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I don't understand what's written in the datasheet (see here on page 3):

Input Voltage Range: min 0.9 Max: Vf-0.2 (Note 1)

Note:  1. VF - LED forward voltage.

What does that mean?

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  • \$\begingroup\$ It means you need to give it at least 0.9V but not more than the voltage the LED needs minus 0.2V \$\endgroup\$ – Trevor_G Nov 16 '17 at 11:42
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First you need to understand what this device does. From both the description and the typical application circuit, we can see that this is a boost converter with output current regulation. That makes sense since it's purpose is to drive a LED from one or two batteries.

One important parameter you need to know is what input voltage range it works over to drive a particular LED. The part you quote tells you exactly that, and quite unambiguously. The answer is that the device needs at least 900 mV in to operate, and the input voltage can be up to 200 mV below what the LED runs at.

For example, if you are driving a LED that drops 3.4 V at the regulated current point, then the input voltage needs to be 900 mV to 3.2 V.

I didn't read all of it, but the datasheet seems to be well written and clear. I really don't see where the confusion is.

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  • \$\begingroup\$ +1 For taking the time to write so much for such an odd question. Nice little part huh. I need to try to remember that one. \$\endgroup\$ – Trevor_G Nov 16 '17 at 11:46
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    \$\begingroup\$ @Trevor: I didn't know this part existed either. Yes, I'll definitely try to remember it too. The first application that occurred to me was my LED headlamp, since that runs from 2 AA batteries in series. Unfortunately this part can only go up to 6 V output, which is no good for the 13 V or so my LED string requires. I guess this part is aimed at things like single-LED flashlights. \$\endgroup\$ – Olin Lathrop Nov 16 '17 at 11:48
  • \$\begingroup\$ Ya seen a number of questions on here where folks have had to add another battery or a boost regulator just to get the V to drop a resistor to a single white LED. This thing would have been a great answer to those. \$\endgroup\$ – Trevor_G Nov 16 '17 at 11:57
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    \$\begingroup\$ @OlinLathrop Thanks for your perfect answer. I was confused by the "-" char in the note. "Vf-0.2" is very close to "Vf - Led forward voltage", it may leads to led forward voltage is 0.2". May be it should be "Note: Vf is led forward voltage. Anyway, it's clear now \$\endgroup\$ – Julien Nov 16 '17 at 12:08

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