED strings without any LED driver (supply power), so in order to made them light must be connected external driver.

Here is sample of screen datasheet when we see a raw LED output of Anode and Cathode to power up using external LED driver.

The problem is that every screen, depends from diameter, manufacturer, LED types etc..., have different LED types, different LED quantity and different connection tree of LED strings.

That means, that every screen need other Voltage and Current, to made LED backlight work.

The bigger monitor's screens, usually have standard monitor LVDS connector and LED backlight cable outputs are usually somewhere at one of screen's corner.

Usually, LED driver is integrated with factory logic controller board and works with controller signal main chip (step-up converter's FET transistor have Gate connected to main signal chip) , so if controller board is bad that means LED driver can not be reused also, there is no LED driver for reuse.

Below is sample screens comparison chart, including nominal values of Voltage and Current for their LED backlights.

They show also, how many pins are in LED socket and which is Anode and Cathode.

As we see in charts, Current vary from 160mA to 440mA and Voltages range from 24V to 66V .

As on charts, LED driving Voltage is always higher than controller's supply voltage (usually 12V) , that way we need driver/inverter, whatever we call step-up converter, it make from regular Vcc (12V) voltage, voltage able to supply and light up LED strings.

This means, LED drivers are NOT interchangeable between screens.

So, LED driver properly working with one screen type, most possibly will burn LEDs in other screen type or most possibly will not light this screen.

Most problem become, because for newer bigger screens are not available any datasheets, so many times I have to test LED backlight parameter by myself.

That is the reason, we must tread LED backlit screens very carefully, normally we must to test each particular screen separately and be sure to NOT overCurrent or overVoltage LED strings, as LEDs will simply burn off.

In some internet shops, are available LED drivers designed for particular screen types, as well as universal LED drivers, but be careful of above.

There is very big chance that such universal LED driver will burn your LEDs or will not drive them to light.

Some of those universal drivers have on-board places(pads) for I-Sense resistors mount/remove, and/or short-pads to regulate Current for particular LED backlight, this made possible to match LED driver's Current parameter with particular screen. However such work need some experience and knowledge about DC/DC step-up converters, to avoid any LED burning disaster.

In practice, is not so difficult, because in monitor's screens are usually 2 - 6 strings, in fact all strings practically have almost equal Vf , so in all those "universal" drivers, Cathodes of strings are connected together.

So there is only one I-Sense Resistor for total current, so if we know the driver's chip reference voltage and proper Current value for particular screen backlight, we can count Sense Resistor for particular current, as from Ohm's law R=U/I , but we have to know exactly the Current of particular LCD display we use, and reference (feedback threshold) voltage of IC used in driver (usually 150mV to 500mV depend on IC type and manufacturer) .

If anyone have such experience, than can buy universal LED driver type with constant Current,

if not, please read what next on this subpage b. step-up converter as LED driver

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in progress