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Old 1st February 2010, 03:49 PM   #1
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Default LM22678 switching relay,power supply

I would like to know if I could use a LM22678 as a power supply to supply LEDs from a 12 battary. The LEDs are 3 watt each 700ma draw and forward V. is 3.5 to 4 vdc. Seeing that it will supply 5 amps.Could I just ground one leg of the LEDs and then connect each of the other legs of the LEDs to the out put of the LM22678. I never used any of these,I have only use the small 3 leg voltage regulators. I dont know how to make cir. but I can build from the schematic. I want to connect 6 or 7 of these leds together.
Thank you
John
Would be nice to find a schematic for the above.
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Old 1st February 2010, 06:37 PM   #2
batee is offline batee  United States
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LM1084IT-ADJ is a three terminal regulator that'll handle 5A.

Most people use a constant current circuit rather than a constant voltage to drive higher current LEDs.

Bryan
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Old 2nd February 2010, 02:48 AM   #3
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Default led regulator

Hello batee, Im just looking for a cir. that would run the led,do you know of one.?
It seems that not many people know how to make up this cir. to run like 7 // 3watt leds together.
tks
John
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Old 2nd February 2010, 04:44 AM   #4
batee is offline batee  United States
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OK, so if you're set on using the LM22678, the datasheet probably has a sample circuit. The datasheet is located here:
http://www.national.com/ds/LM/LM22678.pdf
Put the LEDs in parallel, set the output to 3.8V, and it'll probably work.


I haven't used any of the LED driver ICs, but if you're not stuck on one chip, I think they're a better choice. They require a few external components, but the schematic isn't complicated. They allow you to program the current needed and then regulate the voltage accordingly.
LM3404 - 1.0A Constant Current Buck Regulator for High Power LED Drivers

There are schematics in the datasheet for these ICs.

You can also buy power supply modules for a reasonable price:
PowerPuck - LED Supply.com

Bryan
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Old 2nd February 2010, 10:05 PM   #5
TechGuy is offline TechGuy  United States
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Quote:
Originally Posted by conntaxman View Post
I would like to know if I could use a LM22678 as a power supply to supply LEDs from a 12 battary. The LEDs are 3 watt each 700ma draw and forward V. is 3.5 to 4 vdc. Seeing that it will supply 5 amps.Could I just ground one leg of the LEDs and then connect each of the other legs of the LEDs to the out put of the LM22678. I never used any of these,I have only use the small 3 leg voltage regulators. I dont know how to make cir. but I can build from the schematic. I want to connect 6 or 7 of these leds together.
Thank you
John
Would be nice to find a schematic for the above.
You need to convert it provide constant current instead of constant voltage. This could probably be done using a current sensor resistor and a current sense amplifer. Most of the National switchers are designed with feedback that operates on a 1.2x Volts (x is between 0-9) comparator. You would need to design your current sensor output so that it matches the comparator voltage at your desired output current.

Generally, LEDs for lighting are connected in series instead of parallel.

There are also LED drivers designed for this application. Although they are usually limited to Current Output Try searching for "LED driver" or "Constant Current Power Controller"

Linear Tech has several High Current LED drivers

Linear Technology - LT3756/LT3756-1/LT3756-2 - 100VIN, 100VOUT LED Controller
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Old 27th February 2010, 09:27 PM   #6
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Default leds

Well the cir. that I found on the internet works very well. It lets you connect up 3 three watt led in series ,and they are very bright ,here is the link if anyone was looking to do this.
1.5A Constant Current Linear Regulator for LEDs for
Tks to all
Johnny

Quote:
Originally Posted by TechGuy View Post
You need to convert it provide constant current instead of constant voltage. This could probably be done using a current sensor resistor and a current sense amplifer. Most of the National switchers are designed with feedback that operates on a 1.2x Volts (x is between 0-9) comparator. You would need to design your current sensor output so that it matches the comparator voltage at your desired output current.

Generally, LEDs for lighting are connected in series instead of parallel.

There are also LED drivers designed for this application. Although they are usually limited to Current Output Try searching for "LED driver" or "Constant Current Power Controller"

Linear Tech has several High Current LED drivers

Linear Technology - LT3756/LT3756-1/LT3756-2 - 100VIN, 100VOUT LED Controller
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