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LM3410: Looking for a solution to power white LED parallel string from fixed 3V3 supply

Daniel Schouten
Prodigy 160 points
Part Number: LM3410
Other Parts Discussed in Thread: TPS61099

Hi there,

I am using a TFT display with a white LED backlight. The internal backlight is build up with 8 white LEDs (Vf=3.2V) in parallel (common anode). There is only a 3V3 supply available in my design. And as this voltage level is too close to the nominal forward voltage of the white LEDs, I would like to boost the voltage up a bit so that the LEDs can be properly powered and (current-) controlled.

I have noticed that most available LED drivers (current controlled boost converters) are optimized for series strings of LEDs. My question is, can a chip like your LM3410 be used for a white LED parallel string as well? Or does the chip have difficulties with that, in terms of duty cycle range or similar?

I need a current controlled boost converter for my parallel LED string that can handle at least 160mA continuously and has a possibility for dimming (preferably by external PWM control).

Thanks in advance!

  • 0
    TI__Mastermind 28655 points

    Hello Daniel,

    It can run parallel leds it really doesn't know the difference.  It has to be able to boost so the output has to be higher than the input voltage.  Even if it is close you may run into the leds staying on when the LM3410 is off due to how a boost works.  The input is fed right to the output through the boost diode so the output voltage can never be lower than Vin minus the boost diode.  Vf of the led varies with current which you can see in the LED datasheet.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 160 points

    Hi Irwin,

    Thanks for your quick answer and good suggestion about the power path through the boost inductor and diode which remains 'open' with the converter turned off. I will have to check the datasheet of the white LEDs used in the TFT module to see if Vf can go below 3V3 - Vboostdiode = approx. 2.9 - 3.0V.

    Another option for my application would be to use a standard low power boost converter, like the TPS61099. Then boost up till somewhere between 4V and 5V and use a linear current mirror type of approach, to regulate the current through the paralleled LEDs. This should work fine too don't you think?

    Thanks.

  • 0 in reply to Daniel Schouten
    TI__Mastermind 28655 points

    Hello Daniel,

    Yes, but it may be easier just to add more diode drops in series with the LED, that accomplishes the same thing.  Also you plan on PWM dimming.  Another thing to look at is the SEPIC design on page 26 of the datasheet.  This allows the output voltage to be higher or lower than the input voltage.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 160 points

    Hello Irwin,

    Good suggestion about adding more diode drops in the LED section. One standard 1A silicon diode in the common anode line should do the job then.

    About the SEPIC topology, it was considered shortly but actually further ignored due to the additional parts count. But this was before learning that the boost topology with only 1x Vforward of the LED was pushing the bottom end limits. Is the SEPIC approach on page 26 also compatible with PWM dimming?

    Thanks.

  • 0 in reply to Daniel Schouten
    TI__Mastermind 28655 points

    Hello,

    Yes it is.  Note they also make coupled inductors which work for SEPIC designs, this takes two inductors and puts them into one package.  The calculations change for the inductor, you only need 1/2 the value since it is coupled.

    Best Regards,