Assemble led chips in an array + driver design

Thread Starter

Leila.Montazeri

Joined Jul 14, 2016
7
Hi, I bought led chips and want to assemble them in a small array of 2x2, 3x3 or 4x4 using wirebonding and packaging. Do I need to connect the anodes of all leds in the same row together? Can I connect the cathodes of all leds directly to the ground or I have to use current sinks?

After packaging my leds, I need to design a PCB in order to drive them. I would appreciate if anyone can guide me through the process.

each led chip is operating at 470nm wavelength (blue), forward voltage of 3.3V @ 20mA.
 

AnalogKid

Joined Aug 1, 2013
10,987
What is your intent for the array?
All LEDs on at the same time, or individually addressable, or what?
Constant brightness or dimmable?
If you are connecting LEDs together, you are in control. You can connect them as common cathode or common anode - the LEDs don't care.

ak
 

hp1729

Joined Nov 23, 2015
2,304
Hi, I bought led chips and want to assemble them in a small array of 2x2, 3x3 or 4x4 using wirebonding and packaging. Do I need to connect the anodes of all leds in the same row together? Can I connect the cathodes of all leds directly to the ground or I have to use current sinks?

After packaging my leds, I need to design a PCB in order to drive them. I would appreciate if anyone can guide me through the process.

each led chip is operating at 470nm wavelength (blue), forward voltage of 3.3V @ 20mA.
So many possibilities. If you are going for simplicity, no resistors. Suggested minimum is just resistors. Better is to use a constant current source. Many variations are possible. What parts do you gave to work with?
 

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Thread Starter

Leila.Montazeri

Joined Jul 14, 2016
7
What is your intent for the array?
All LEDs on at the same time, or individually addressable, or what?
Constant brightness or dimmable?
If you are connecting LEDs together, you are in control. You can connect them as common cathode or common anode - the LEDs don't care.

ak
 

Thread Starter

Leila.Montazeri

Joined Jul 14, 2016
7
So many possibilities. If you are going for simplicity, no resistors. Suggested minimum is just resistors. Better is to use a constant current source. Many variations are possible. What parts do you gave to work with?
Well, I am interested in array of leds that can be individually addressable. I did not decide on parts yet.
 

AnalogKid

Joined Aug 1, 2013
10,987
Individually addressable can be done several different ways. Using a 4x4 array for example, one way to do it is to tie all 16 anodes together and bring out all 16 cathodes separately through 16 current limiting resistors that set the max brightness. That's 17 wires. If you wire them as crosspoints in an x-y wiring array, then you need only 8 wires but the driving methods are more complicated.

ak
 

Thread Starter

Leila.Montazeri

Joined Jul 14, 2016
7
I prefer the first method (simpler). All connected anodes can be directly connected to the ground?

Which parts I need to drive the array using PWM brightness control?

Which parts I need to drive the array using current steering?

Thanks
 

hp1729

Joined Nov 23, 2015
2,304
Well, I am interested in array of leds that can be individually addressable. I did not decide on parts yet.
Ah, like a TV screen.
Processor controlled? Hw mucg degree of control do you want? Would an 8-bit DAC be sufficient? 8-bit PWM? 256 possible levels of brightness?
 

Thread Starter

Leila.Montazeri

Joined Jul 14, 2016
7
Actually, I have a very small array of 4x4 and 8-bit PWM is more than enough. Each pixel of the image is expressed in 3bits and hence, I can use a 8-bit PWM.
 

AnalogKid

Joined Aug 1, 2013
10,987
I prefer the first method (simpler). All connected anodes can be directly connected to the ground?
Nope. All connected anodes are connected to the positive voltage source. The cathodes are connected to GND either through a PWM circuit or a simple switching transistor or a variable current sink circuit.

ak
 

hp1729

Joined Nov 23, 2015
2,304
Nope. All connected anodes are connected to the positive voltage source. The cathodes are connected to GND either through a PWM circuit or a simple switching transistor or a variable current sink circuit.

ak
So in the 4 x 4 you have 16 PWMs, or which ever?
That works.
 

AnalogKid

Joined Aug 1, 2013
10,987
Depends. Even though the 16 LEDs have all anodes connected together, the cathodes still can be multiplexed. So in theory one PWM circuit could be modulated as the multiplexer is addressed for a different brightness for each of 16 LEDs. Messy, but doable on paper.

ak
 
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