11. January 2015
Tiny LED booster
I own an older head-lamp (one that has a rubber band which you wear on your head) that still operated with a light bulb.
But LED technology has made impressive progress in recent years and therefore I had the idea to transform this lamp to a LED light source.
From TI there is a device LM2731 in a compact SOT-23 package.
The standard application (see above) is quite usable for LED applications with only a small modification.
The LED needs to be connected between the output diode (D102) and the feedback input (pin 3). A resistor between the feedback pin and ground then defines the LED current.
The resistor value is calculated from the desired LED current and the feedback voltage of 1.23 V.
With a LED current of 25 mA the resistor value results in 1.23 V/0.025 A = 49.2 Ω.
A higher value reduces the LED current, a lower value increases it.
This modification does not care about the number of LEDs connected in series because only the current through the sense-resistor controls the output voltage. The LM2731 can generate up to 22 V output voltage and therefore drive up to 6 white LED in a single string.
Coil L102 needs to be modified accordingly, naturally.
Of you want to protect the circuit against an interruption in the LED string (and therefore producing a too high output voltage) you can attach a resistor parallel to the LED (string) directly on the board (example value 890 Ω when using a 50 Ω sense resistor).
