High power IR LED driver - Battery powered

[Magnets]

I want to drive a 3w IR LED using AAs, I need a minimum of 3xAAs to power a micro.

The LED is rated 1.4-1.6v 700mA but I don't mind if I only run it at 350-500mA

I've tried using 3xAA (~4v) + a linear AMC7135 (PDF: http://led22.ru/ledcat/ledsource/amc7135.pdf) but obviously the power wasted through the driver is more than the LED uses.

Using 4xAA (5.5v) + a small buck (mp1584) + AMC7135 is better but the AMC7135 needs 2.2v to work - it's rated at 2.7v (Vs) so it's still not very efficient. I get around 60%

Are there any small linear drivers that will run from 1.5-2v supply? The only ones I could find are the BCR321U and BCR320U but I'm not entirely sure what their minimum supply voltage is, the infineon site says 1.4v min, but the datasheet doesn't mention it. Or maybe some off-the-shelf switch mode drivers?

I think the best bet is to just get 2x3W leds in series and use 1xAMC7135 but I would rather just use 1 LED.

[Kleinstein]

When near empty 3 Alkaline cells is something like 3.3 V. This is well enough for 2 IR diodes in series with a linear driver. When not using the maximum current, the voltage should be more like 1.1-1.4 V. For linear current control one could also use an standard current sink circuit with OP and low level MOSFET.

For just 1 LED one could used a switched mode current sink, not voltage regulator and linear current regulator. Many switched mode regulator chips can also work in constant current mode, preferrably those with a low voltage at the FB pin.

[bktemp]

TS19377 should work fine at 3-4.5V and the LED brightness can also be controlled by PWM.
http://www.taiwansemi.com/products/datasheet/TS19377_A11.pdf

If you look for other LED drivers, some boost converts can also drive LEDs with a low forward voltage by connecting the LED cathode to the input voltage instead to GND. There are probably more boost converts capable of running at 3V than buck converters.

[Magnets]

The TS19377 looks good, I see there are clones (AX2002) which come in full modules