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I want to supply constant voltage to my LED's but the supply varies from 3.7V to 4.2V (Li-ion battery). I cannot use 3.3V voltage regulator as it will need at least 4.3V voltage at the input but my battery can supply only 4.2V max. so is there any other way I can supply regulated 3.3V to my LED.

  • RGB LED = 3.3V, 60mA
  • Battery = 3.7V - 4.2V, 3000mA
  • Number of LEDs = 24
  • The LED's are connected in parallel
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    \$\begingroup\$ You can get an LDO regulator with less than 400mV dropout voltage- or make one. 400mV is probably plenty - assuming constant voltage is really what you need. Usually controlling LED current is what you should do. Feeding 3.3V to the RGB LEDs will not give good results (too bright + damage or too dim). \$\endgroup\$ Aug 28 '17 at 5:32
  • \$\begingroup\$ What is the RGB LED, in particular? Do you intend on PWM (each or all at once?) Or is this all going to be ON/OFF? How much variation of current are you allowing yourself, one LED to another? One color to another? Do you need any accuracy or do you just need a very rough approximation across the board? What do you care about (IC/discrete, cost, size, efficiency, etc.) (Note that you don't have much overhead control voltage available.) \$\endgroup\$
    – jonk
    Aug 28 '17 at 6:10
  • \$\begingroup\$ iam using ws2812b (RGB)... pwm all at once. (same color) accuracy is main concern... \$\endgroup\$ Aug 28 '17 at 16:03
  • \$\begingroup\$ use a DC-DC boost+buck converter (all in one) so any input can be any output. they can be had for a few USD. bonus points for getting one with a CC potentiometer. \$\endgroup\$
    – dandavis
    Aug 28 '17 at 19:44
  • \$\begingroup\$ like TPS63027 from TI ? i planned to use that but the ic is too small with BGA and i dont have the right tool so ended up destroying the ic. Do you know any other ic which has DIP package?? \$\endgroup\$ Aug 29 '17 at 5:46
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LEDs shouldn't be driven at constant voltage, they should be driven with specified current, leaving the forward voltage whatever it will be.

The reason is that the LED's forward voltage drops with increase in temperature, see, for example, this white paper from Osram. As an example there, the Vf drops by 0.2V with delta T of 60C, so a 3.3V LED will have 3.1V at the same current. This means that continuing to drive the LED with 3.3V will move the working point (current) up. With a typical steep I-V curve of LED it might draw maybe 2X of initial current, which will heat up the chip more, and thermal runaway may occur.

In this particular case (24 LEDs from one Li-ion battery), one solution (engineeringly correct) is to use a boost converter, with multiple outputs into several chains of LEDs. The other simplistic "solution" would be to put resistors in series with every LED, 5-7 Ohms value, and let it go naturally.

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  • \$\begingroup\$ i understand that i have to add a resistor for each led in parallel to limit the current but when one finds a value of a resistor he knows the input voltage supply(say 5v) and in my case the voltage will drop from 4.2v to 3.7v ... so how do i find resistor value ?? \$\endgroup\$ Aug 28 '17 at 15:56
  • \$\begingroup\$ @NitinSharma, since the supply voltage is variable, a resistor limit will not work well. That't why you need a "LED Driver", a constant-current supply per each LED. Using a simple resistor will be very suboptimal. You need to start with worst case, 4.2V. Minus 3.3V makes the drop of 0.9V. For 60 mA you will need 900/60 = 15 Ohm resistor. But when the battery goes down to 3.6 V, your current will be just 300/15 = 20 mA, and brightness will change. \$\endgroup\$ Sep 2 '17 at 16:58
  • \$\begingroup\$ thanks for your replies everyone.. i will use buck/booster and led driver to see which gives me the best result. \$\endgroup\$ Sep 4 '17 at 5:33
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No, you don't want to supply a constant voltage to your LEDs, you want to supply a constant current.

You will let the LEDs decide what voltage they'll have at that current.

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  • \$\begingroup\$ i understand that i have to add a resistor for each led in parallel to limit the current but when one finds value of a resistor he knows the input voltage supply(say 5v) but in my case the voltage will drop from 4.2v to 3.7v ... so how do i find resistor value ?? \$\endgroup\$ Aug 28 '17 at 15:57

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