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I am having some issues making my boost converter (TPS61085PWR) output the correct amount of current to 3 LEDs connected in series. I am using a 600 mAh 3.6 V lipo battery to power the circuit. When the circuit is being powered from 3.6 V, the boost converter is outputting ~11.5 V, and the LED's are consuming ~0.011 A. When I increased the input voltage to 7.6 V, the boost converter outputs ~11.6 V, but the current goes up to 0.09 A (where it should be). I am running this IC at 650 kHz.

TPS61085PWR specs.

Inductor specs.

LED specs.

If anyone could give me some insight to this issue I would be forever grateful!

enter image description here

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    \$\begingroup\$ How do you regulate the output to produce constant current? \$\endgroup\$
    – winny
    Commented Nov 17, 2016 at 18:27
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    \$\begingroup\$ To begin, you are not feeding the LEDs correctly. The LEDs are supposed to be controlled by current, not voltage. You need to convert your circuit into current-feedback booster, as all professional LED drivers do. \$\endgroup\$
    – Ale..chenski
    Commented Nov 17, 2016 at 18:29
  • \$\begingroup\$ My plan was to use a resistor to limit the current. I have played around with different resistor values, but I just wanted to show that the converter is not outputting enough current with no means of current control. Perhaps I should use a different boost converter? (one with a constant current mode) \$\endgroup\$
    – kevinconnectable
    Commented Nov 17, 2016 at 18:33
  • \$\begingroup\$ To operate the IC at 400kHz, the EN pin must be grounded, not as shown. Your chip is running at 1.2MHz, and 6.8uH inductor might be too much. \$\endgroup\$
    – Ale..chenski
    Commented Nov 17, 2016 at 18:34
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    \$\begingroup\$ For gnd upwards people go to hell \$\endgroup\$
    – user76844
    Commented Nov 17, 2016 at 18:40

4 Answers 4

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You should use a current-controlled boost converter designed specifically to drive series of LEDs instead of simple voltage boost. As an example, the AP3031 chip provides current sense feedback with just 200mV level, so little power will be wasted, which is important in battery-operated environment.

As for the circuit odd behavior, the converter might need some minimal load for the booster to switch nicely.

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  • \$\begingroup\$ I ended up using the AP3031, thanks! Much cheaper than the boost converter I was using. \$\endgroup\$
    – kevinconnectable
    Commented Nov 21, 2016 at 16:40
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You need some resistance between your converter and the leds. If you are thinking that 90 mA is correct because of 3x30 mA this is not the case here. The leds in series should all be using the same 30 mA. You need to take 11.5-3XLED forward voltage divided by 30 mA to get the resistor you need.

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  • \$\begingroup\$ 3x overdrive makes nice toasty LEDs. I'm a bit surprised they didn't burn already. \$\endgroup\$
    – Barleyman
    Commented Nov 17, 2016 at 18:53
  • \$\begingroup\$ Max current is listed as 100mA, so the toast is likely delayed, depending how the LEDs are mounted/soldered to a PCB substrate. \$\endgroup\$
    – Ale..chenski
    Commented Nov 17, 2016 at 19:09
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You can convert this circuit to a constant current generator just as you can with any chip like this. The FB input acquires a voltage level of 1.238 volts when regulating and, if you require a LED current of 90 mA then make R7 = 1.238/0.09 ohms i.e. 13.75 ohms and put the LEDs in place of R6. They've even got three application circuits in the data sheet. here's the last one: -

enter image description here

This should give you some idea and it might be also a good idea to fit R1 (to provide a little closed-loop stability.

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Texas has excellent online design tool. Just open the product page and start webench from there. It'll sort you out faster than anything else.

http://www.ti.com/product/TPS61085

WRT your particular problems, it could be any number of things. Check what Webench thinks of your design parameters.

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