After I smashed about 5 mobile phones and collected some white LEDs, I decided to build some some LED searchlight. There are two ways to solve this problem: first, Chinese version, just connect some white LED’s to battery and use some ballast resistor. And watch you spotlight fading while the battery is loosing it’s power. Ante the second, hi-tech, way. To build special LED driver.
almost all recent white LEDs are in fact blue LEDs with special coating to emit missing wavelengths and the result is white color. To use LED and full efficiency, you must supply constant working current to LED. It is possible to connect LED in daisy chain way using only one current ballast, but the voltage must be quite high (~3V per LED) or connect them in parallel. But in parallel mode you must use separate ballast for each LED.
Chip manufacturers make special chips- LED drivers. This chip lets you to connect LEDs in daisy chain and is working from single cell. I received sample from Linear Technologies. It is LT3486, dual channel white LED driver from single Li-ion cell.
It is generic step-up converter with feedback from current shunt resistor and over-voltage protection. In sample circuit there are 2×8 white LEDs connected to chip. But other example is with 2×16!
I thing the limit is only max power on the chip, max voltage on the output (35V) and max current (~25mA typical) per channel. And the number of diodes are not very significant value.
The only bad thing is that chip is “new style”, lead-less and very small…
The circuit is very simple. All component are easy to get. As I love using second hand components, I had some difficulties collecting very small 10μH inductors. Thanks to Samsung digital cameras, I removed inductor from camera’s PCBs. Other component was fast schottky diode- recommended one was ZHCS400 (40V, 400mA), but I placed IR1F4 (10BQ040), 40V, 1A. I don’t know if it is fast enough.
This is first run. Only one channel is working- 9 White LEDs from mobile phone back-light. Power supply is one LiIon cell (~4V). The voltage across LEDs are 29V, empty circuit voltage- 36V. White LEDs are getting quite hot- I made some mistake in calculations and LEDs are overloaded.
The PCB is with errors- I made connection of start pin to Ref pin. But when device is not started, Vref is 0 and device didn’t autostart. I replaced this connection with big blue resistor to Vcc.
This is third time I am re-soldering the chip. It is good that chip is new technology, without LED, RoHMS, so the chip body is very sturdy.
This is populated PCB:
The supply current using one cell is 0.6A, while using 2 cell battery- 0.28A.
Here is full 18 LEDs. It is quite bright device, but I can’t show the effect with my photo camera. The only thing my camera decided, that exposure must be 1 from 500…
After some days…
I tested 6 pcs 40000mCd, 50mA white LED. I can’t compare the output power, but the output is more in one spot, as these LEDs have only 20 degrees output.
