I have been itching to start designing my LED driver for 1 and 3 XM-L LEDs. I found a really good bin of them from a distributor and want to use them since I would get about 4500K color at a high Lumen rate for 700mA of current.
I was searching DealExtreme for drivers and noticed many of the boards had very few chips and some with no inductors at all. Then presto, found one the Chinese manufacture had forgot to scratch the top of the IC off for the picture.
It was an ATMEL TINY microcontroller and what appears to be a diode, some voltage divider resistors probably to power the micro and 3 FETs (7135 35A) is the markings.
So my thought on this is simply this driver is direct drive and the micro is simply using a PWM signal to switch the MOSFETs? :thinking:
But wouldn't that also mean the same "mode" on the flashlight would get dimmer as the single cell battery depleted? :thinking:
So this would not provide for constant current then through the life of the battery???
I think I can hit the ground running once I determine what is considered a "good" driver, but I was hoping to get some input on here.
My thoughts for a 3 XM-L light with the LEDs in series would be I would first need a boost circuit if running off a 2 cell 18650 in series. From the boost circuit would it be as simple as putting a micro controller running a MOSFET with a current sensing resistor to maintain constant current? :thinking:
Or since the boost circuit is maintaining a constant voltage, could I just designate a 99%, 50%, 20% duty cycle PWM signal and hope the current would remain constant due to the constant output voltage from the boost circuit? :thinking:
I have read that PWM allow the color of the LED to remain consistent even at lower outputs like 50% and 20% so what is so bad about PWM????
If it is related to flicker, then why can't the PWM frequency be higher like 120Hz, 200Hz, 30kHz, or 100kHz? :thinking:
Is there a limitation to how high a frequency the XM-L LED can be PWM'd at?
This brings me to the next method using a constant current voltage regulator. Something like this LT3080 configured to deliver a constant current like 3Amp and then tweeking it so that the constant current setting can be adjusted. Is something like this a good method? :thinking:
Datasheet: http://cds.linear.com/docs/Datasheet/3083fa.pdf
What could be the disadvantages other than the LED color shift at lower currents?? :thinking:
The last option I have thought about is using an actual "LED Driver" such as the LT3478
Datasheet: http://cds.linear.com/docs/Datasheet/34781f.pdf
This looks to have the DC to DC converter built in, but this has once again PWM dimming.... so is that bad even though it is at 200kHz or higher?
Or another chip with similar operations, the LT3755
Datasheet: http://cds.linear.com/docs/Datasheet/37551fd.pdf
Thank you for your input.
Clint
I was searching DealExtreme for drivers and noticed many of the boards had very few chips and some with no inductors at all. Then presto, found one the Chinese manufacture had forgot to scratch the top of the IC off for the picture.
It was an ATMEL TINY microcontroller and what appears to be a diode, some voltage divider resistors probably to power the micro and 3 FETs (7135 35A) is the markings.
So my thought on this is simply this driver is direct drive and the micro is simply using a PWM signal to switch the MOSFETs? :thinking:
But wouldn't that also mean the same "mode" on the flashlight would get dimmer as the single cell battery depleted? :thinking:
So this would not provide for constant current then through the life of the battery???
I think I can hit the ground running once I determine what is considered a "good" driver, but I was hoping to get some input on here.
My thoughts for a 3 XM-L light with the LEDs in series would be I would first need a boost circuit if running off a 2 cell 18650 in series. From the boost circuit would it be as simple as putting a micro controller running a MOSFET with a current sensing resistor to maintain constant current? :thinking:
Or since the boost circuit is maintaining a constant voltage, could I just designate a 99%, 50%, 20% duty cycle PWM signal and hope the current would remain constant due to the constant output voltage from the boost circuit? :thinking:
I have read that PWM allow the color of the LED to remain consistent even at lower outputs like 50% and 20% so what is so bad about PWM????
If it is related to flicker, then why can't the PWM frequency be higher like 120Hz, 200Hz, 30kHz, or 100kHz? :thinking:
Is there a limitation to how high a frequency the XM-L LED can be PWM'd at?
This brings me to the next method using a constant current voltage regulator. Something like this LT3080 configured to deliver a constant current like 3Amp and then tweeking it so that the constant current setting can be adjusted. Is something like this a good method? :thinking:
Datasheet: http://cds.linear.com/docs/Datasheet/3083fa.pdf
What could be the disadvantages other than the LED color shift at lower currents?? :thinking:
The last option I have thought about is using an actual "LED Driver" such as the LT3478
Datasheet: http://cds.linear.com/docs/Datasheet/34781f.pdf
This looks to have the DC to DC converter built in, but this has once again PWM dimming.... so is that bad even though it is at 200kHz or higher?
Or another chip with similar operations, the LT3755
Datasheet: http://cds.linear.com/docs/Datasheet/37551fd.pdf
Thank you for your input.
Clint