Constant Current Source for the control of 960 Leds - Page 1
Forum section: electronics / beginners

author: firstoption
author_rank: Newbie
timestamp: April 22, 2014, 08:57:46 am
content: Good day to all,Please i need your guide and support in the design of a Constant current source for the control of 960 Low power Leds Brightnes using PWM Signal.I have 8 pieces of PCB boards with each board having 120 Leds on it.The Leds are arranged in Parallel and Series.So each board has 24 Clusters x 5(a Cluster is made up of 5 leds in parallel)which is equal to 120 leds.Each of the Cluster is connected to another Cluster in series.The Led operating voltage is 3.2V @ 20mA.From this information i calculated the required voltage for each board to be 76.8V7/100mA.After going through the Internet i came across this Led Controller(LT3756) from linear Technology which has an Input volatage range from (6V-100V)DC and an Output voltage up to 100vdc with current-source Regulation.My assumption is that this Led Controller(LT3756)will be suitable for my design.But i have two question concerning the use of the LT3756.1--Do i need to use 8 pieces of LT3756(one for each board) for the design OR one Piece of LT3756 will be enough since i want to use a constant-current source to drive the leds.2--I have a power supply which i made use of in my last design,this power supply has an Input range of (3.3V to 32V)dc and an Output volatage range up to 30Vdc with constant Output current of 6Amp.Will this power supply be enough to power the LT3756 led Controller which will then drive and control the led brightness using the PWM Signal it(LT3756) generates.I would be very grateful if somebody could put me though with respect to this design.thank you for the usual suppot.best regards.Firstoption.

author: Juun
author_rank: Newbie
timestamp: April 22, 2014, 12:31:00 pm
content: Hello firstoption,It's never advised to power LED's in parallel with a constant current source, the spread of the current won't be equal between the various "clusters", as the voltage drop on every LED will be somewhat different.If you have a look at the data-sheet of the LT3756 you won't find a schematic with LED's in parallel.Maybe it would be easier for you to wire the LED's up as in a standard LED-String (the ones you can by on wheels and cut every ~10cm) They use a constant voltage supply and a resistor for each "cluster" of LED's. Afterwards you can PWM the whole assembly.If you want to stick with the LT3756, I would advise you to use one for every string of LED's and work with the highest voltage possible. Don't forget that the actual switching is not done in the LT3756 itself. It uses an external MOSFET, which you have to choose.Good luck!

author: firstoption
author_rank: Newbie
timestamp: April 22, 2014, 01:46:11 pm
content: Sir,thank you for the response and the suggestion,i really appreciate your Kind gesture.i equally understand that parallel arrangement of Leds is not a good Option but the problem i am having is that the boards were not made by me.The board had already been made,i was only asked to design a circuit that can be used to control the brightness of the leds using PWM signal.That is why i am looking for possible configurations that will enable me to control the led brightness.Any Suggestion and idea in this regard will be hihgly appreciated.once again thank you for the suppport.Best regards.Firstoption

author: Juun
author_rank: Newbie
timestamp: April 22, 2014, 02:19:23 pm
content: You're welcome.In this case the use of the LT3756 is actually a reasonable choice, because you need the high voltage for the LED's.For the power supply: Why do you want to use a DC to DC supply in front of the LT3756, which has already buck-boost capabilities? Is it battery powered?Anyway you need 960 times (3.2V*20mA) 64mW = 61.44W of power. Let's say the overall efficiency is 80% (to be save), so your power supply must deliver about 77W at max.