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LCD Backlight Control - Multiple LCD Types LM3405/LM3406

Kabir Alli
Prodigy 40 points
Other Parts Discussed in Thread: LM3405, LM3406

Hi, I'm trying to power an LCD backlight LED with a current source and control it with PWM. I was considering the LM3405 but now I may have to change over to LM3406 (due to increased current Requirements)

The problem is that my led backlight source/sink needs to be universal, work with multiple LCD's with different LEDs (currents requirements from 200mA to 1.2A)  which need to be plug and play, without having to change a drive resistor .

From reading the datasheet, I want to say that the device does not support that as it only allows for a fixed current output. Am i correct in thinking this ? (that the output will source that current but increase the drive voltage to compensate for the impedance of of the LED)

 Could anybody suggest a way to do this, make a marginally univeral LED backlight driver for a range of LCD's requiring varying currents (200mA to 1200mA) ?

Thanks in advance for your help

  • 0
    TI__Expert 5130 points

    Hello Kabir,

    That is an interesting application.

    What is your supply voltage? Is there any specific requirement such as contrast ratio and dimming ratio?

    According to what you have described, my understanding is that you want to use the fixed driver for LCDs with different output load.

    It would be easier for us to help if you could share that information to us.

    Thanks and best regards,

    Victor

  • 0 in reply to Victor Chen_HK
    Prodigy 40 points

    Hi Victor,

    Supply is 12Vdc, I have no other special requirements (the rest of the LCD drive is taken care off by other circuitry) . Basically the LEDs of various types of mono-graphic+character LCD's need to be driven. 

    Yup, i want a single driver that i can adjust the drive to suit various LCD's (backlighting).

    For example I have an LCD (from manufacture1)that has White backlight, 860mA @4.2V, then a second (from manufacture2) that has yellow backlighting @2.8V@280mA ...both of these need to be driven.

    I want to control the brightness via PWM.

    I've drawn a block diagram of what im trying to accomplish(see below)  

    my knowledge of these type of applications(High current source LED drive) is purely theoretical, so im not sure if the LM3405 will be able to supply variable currents WITHOUT changing the feedback sense resistor. 

  • 0 in reply to Kabir Alli
    Prodigy 40 points

    Cost and IC-obsolescence  are also a big factors

    Appreciate the interest :) 

  • 0 in reply to Kabir Alli
    TI__Expert 5130 points

    Hi Kabir,

    It may work if you only swap the LCD. But there is one thing you should pay attention to that you can change average current but the maximum output current will always be fixed.

    As shown in the figure below, the maximum current is set to 1.4A (as you said your current is from 200mA to 1.2A). You change the PWM duty cycle to make the average output current to 280mA (the dot line). However, during the LED turned on period, the current through LEDs is always 1.4A. This could damage your LED as the maximum current allowed of 280mA LED may be 500mA.

    Regards,

    Victor

  • 0 in reply to Victor Chen_HK
    Prodigy 40 points

    That does tell me what i needed to know.

    i think i will now be looking at adjustable current sources(tha i can control via pwm or analogue) to achieve this.

    Thank you, appreciate the clarity.

  • 0 in reply to Victor Chen_HK
    Intellectual 370 points

    Why not choose the sensor resistor to draw 1.2 Amps max and then use PMW to adjust/reduce current according to user's need?  As to the original question, I don't think LM3406 is designed to accommodate PnP type varying load, I just don't see the benefit of having one chip to fulfill various output. 

    All LEDs have different forward voltage drops that will change the selection of output capacitor and inductors for optimization; I thinks its best to make a separate driver circuit for each specific type of LED, and this will simply the how the system can choose the LEDs.