What happens to the AX2002 when input voltage is below 3.6V?

Nil Einne

Newly Enlightened
Apr 15, 2010
New Zealand
As the title says does anyone know what the AX2002 does when input voltage is below 3.6V? I'm obviously thinking of the case when Vf isn't a factor. In theory it should drop out of regulation but how?

Some background, I'm looking at using the AX2002 for driving a (or multiple) red power LEDs. Since these have low Vf usually under 2.8V, Vf isn't really a factor with a single li-ion or 3xNiMH so with the right driver it should be able to stay in regulation until there's very little power left in the cells. However the AX2002 datasheet says it requires a 3.6V input voltage. With white leds this isn't usually a factor since with the 0.25V drop and high Vf, you don't need to reduce voltage further to limit current, your LED may dim is all. And actually I think I've seen some testing suggest you really need about 0.6-0.7 Vout from Vin for proper regulation.

But with red LEDs things aren't so simple. Even at say 3 V you may still have headroom let alone 3.5V even if you need 0.6-0.7. So if the AX2002 basically stops regulating current and passes the voltage (- voltage drop), I'm guessing you may kill the led or at least it'll get very hot (and possibly perversely brighter) unless you put a resistor or something to protect it (thereby likely reducing the efficiency).

I've looked at the datasheet and didn't see anything which offers any explaination but I'm not that great with electronics. Couldn't find anything from searches either. Does anyone know? It has occured to me that Axelite may just have never really thought much about the red/yellow LED market much, nor those using li-ion and 3x NiMH therefore it's actually fine below 3.6V just not officially supported.

P.S. I appreciate that there doesn't tend to be much power left in li-ions below 3.6V although I think there may still be some in NiMH depending on the type of cell. In any case it remains important.
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