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I have a LED strip (16ft Hitlights Luma5 warm-white high-density) powered by a constant voltage supply. I'm considering adding PWM dimming with a Maxim MAX16820 constant current driver. The PWM signal is generated by a microcontroller monitoring a pot for brightness control. One difference from the reference circuit is that the LED strip contains parallel sets of three series LEDs with a current limiting resistor.

Some sources suggest that a hybrid architecture with a constant current driver and a constant voltage supply are beneficial (e.g. this reference).

In my case, is there anything that makes this a bad topology, or traps to consider? And what are the benefits of this setup?

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  • \$\begingroup\$ Have you considered just using a FET? As in here: electronics.stackexchange.com/questions/67775/… It will be much simpler and cheaper. Since its only 16ft you are controlling I dont see why make it much more complex. \$\endgroup\$ – Wesley Lee Mar 3 '15 at 3:15
  • \$\begingroup\$ @WesleyLee--I would still need a MOSFET but thought I could reduce the component count (no separate voltage regulator needed for the micro or snubber/compensation), board size, and layout complexity (especially if I want to add more drivers to one board to control several strings). \$\endgroup\$ – jt lancer Mar 3 '15 at 14:29
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The point of using a constant current driver is to avoid the use of less efficient current limiting items (such as resistors). Since you already have resistors in the LED strip acting as the current limiter, adding a constant current IC and other necessary components would be more detrimental than beneficial in your application by adding not only the complexity but also the inefficiency of both methods. Also with a constant current driver you will be unable to change the length of the strips you are using without having to readjust the constant current. With a constant voltage you can just add or remove strips or length to the same driver provided it can handle the load, since the current limiting will be embedded in the strip. Using a fet to drive the strips is a common method, if you are worried about this method of control.

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  • \$\begingroup\$ Adjusting the current for different lengths did seem a pain. I hadn't considered the efficiency aspect. Thanks. I'll stick with FETs for these strips. \$\endgroup\$ – jt lancer Mar 5 '15 at 0:55
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I'm going to suggest that the constant-current driver isn't appropriate for your particular LED strip. PWM is your better choice.

The reason that I'm suggesting that constant current isn't appropriate can be seen if you connect your LED strip to a variable-voltage power supply and slowly drop the voltage. You will see the brightness start to drop and then you will see that some LEDs are brighter than others.

The lower the supply voltage, the more the brightness disparity will be.

The reason for this is that the LEDs are not identical. Even if they came from the same manufacturing batch (they most likely did), there are still minor variations in their forward voltage. This isn't a problem when there is significant voltage across the current-limit resistors. However, reducing the supply voltage reduces the voltage across the series resistors and now the individual LED forward voltage variations become noticeable.

And that's what that variable current supply is going to do. As you drop the total current into your LED strip, the voltage also drops.

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  • \$\begingroup\$ My understanding of the driver from the datasheet (pages 4 and 5) was that it outputs a fixed current (with about 10% ripple) in sync with the PWM signal. So the current is either all on or off as dictated by the PWM input, and not variable. Am I not understanding this correctly? \$\endgroup\$ – jt lancer Mar 3 '15 at 14:17
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Powering LED Strips with a Constant Voltage(CV ) Supply and Constant voltage Driver is best suitable matching.

The dimmimble led strip is available for CV type,also Constant Currenct (CC) type. Just for difference driver .

CV led strip 12V/24V/36V/48V and so on, and CV driver with output voltage 12V/24V/36V/48V , out put current is 0~ 5.0A 0~3.0A , 0~1.7A

CC led strip is 350mA/700mA/1050mA , CC driver with output voltage 350mA/700mA/1050mA,output voltage is 24-48v, 16-24v,12-16v

Please kindly try to compared the constant current and constant voltage driver . No matter the led products is LED strip, led down light and or other led light , it is same way to Choose the corresponding power type.

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