# How to best drive 100W RGB LED - any suggestions for ICs?

#### -live wire-

Joined Dec 22, 2017
912
I have this 100W RGB LED, with each color having a max current of ~1A and a voltage of 30-35 Volts (except red which is ~25 V).

Here are my goals:
-I want to drive each color independently with a constant current of 1A at those voltages, as opposed to using a resistor or some other means of driving them.
-I want to be able to control it with the ESP-32, which I believe outputs 3.3V. So I need a circuit to convert it to a 5+V signal or an IC that takes 3.3V input.
-it should be a reasonably efficient circuit where only the LED needs active cooling, not any of the components controlling it

So here is where I need help. Is there an IC that can output those currents and voltages with just, maybe a 40-60V DC power supply and some passive components? Are there some that take 3.3V input? Also, it would need to be through-hole and not SMD because I'm not that good at soldering. I tried looking on mouser but I wasn't really sure if the ICs that were showing up would work, and I didn't know what some of the specifications meant.

Also, all of the constant current boards with those output currents and voltages were $20+ and bulky, and I don't want to spend$60 when there is an easy \$5-10 DIY solution.

#### -live wire-

Joined Dec 22, 2017
912
Okay, thanks. I'll have to look into that more. What value inductor should I use, though? I didn't see anything in the datasheet.

#### Sensacell

Joined Jun 19, 2012
2,673

#### -live wire-

Joined Dec 22, 2017
912
I'm confused, doesn't Vout vary based on the load if it's constant current, and also doesn't the constant current you want vary based on the input signal?

#### -live wire-

Joined Dec 22, 2017
912
Do I just use the LED drop voltage or what?

#### Sensacell

Joined Jun 19, 2012
2,673
Near the end of the application note, they break down the calculations.

You will need to know the Vf of your LED.
Follow the reference design circuit as a start, calculate a new inductor for the LED Vf and supply voltage you are using.

The 2 critical things to get right are the value of Rcs and the inductor value and saturation current.
A Schottky diode is a must.

#### -live wire-

Joined Dec 22, 2017
912
Near the end of the application note, they break down the calculations.

You will need to know the Vf of your LED.
Follow the reference design circuit as a start, calculate a new inductor for the LED Vf and supply voltage you are using.

The 2 critical things to get right are the value of Rcs and the inductor value and saturation current.
A Schottky diode is a must.
Since it's supposed to be efficient, a 360 mOhm resistor for Rcs with a power rating of 3W is almost overkill, right? And I attached an image of the Vfs. The way the LEDs are configured its a 10s 3-4p grid for each color.

Also, is diode D a Zener or shottky diode? Looking at the image, It's hard to tell which way the lines are going and everything, and I haven't done much EE stuff in a little while.

Finally, is it wrong to assume that an 860uH inductor is the best here because on this graph for different Vfs it appears to have the highest efficiency?

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#### Sensacell

Joined Jun 19, 2012
2,673
A 360 milliohm resistor with 1 amp flowing through it will dissipate 360 milliwatts. (I squared over R)
A 1/2 watt unit is sufficient.

Diode is a Schottky - see the reference circuit BOM.

Yes- it's naive to assume that an "860 Uh" inductor will produce the best efficiency.

Efficiency is about losses- which come from every part in the circuit.
The wrong inductor core material, too low saturation current rating, the wrong diode, etc - can all substantially reduce efficiency.

Build the reference circuit- a good place to start.

Making this work is relatively simple- making it efficient requires deep optimization, which requires a deep understanding of the circuit operation.

#### -live wire-

Joined Dec 22, 2017
912
I don't need 99% efficiency, just enough that the amount of heat is reasonable. But are you saying to even get 80-90% efficiency you have to understand it very well?

#### Papabravo

Joined Feb 24, 2006
14,846
A 360 milliohm resistor with 1 amp flowing through it will dissipate 360 milliwatts. (I squared over R)
A 1/2 watt unit is sufficient.

...
I'm sure you meant $$I^2R$$ and not $$\frac{\;I^2}{R}$$

#### Papabravo

Joined Feb 24, 2006
14,846
I don't need 99% efficiency, just enough that the amount of heat is reasonable. But are you saying to even get 80-90% efficiency you have to understand it very well?
Absolutely - this is not really a good place for dabblers.

#### -live wire-

Joined Dec 22, 2017
912
So realistically what kind of efficiency would I get then?

#### Papabravo

Joined Feb 24, 2006
14,846
So realistically what kind of efficiency would I get then?
On my 1st SMPS I was aiming for 85% and I ended up around 65%. This was mostly due to poor choices for the inductor and not making a PCB.

#### -live wire-

Joined Dec 22, 2017
912
If I'm driving a 100W LED, and it's 60-70% efficient, then where's the 50W of power being dissipated? In the IC? The inductor?

#### Papabravo

Joined Feb 24, 2006
14,846
If I'm driving a 100W LED, and it's 60-70% efficient, then where's the 50W of power being dissipated? In the IC? The inductor?
In the LED itself. According to the datasheet you have 30-34 Volts at 2800-3500 mA which gives a range of 95-105 Watts. That is your power output. Your power input will be larger. If your efficiency is 60% then it will be 100/0.60 = 166.67 Watts. That difference will be consumed by everything except the LED, including the resistor, the inductor, and any IC's that are in the path between the input power and the LED.