I decided to write this quick tutorial for two reasons: First because there are many people who would like to know more about driving and controlling LED lights, and second because i was provided an excellent LED driver chip from Farnell for test, and i wanted to put it under the microscope. So i will place this chip against some other LED drivers to see how good it is.
The chip that I'm talking about is the A6210 from Allegro Microsystems. It is a Buck-Regulating LED Driver able to drive up to 3A load with constant current, with switching frequencies up to 2 MHz and supply voltage from 9 to 46 volts. It has an optional PWM input to control the brightness of the LED. The sense voltage is down to 0.18 volts for higher efficiency.
The previous description may sound Greek to you, but after reading this tutorial you will be able to design your own LED driver. Special thanks to Farnell and Element14 for offering three of these chips for test.
After having explained the basic methods to drive LEDs with constant current, i explain how can someone modify these circuits to control their brightness
@Ian for 90% efficiency you will need other type of circuits, not a linear driver. An SMPS instead
At 14 January 2013, 5:22:32 user Ian wrote: [reply @ Ian]
Hi Giorgos,
I'm wishing to use currents between 10mA and 1A depending upon the type of LED connected. I'd like the efficiency to be 90% plus if possible. I've seen Rds(on) values as low as 0.021 ohm, which I'd guess would translate into less power losses?
I'm guessing that from a 5v PIC micro that a logic level mosfet is the only way to go as opposed to a normal mosfet?
@Ian depends on the current and your efficiency requirements. Typically, on resistance of mosfets is very low. In many cases it is lower than 1Ohm (not high current mosfets). What current requirements you have? More than 1 amp?
At 11 January 2013, 9:49:19 user Ian wrote: [reply @ Ian]
Hi Giorgos, Thank you for your help with current measurement. Can you also tell me in the selection of a power MOSFET if Rds(on) should be as low as possible or doesn't it matter? I'm presuming the MOSFET is operating in saturation mode or would be if PWM was applied to the gate? Thank you, Ian
Circuits are plenty for pwm control of led brightness. It is agreed that there cannot be a linear control of led brightness. Assuming that the pwm controls applied and the led current exceeds the 20ma limit in case of 4mm and 5mm Leds and 350ma in one watt, how a feed back control either shuts off the led or keeps the led current at maximum is yet to be published
At 2 October 2012, 8:17:29 user Ajay wrote: [reply @ Ajay]
Can you prove it's stability ? Just by looking at the diagram I think it might oscillate, eventually at high frequencies, and eventually amplify the thermal noise.
@Lupin Thank you very much for pinpointing those mistakes. You are right for both. The first one is already corrected, as for the second i will recompile the video. Thanks for noticing it soon.
At 25 February 2012, 14:54:05 user Lupin wrote: [reply @ Lupin]
Just wanted to mention, tha in the example it seems you forgot to subtract V_Z in the calculation of I_RB_MAX.
On the other hand, I got an issue on how you calculated P_RB in the video (around 5:50). You added the base current to I_RB_MAX. Isn't that already included?
Still many thanks for all the explanations and videos! I'm teaching a beginners electronics course (diodes, transistors, opamps) next semester at uni. Teaching applications, when you got to do theory first always falls short. I guess I'll refer some students to your page if they want to know more.
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