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Newly Enlightened
Hello guys, some more findings..
I built constant current drivers for 1 watt and 5 watt LEDs using ST890(or MAX890) which worked really great with higher efficiency, stable light output through out the discharging period and a very small footprint.
I assmebled this circuit on a 10mm by 8mm universal PCB with leaded components. It would be possible to make it even smaller with Surface mount devices for sure.
1) I assumed to use 3 Ni-MH or 1 cell Li-Ion rechargeable batteries for 1 Watt and 6 Ni-MH or 2 Li-Ion for 5 watt (or two 1 watt LED in series) as this device is a linear voltage dropping device and so I have to use batteries which has voltages slightly higher than forward voltage(Vf) of LEDs. As the Vf of 1 watt is 3.2V and that of 5 watt is 6.4 volt, the rated voltage of 3.6V with 3 cell Ni-MH or 1 cell Li-ion is good for 1 watt, rated voltage of 7.2V with 6 cell Ni-MH or 2 cell Li-Ion is ideal for 5W device with the efficency of 88% at rated voltage. As the On resistance of output device of this IC is 0.09ohm at 3V input, it maintains the constant current capability as far as the battery voltage is even 0.1V higher than Vf of LEDs. With the LM317, you need at least 1.2V higher battery voltage to maintain constant current characteristic.
2) Even though the absolute maximum rating of supply voltage is 6V, ST890 worked fine at slightly over 8V which is neccesary for 6 Ni-MH ot 2 cell Li-ion. As I didn't test a Maxim device, I am not sure if MAX890 also works fine or not.
3) With 3 cells of 1800mAH Ni-MH or 1 cell 1850mAH Li-Ion battery, the 1watt LED can run approx 5 hours. With 6 cells of 1800mAH Ni-MH or 2 1850mAH Li-ion batteries, the 5 watt LED can run approx 2.5 hours.
4) The number of external components is very few. You need 1uF (Multi layer ceramic prefered due to size reason) for input filter, 0.1uf ceramic(same reason as 1uF) for output filter, no resistor at fault pin, ground the ON pin, 4.7KOhm at current setting pin for 1 watt or 2.3KOhm for 5watt. You may need to adjust the resistor value to precisely set the current. Simply adding a 100KOhm Potentiometer in series to the current setting resistor makes the light dimmable.
5) You can also put three 1W LED in parallel with 3 cell Ni-MH or 1 cell Li-Ion by tripling the current setting. You may need to insert a small value resistor in series with the LED to balance the Vf variation.
As I do not have my own homepage, I don't know how to post the pictures I've taken. If somebody would like to post the pictures in behalf of me, give me an e-mail ID so that I can send pictures thru e-mail.
Cheers..
I built constant current drivers for 1 watt and 5 watt LEDs using ST890(or MAX890) which worked really great with higher efficiency, stable light output through out the discharging period and a very small footprint.
I assmebled this circuit on a 10mm by 8mm universal PCB with leaded components. It would be possible to make it even smaller with Surface mount devices for sure.
1) I assumed to use 3 Ni-MH or 1 cell Li-Ion rechargeable batteries for 1 Watt and 6 Ni-MH or 2 Li-Ion for 5 watt (or two 1 watt LED in series) as this device is a linear voltage dropping device and so I have to use batteries which has voltages slightly higher than forward voltage(Vf) of LEDs. As the Vf of 1 watt is 3.2V and that of 5 watt is 6.4 volt, the rated voltage of 3.6V with 3 cell Ni-MH or 1 cell Li-ion is good for 1 watt, rated voltage of 7.2V with 6 cell Ni-MH or 2 cell Li-Ion is ideal for 5W device with the efficency of 88% at rated voltage. As the On resistance of output device of this IC is 0.09ohm at 3V input, it maintains the constant current capability as far as the battery voltage is even 0.1V higher than Vf of LEDs. With the LM317, you need at least 1.2V higher battery voltage to maintain constant current characteristic.
2) Even though the absolute maximum rating of supply voltage is 6V, ST890 worked fine at slightly over 8V which is neccesary for 6 Ni-MH ot 2 cell Li-ion. As I didn't test a Maxim device, I am not sure if MAX890 also works fine or not.
3) With 3 cells of 1800mAH Ni-MH or 1 cell 1850mAH Li-Ion battery, the 1watt LED can run approx 5 hours. With 6 cells of 1800mAH Ni-MH or 2 1850mAH Li-ion batteries, the 5 watt LED can run approx 2.5 hours.
4) The number of external components is very few. You need 1uF (Multi layer ceramic prefered due to size reason) for input filter, 0.1uf ceramic(same reason as 1uF) for output filter, no resistor at fault pin, ground the ON pin, 4.7KOhm at current setting pin for 1 watt or 2.3KOhm for 5watt. You may need to adjust the resistor value to precisely set the current. Simply adding a 100KOhm Potentiometer in series to the current setting resistor makes the light dimmable.
5) You can also put three 1W LED in parallel with 3 cell Ni-MH or 1 cell Li-Ion by tripling the current setting. You may need to insert a small value resistor in series with the LED to balance the Vf variation.
As I do not have my own homepage, I don't know how to post the pictures I've taken. If somebody would like to post the pictures in behalf of me, give me an e-mail ID so that I can send pictures thru e-mail.
Cheers..
