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LED backlight driver for big screen (hight power)

Paul Grangeray
Prodigy 40 points
Other Parts Discussed in Thread: TPS92692

Hi everyone,

I'm new here!

I am currently working on a design, it's a LED backlight driver for a big screen (43").

Common products don't suit for my application :

Low luminosity limit must be very low

I need a LED voltage supply around 100V, 1A. The driver must control the current (PWM).

if possible, this driver must be used for smaller screen (21") : around 40V voltage supply, and 300mA.

I saw different chip, and this example seems to be good for me : eng.eewiki.net/.../get_file.php

But LM3430 is discontinued!

Any suggestions?

Thank you!

  • 0
    TI__Mastermind 28655 points
    Hello Paul,

    Not sure I can help with this application but I'll ask questions and see, otherwise it may go to another group.

    What is Vin (full range needed)?
    What is Vout range (full range needed)? Note, LM3430 is not rated for 100V output.
    What is Iout range (also full range needed and dimming capability)?
    What PWM range and frequency is desired?
    What topology are you interested in, boost only? TPS92692 may work for your application.
    Are you open to more than one design (for optimization)?

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hi,

    Vin is 12Vdc or 24Vdc.

    Vout is from 30Vdc to 100Vdc.

    Iout is from 100mA to 1A.

    PWM is from 0% to 100%, with high precision for a good luminosity variation (it must be used for nigh vision, so very very low luminosity). Frequency can be adjusted as desired.

    Topology, boost or other, i don't really know!

    Thanks for your help.

    Paul

  • 0 in reply to Paul Grangeray
    TI__Mastermind 28655 points

    Hello Paul,

    I would look at the TPS92692.  This is fairly high power and the input current will be high with 12V input.  I've created a CV (constant voltage) design with the TPS92692 that is near this power level so it can be done.  I would go with boost as long as your 30 VDC output doesn't have issue when 24 VDC in present (LEDs Vf varies with current and they may still be illuminated).

    The current sense will need to be done on the bottom of the LED stack which requires some different calculations (slope compensation).  There may be other options but you want to PWM dim which limits some of the higher power options.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hi Irwin,

    I can't  find in the TPS92692 datasheet the maximum output voltage for LED string.

    Is it decided by the external component (MOSFET etc.)? And what is the max outpout voltage?

    I need 100Vdc.

    For my application i need a very very low luminosity (adjust from 0 to 100% with 0.1% steps).  Do you know if this precision is possible?

    Thank you

    Paul

  • 0 in reply to Paul Grangeray
    TI__Mastermind 28655 points

    Hello Paul,

    You can use it up to the voltage rating of the external parts, MOSFET, etc.  This works only if the current sense is done at the bottom of the LED stack which requires a different slope compensation calculation.  It probably doesn't matter regardless since it will not be as accurate as you need.  To get that accuracy you will probably need to design your own current sense circuit.  I guess I should ask first, are you planning on analog or PWM dimming or a combination of both?

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hi Irwin,

    Thanks for your reply.

    I plane to use PWM dimming, but if necessary I can use both of them.

    I have a PIC24 for dimming control.

    Best regards,

    Paul

  • 0 in reply to Paul Grangeray
    TI__Mastermind 28655 points
    Hello Paul,

    You will have to use the datasheet calculations to see if you can be that accurate. You could analog dim down and add PWM dimming. To get to 0.1% accuracy is difficult and will probably require external circuitry. If you plan on trying to be that accurate over your operating range it will be even more difficult. Are you concerned about color shift due to analog dimming?

    Best Regards,