I need a very small and light 3 W flashlight/headlamp for a kind of specific purpose.

I want to use two AAA Eneloop cells.

In the datasheet there is discharge curve at 1.6 A, so I guess I can discharge them safely with that current.

1.6 A at 1.2 V is 1.92 W per cell, so when I use two, there will be 3.84 W available power. If I can archieve ~80% efficiency, I can use a 3 W LED and batteries will last for 30 minutes (it is enough for the purpose of that flashlight).

What I have found so far:

I found the LT1618 (datasheet link) converter which looks just perfect for me:

  • Minimum input voltage 1.8 V
  • 1 A current
  • May work as a constant current and constant voltage source
  • High efficiency (maybe I could reach 80%, if not - 70% will satisfy me)

My problem:

It is hard to get LT1618 where I live.

My question is:

What other options I have?

  • \$\begingroup\$ You've done a good job of describing your application. Let's hope that responders focus on circuit designs that meet your needs, rather than product recommendations, which would be off-topic. \$\endgroup\$
    – Dave Tweed
    Nov 8, 2014 at 14:51
  • \$\begingroup\$ I know what you mean, but I think rules about recommendations on this site exist to prevent manufacturers or marketing companies to glorify their products and to prevent arguing between fans of diffrent manufacturers. I'm not asking "which product is better" or "shall I buy PC or Mac" :) \$\endgroup\$
    – Kamil
    Nov 8, 2014 at 17:14
  • 1
    \$\begingroup\$ No, that rule exists because such answers quickly go out of date as manufacturers introduce new products, and have no lasting value for the site overall. \$\endgroup\$
    – Dave Tweed
    Nov 8, 2014 at 17:34
  • \$\begingroup\$ "What other options I have?" How would anyone know what is available in your part of the world? \$\endgroup\$
    – Andy aka
    Dec 8, 2014 at 18:23
  • 1
    \$\begingroup\$ You've told us what you can't get; what about what you can get? Otherwise, we're going to suggest things, and you're just going to say, "Nope, can't get that." For example, can you get discrete MOSFETs and small microcontrollers (e.g., PIC10Fxxx)? \$\endgroup\$
    – Dave Tweed
    Dec 11, 2014 at 1:32

2 Answers 2


Design is largely about choices and options. To get anything done there will first be some baseline choices that narrow down the universe of options. Given an objective, some choices are obvious and others, especially the later more detailed, are not obvious at all. If the options suggested by the baseline choices turn out to be undesirable, it's time to change the baseline choices.

In this case the baseline choices are: using 2 Eneloop AAA batteries in series to power 3W worth of visible white light LEDs. The batteries are relatively well defined here, but explicitly, range is 1V < \$V_B\$ < 1.6V at 750mAh. Or, a source voltage range of 2V < \$V_S\$ < 3.2V. For the LEDs, just to keep things moving, figure use at 3V and 1A. We'll look at both of the input and output choices and see what kind of options these suggest. This will involve looking at source and load characteristics to decide what power topology will be most effective to use.

  • Input voltage less than 3.2V to supply ~3V says that a Flyback, SEPIC, or Boost (barely) would work.

  • Batteries like, as much as possible, constant current loads. So, to keep the input capacitance to a minimum (maybe zero), the Flyback is out, leaving the Boost (barely) or SEPIC. As an aside, should also leave out the output capacitor, since LEDs are current devices. This will also make any loop compensation a little easier.

  • Desire to operate the batteries down to the nubbin of 1.8V will mean minimizing conductive loss in the power circuit. That means no extra line length or switch to turn the thing on or off. So, the Boost is out since turning a Boost off requires an extra switch. SEPIC it is, since on/off function can be a micro switch to pull the chip enable down.


You would look for a SEPIC capable IC of either generic power supply or LED specific form. Preference goes to the LED specific form, because that will be minimal parts. The generic form would require adding a current sense amplifier. For more information about SEPICs this app note (slyt309) from TI is a good place to start.

Some numbers to narrow the search:

  • Duty Cycle (DC) at \$V_S\$ = 1.8V

    DC = \$\frac{V_o}{V_o+V_S}\$ = \$\frac{\text{3V}}{\text{3V+1.8V}}\$ ~ 0.63

  • Inductor ripple current (coupled Inductor): \$\text{$\Delta $I}_L\$ = \$\frac{\text{DC } V_S}{2 L f_{\text{SW}}}\$ = \$\frac{\text{1.8V 0.63}}{\text{2(5uH)(500kHz)}}\$ = 0.225A

  • Peak switch current: \$I_{\text{Qpk}}\$ = \$I_o\$ \$\frac{1}{1-\text{DC}}\$ + \$\text{$\Delta $I}_L\$ = \$\frac{\text{1A}}{\text{1-0.63}}\$ + 0.225 ~ 2.9A

Look for a SEPIC capable LED controller that can operate down to 1.8V input with a peak switch current of 2.9A. These requirements are both on the margin. There are chips that will start at about ~2.3V and operate down to 1.8V. Also an external switch might be needed as ~3A on an internal switch is rare.

Here is a sample search for LED controllers at DigiKey. This is just an example of what search results would look like. Other suppliers like Farnell (Newark) or Mouser would give similar results.

No one, except maybe your next door neighbor will know what parts you can actually get where you live. It's up to you to find a part. If it turns out that you can't find a suitable part, it's time to change the starting point. Most simple would be to add a third battery to up the source voltage. A SEPIC would still be a good choice here.

If it turns out that you absolutely have to operate down to 1.8V and it is not possible to get an IC where you are that can do that, you might have to go to crazy options. As an example of crazy option there is this solar-cell light, a discrete circuit.


You didn't specify the forward voltage on the LEDs, which makes a huge difference. Since two rechargeable AA batteries in series will have an open circuit voltage of 2.4V, if your LED(s) has a Vf above that then you may need to use a different solution than if it is below. HBLEDs have a high Vf; for a 3W LED a Vf of 3.5V or higher is normal.

So, if you have a supply voltage of 2.4V from the AAs, and a Vf of 3.5V for the LEDs, then you will need an LED driver IC that is a boost converter like the LT1618. Or make your own, but it's much easier to just buy one; there are many out there. Generally these are more of a pain than buck converter drivers. If you want to make your design easier, then add another AA battery so that your supply voltage is 3.6V; then you can use an easier buck converter IC.

As for which IC is best, there are many options out there, from vendors like TI, National Semi, etc.


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