efficient LED driver

I am planning to mod led lenser K3 w/ red XPE. Vf is 2.1V @350mah and 2.3V@700 mah (max). It will be powered by single 10280: 3.6v/180mah

My req:
>It has to be tiny because the only space is between Led PCB and pill. see 4th and 5th pic:
https://www.candlepowerforums.com/vb/showthread.php?t=244570

>It has to be as efficient as possible.
I am considering TPS62225 buck converter. It is small and requires only 3 other parts: inductor, input capacitor and output capacitor.

High-Efficiency Synchronous Step-Down Converter With up to 95% Efficiency
2.5-V to 6-V Input Voltage Range
Fixed 2.2 Output Voltage
Up to 400-mA Output Current
1.25-MHz Fixed Frequency PWM Operation
Highest Efficiency Over Wide Load Current Range Due to Power-Save Mode
15 µA Typical Quiescent Current
Soft Start
100% Duty Cycle Low-Dropout Operation
Dynamic Output-Voltage Positioning
Available in TSOT23 Package

https://focus.ti.com/docs/prod/folders/print/tps62225.html


>Over discharge protection/warning. The above regulator needs min 2.5Vin. what happens when the battery voltage drops below 2.5? will it cut off or drop Vout below 2.2v to give some kind of warning?

>This not a req but would be very nice to have: modes. Any idea how I can implement this? I have seen this only in current regulated drivers so I am not sure if it is even possible w/ above buck converter. Is there any current regulator that would meet all my req and offer modes??

Thanks for your help.

Hi,

any converter between power source and LED will consume extra power. Try to get the best solution by reducing the forward current of the LED for maximum battery endurance.

Boncuk

The lithium battery is destroyed if its voltage drops below 3.0V so your circuit must detect a low battery voltage then disconnect the battery.

The LED needs a current regulator, not a voltage regulator.

The real eficiency may be in the 80%. (power output / power input). And yes, you need a current regulation over the voltage output.

Go to diodes.com and look for DN81. It has 107 pages of informaiton on LED power supplies.
IT and Linear have good parts.
PWMs that are used in LED light bulbs have a feed back voltage that is very small. Usually 0.5 to 0.1 volts. (0.05)
To get long battery life and long LED life use a PWM.
I never (almost never) use an undervoltage lockout to "save the battery". {audioguru} In your circuit the LED will start to get dim at about 3 volts on the battery.