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I'm trying to tie together a single cell LiPo battery, an MPPT solar charger and a 3W LED

The problem is that the constant current supplies i'm seeing need 6V+ and the LiPo puts out a nominal 3.7

I understand that a big reason why constant current is preferred is because of the asymptotic nature of current draw as a function of voltage, but if i have a battery providing 3.7 (and let's say I run it through a linear voltage regulator), wouldn't this work just as well? If not - why?

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  • \$\begingroup\$ Lithium battery average voltage is 3.7V. Range is 4.2V down to, say, 3.5 or 3.4V, or even lower, depending on many things. If your LED is red or yellow, you can probably use the battery and a linear regulator, although the regulator will get very hot. If you are doing blue, green or white, it probably will not work well with a linear regulator because the battery voltage is not high enough in the lower part of the range. If you don't need to run the LED at full brightness, you may just get away with it. But at 750mA, probably not. \$\endgroup\$ – mkeith Dec 5 '17 at 18:46
  • \$\begingroup\$ It is an awkward problem. If you really need full power out of a white, blue or green LED, you may need to boost then either linear regulate or buck regulate to drive the LED, or use a flyback topology of some sort. \$\endgroup\$ – mkeith Dec 5 '17 at 18:48
  • \$\begingroup\$ @kolosy: See my answer to electronics.stackexchange.com/questions/342121/…. \$\endgroup\$ – Transistor Dec 5 '17 at 19:30
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    \$\begingroup\$ If you can live with (say) 80% of the LiPo cell capacity, you should be able to come up with a constant current circuit that works down to about 3.3 or 3.4V and consider the cell "empty" at that point. (Bonus : the LiPo cell will probably last longer too) \$\endgroup\$ – Brian Drummond Dec 5 '17 at 19:44
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When an LED is powered from a battery it is best to use a 50¢ linear constant current regulator (CCR) rather than a resistor to limit the current. For current over 350mA you will need to use parallel CCRs. A good choice would be an On-Semi NSI50150ADT4G.

The CCR will hold the current constant as the battery voltage drops from full charge.

A $5.00 350 mA Mean Well LDD-350LS CC driver would work well as it can be powered with an voltage between 2 and 28V with up to 95% efficiency. Better efficiency than a resistor in many cases. Still the input voltage must be higher than the out as this is a step down regulator. You could power a red, orange, or yellow LED with 2V. Use an LDD-700LS for up to 700mA.

A 3.6V Li-on or LiPo is still a very good choice for a battery powered LED.

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  • \$\begingroup\$ appreciate the detail, but still trying to understand why it would be better. is it down to efficiency? \$\endgroup\$ – kolosy Dec 5 '17 at 19:13
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    \$\begingroup\$ vs, a resistor, a CCR will keep the brightness of the LED constant from 3.6V down to the discharge cutoff voltage of about 3V. Efficiency is generally a good thing with battery powered circuits. I doubt you'd get 95% efficiency with the LDD at your voltage levels. 90% efficiency would be difficult with a resistor if not impossible when the voltage is a moving target. Typical efficiency with a 1% or 5% resistor and constant voltage is about 83% when a specific current is needed. \$\endgroup\$ – Misunderstood Dec 5 '17 at 19:25

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