efficient LED driver

[huzefasi]

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.

 

[Boncuk]

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

 

[audioguru]

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.

 

[vmsa]

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

 

[ronsimpson]

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.