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LED Backlight Driver for LCD-TFT Display Panel

Hitesh Kumar
Prodigy 50 points
Other Parts Discussed in Thread: TPS61182, TPS61181, TPS61500

Hi All,

Can anyone suggest me any suitable solution for following backlight driver configuration.

For Display 5.7". It has 3 string of 6 LEDs each and need 200mA current @ 11V.

For Display 7". It has 3 strings of 8 LED on each string and need 60mA @ 24V.

Early reply will be highly apprciable.

 

Regards

H

 

  • 0
    TI__Guru**** 155606 points

    Is that 200mA and 60mA per string or total?

  • 0 in reply to Jeff F
    Prodigy 50 points

    ..Its total current.

    Sorry for not giving full information. Moreover I have to see package style as well. because QFN is very difficult to solder. So if I can find something in SOT or SOP that would be great for me.

    Regards

    H

     

  • 0 in reply to Hitesh Kumar
    TI__Guru**** 155606 points

    For the 60mA total, 60mA/3 =20mA per string backlight, you could use our released part, the TPS61181 or TPS61182 and simply not use the other three strings.  For the 200mA total, 200mA/3=67mA per string design, we have a driver in development that would work.  See attached concept.  Samples available August 2010.  Or if the LED current matching of each string is less important, then you could use the TPS61500 with ballast resistors on each string to improve the matching.

  • 0 in reply to Jeff F
    Prodigy 50 points

    Thanks Jeff for your prompt reply. Even I found TPS61500 is more closure to the specification and can solve my purpose. I already ordered a sample of it. Lets see how it will work. Once again thanks for your kind help.

    Regards

    H

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi !

    i am using the TPS61181 - QFN - and i have promlems to activate it !

    i using the TPS61181 to activate LCD backlight - 8 LEDs for one string and total  3 strings  -each string need 20 mA -> Total 60mA  

    i designed as the application note recommandded and have problems whith the output caps - the current consptions is go up to 800 mA However when i removed the output caps the consiption is ok ->60mA but have blinking LCD -> Ripples 

    do any body have an idea How to fix the problem ?

    Note - ??

     

  • 0 in reply to olvik josh
    TI__Guru**** 155606 points

    Can you send a schematic or provide the number of the app note to which you are referring?  Please include the input voltage.

  • 0 in reply to olvik josh
    TI__Guru**** 155606 points

    I don't see any big errors with the schematic.  You might increase the output capacitor to the 4.7uF and see if that makes a difference.  If you are new to soldering QFN, I suggest carefully inspecting the soldering of the pins.  It is possible that one of the pins is shorted to gnd underneath the IC.  You might try measuring the resistance of each pin to gnd and see if one is shorted.

  • 0 in reply to Jeff F
    Prodigy 100 points

    The Thermal Pads is going to GND according to the layout recomandations of TI ,there is no shorted pins to the gnd ?

    What is interesting that i see the LEDs Blinking but not stablize (whithout caps)-> in my understanding my be it is layout problim becuse the high return current feedbak ?

    what do you think ?

  • 0 in reply to olvik josh
    TI__Guru**** 155606 points

    What is the output capability of the power provided to the TPS61181?  It is a boost converter so it will need more input current than it provides in output current.  Use the efficiency equation to estimate your input current requirements:  eff = Vout*Iout / (Vin*Iin) so Iin = Vout*Iout/(Vin*eff) where eff is 85%

  • 0 in reply to Jeff F
    Prodigy 100 points

    whith this current 60 mA i am getting 75% eff -so my i supposed that youare saying that there is not engh current in input to get effective from the Boost converter !

    i am woundring if the output caps shall be spicaeal requirment such as resistor in seireas whith the output cap - ?

    i am woundring if there is EMI ? 

  • 0 in reply to olvik josh
    TI__Guru**** 155606 points

    I have had other customers whose input power supplies which could not provide enough input current or which dropped below the 4.5V undervoltage lockout threshold and saw flickering.   If you are getting enough input current, measuring the input voltage at the IC's VBAT pin and confirm that it never droops below 4.5V.

    Another common problem is an inductor with too low of a current rating.  What is your inductor part number and current rating?

    Regarding the output caps, ceramic capacitors are recommended.

    Regarding EMI, if you have an inductive switcher, you have EMI but I don't think this iyour problem.

     

     

  • 0 in reply to Jeff F
    Prodigy 100 points

    HI !
    My Inductor part number is IHLP2020BZER100M01 of Vishay max DCR is 200 mOhm

    you can see the data sheet at http://il.mouser.com/ProductDetail/Vishay-Dale/IHLP2020BZER100M01/?qs=sGAEpiMZZMsg%252by3WlYCkU%252b3xySivvrlQPpu4vHQaNlk%3d

    i think i am to high resistance ? do you recommend on any other solutions casue the problem - i will try to change the inductor !

  • 0 in reply to olvik josh
    TI__Guru**** 155606 points

    The inductor DCR is higher than I typically recommend and will cause lower efficiency but, as long as your input power supply can provide 5V at the required current level, your LEDs should not blink.  Are you certain that the voltage at the VBAT pin never dips below 5V?

  • 0 in reply to Jeff F
    Prodigy 100 points

    well let me cheack !

    you mean under 5 volt ???

  • 0 in reply to olvik josh
    TI__Guru**** 155606 points

    The 61181 is specified for operation only down to 5V.  It has an undervoltage lockout circuit that will shut down the part if Vin drops below 4.5V (max).  So, if your input supply cannot provide the required input current, or your board traces are long and thin and drop the voltage before it reaches our IC, the IC will turn off momentarily until the input voltage returns to 5V and then turn back on.  The quickest way to confirm if this is the problem is to connect VBAT to a higher input voltage and see if that eliminates your problem.