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TPS61043: Backlight driver selection help

dnilsson
Prodigy 180 points
Part Number: TPS61043
Other Parts Discussed in Thread: TPS61169,

Tool/software:

Hi,

Working on a customer design, consumer electronics, with the need to power a LCD display backlight. Display has 6 LEDs connected in parallel, resulting in Vforward of 3.3 +/- 0.2V and Iforward of max 120mA spec on the backlight. Goal is to be able to power the driver directly from the product battery supply, which is 2x 1.5V alkaline AA-batteries in series for a nominal 3V supply.

First prototype used a TPS61169 driver powered from a regulated 3.3V rail, but this didn't work well since the lowest brightness achievable is higher than wanted. So we need to select another driver and I'm looking for advice.

Can TPS61043 be used in this application? I suspect this means I need to limit the LED current to 60mA since "Maximum LED switch current" through Q2 is limited to 60mA ? Are there better driver alternatives? I'd like to remove the backlight driver from the regulated 3.3V rail as well since it causes more strain on this 3.3V rail than wanted.

This is a custom display, so we have the option as well to ask the display manufacturer to connect the total of 6 backlight LEDs in series instead, or maybe a combination of two string of 3 LEDs in series for a 9.9V +/- 0.6V / 60mA specification if that makes the driver selection easier?

Thanks
Daniel

  • 0
    TI__Mastermind 30140 points

    Hi Daniel,

    If the 6 LEDs could be in series, it is ok to use TPS61169.

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Prodigy 180 points

    Hi Patrick,

    Thanks, just doublechecking though... The TPS61169 has a Vin min of 2.7V with an undervoltage lockout of 2.3V max. Is that OK to power directly from two standard alkaline AA-batteries in a scenario with the 6 LEDs in series?

    Regards
    Daniel

  • 0 in reply to dnilsson
    TI__Mastermind 30140 points

    Hi Daniel,

    Do you know what is the lowest voltage of the AA battery you are using? If it is above the UVLO threshold, it is ok to use.

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Prodigy 180 points

    Hi Patrick,

    Well, two AA-batteries in series have energy left all the way down to 2.0V so with the TPS61169 directly connected to the batteries this will be the first function to stop working when batteries are weak. Also for that reason I was looking at TPS61043 since it has a 1.8V Vin, maybe with TPS61043 we would not require altering the display to have the LEDs in series.

    Are there advantages with TPS61169 and the LEDs in series vs TPS61043? I was thinking maybe arranging the LEDs in two strings with 3 LEDs per string in combination with TPS61043 to achieve a solution which allows working all the way until the batteries are fully discharged.

    Thanks
    Daniel

  • 0 in reply to dnilsson
    TI__Mastermind 30140 points

    Hi Daniel,

    If you separate the LEDs in two strings, the configuration will be 9.9V +/- 0.6V / 2*60mA? The concern is that the switch current limit of TPS61043 is 320mA@Min.

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Prodigy 180 points

    Hi Patrick,

    Each LED i 3.3V/20mA (max) so configured as two strings of 3 each should be 9.9V and 2*20mA = 40mA (max). I don't need the full brightness though, we can configure the current limit on the driver (Rs) lower than the maximum the LEDs can handle if that helps?

    Is the switch current limit of TPS61043 still a concern in that case?

    Regards
    Daniel

  • 0 in reply to dnilsson
    TI__Mastermind 30140 points

    Hi Daniel,

    I would say for most chips it will be fine.

    If considering below worst condition, the 320mA current limit might cause the LED current only up to ~33mA. Is this acceptable to you?

    • VIN = 2V
    • VOUT = 10.5V
    • L = 22uH
    • Fsw = 500kHz
    • Isw = 320mA
    • Efficiency = 70%

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Prodigy 180 points

    Hi Patrick,

    Yes, 33mA is acceptable max current (~16.5mA per string / LED). Now, final question:

    With TPS61043 could I as well keep the LEDs all in parallel (current display configuration) so that LED characteristics are 3.3V +/- 0.2V and lets say around 100mA (6x ~16.5mA) max current instead? Or is rearranging the LEDs in the display still preferred due to regulation inside the driver?

    VIN on TPS61043 would be in the range of 2V (min) to 3.1V (max).

    Thanks
    Daniel

  • 0 in reply to dnilsson
    TI__Mastermind 30140 points

    Hi Daniel,

    Theoretically this is also acceptable.

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Prodigy 180 points

    Thanks, we have designed and ordered prototype to evaluate the solution with TPS61043