WS2801 w/ 3LEDs on single chip

Hello,

I want to control 3 RGB 5050 LEDs with single WS2801 chip on a 12V. I know I need to use 5v zener diode, resistor and a filter cap to make chip work with 12V supply. Although, few things concern me:

  1. Should connect resistors to GFB/RFB/BFB (feedback) pins? As datasheet says, resistors on these pins are used to control current output. If I do not need control of output corrent, I can leave them float or I should connect them to gnd?
  2. How to connect LEDs? What about limiting resistors on LEDs? I can’t use resistors. As far I know, there are two options:
    a) Connect LEDs in series. I have seen this type of connection on LED pixels. There are 2caps, diode, resistor and extra 3 resistors as current limiting ones for LEDs. (e.g.)As I see from WS2801 datasheet, max. output voltage is 7V. If min. voltage for 5050 led is 2.8V, 3 leds in series give 8.4V and that exceeds 7V(red should work fine, right? 1.7V*3 = 5.1V). What are these limiting resistors for in that case?
    b) Connect single LEDs to chip’s output. In that case, every LED would require each own 3 limiting resistors, resulting in 9 extra resistors which does not seem as great solution.

Basically, I’d love to hear corrections/suggestions on things I have wrote up there. I believe they are not all correct.

I have attached a schematic at my current stage of develop.

Thanks!
David

p.s.: here’s link to datsheet

The datasheet have all the answers you're looking for. Sorry to be short but you should spend some time reading it.

Some OEM training material might be helped.

  • unique IN-RUSH current-feedback architecture, compatible with constant voltage/current, dual mode.
  • directly sampling current resistor.
  • direct feedback, sampling precision and consistency.
  • via connect feedback to grounded, constant voltage drive mode is selected.
  • CMOS technology, the maximum voltage up to 6V, and current up to 50mA per channel.

  • drive an LED can be set to either constant current or constant voltage mode.
  • when led voltage> 6V (more than one LED!), by use external transistor constant current or constant voltag mode could be set.
  • to power ws2801 itself at > 6V voltage power supply environment, you can use a simple resistor divider or the zenor.

Multi-LED series drive by external transistor in constant current mode, current <=50mA. The one for you.

Big current LED drive by external transistor in constant voltage mode

Highway bridge LED decoration by WS2801

Thanks on your answers! It isn't that I'm lazy(actually I printed and went through whole datasheet), I just not fully understand it. For example, what's difference in usage of constant voltage and current?

If feedback pins are on gnd, I will get constant voltage mode. Does that means output voltage will be same all the time and brightness of LEDs will be controlled by current?

About higher voltage, I understand usage of transistor, but I want to avoid it as Chinese manufacturers did. I don't understand how to do that, since max. output is 6V. How did they did it?

what's difference in usage of constant voltage and current?

The clue is in the name. What do you want to be constant? If it is voltage then the current will change according to the load. If it is current then the voltage will change ( within limits ) so that no matter what load you have the current is always the same.

For LEDs you mainly need constant current as the load will change with age and temperature. To drive LEDs with constant voltage you need a current limiting resistor to make the constant voltage look like a constant current. With this chip you also need a resistor to set the current so you save nothing on the number of components, just their value. A constant current however is better for you.

Does that means output voltage will be same all the time and brightness of LEDs will be controlled by current?

The brightness of an LED is always controlled by the current. http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

I don't understand how to do that, since max. output is 6V. How did they did it?

did what? With only 6V you limit the number of LEDs you can have in series. Using a transistor gets you over this 6V limit.

For example, what’s difference in usage of constant voltage and current?

At multiple LED situation, constant voltage use when LED current >= 350mA, and constant current for LED current <= 50mA. The between is good question for worldsemi’s field support engineer.

If feedback pins are on gnd, I will get constant voltage mode. Does that means output voltage will be same all the time and brightness of LEDs will be controlled by current?

The brightness is controled by PWM. The RL set max LED current.

About higher voltage, I understand usage of transistor, but I want to avoid it as Chinese manufacturers did. I don’t understand how to do that, since max. output is 6V. How did they did it?

Ok, it is my fault. “OEM = OEM IC” in here. you can not to avoid it. Two key issues;-

  1. CMOS technology, the maximum voltage up to 6V. During start up LED voltage drop is small.
  2. Transistor acts as RL, and take extra voltage drop to make WS2801 working in 1~1.5 V voltage drop which is safe working zone.

At multiple LED situation, constant voltage use when LED current >= 350mA, and constant current for LED current <= 50mA. The between is good question for worldsemi’s field support engineer.

Err no.
Wrong way round. Constant current use when LED current >= 350mA,
Constant voltage where a largish series resistor turns this into constant current < 30mA

Grumpy_Mike:

At multiple LED situation, constant voltage use when LED current >= 350mA, and constant current for LED current <= 50mA. The between is good question for worldsemi’s field support engineer.

Err no.
Wrong way round. Constant current use when LED current >= 350mA,
Constant voltage where a largish series resistor turns this into constant current < 30mA

I love your reply, It is possible worldsemi’s field support engineer at UK is smarter than worldsemi’s field support engineer at HQ? Why not?

I got here by read data sheet in English and Chinese, plus app note from worldsemi in Chinese. still not very clear until found this training slide ( from field support engineer?). We are in same boat! Every one could be wrong. :stuck_out_tongue:

After reading your posts, here’s what I know now:

  • I have to stick with constant current. I first need to calculate resistors on feedback pins to do that. If I assume, I do not need more then 50mA on LED output pins, I can connect 12ohm resistors to feedback pins. Correct?
  • I have to use transistor if I want more LEDs in series. Anyway, I have found piece of 12V LPD6803 strip in my components box. With multimeter, I have found out that connection of LEDs is like on schematic below. I also tested strip and it works fine. Could I connect LEDs in the same way to WS2801?

If you have A 12V strip then that has current limiting built in so all you can do is to drive it with a constant voltage drive. So no exter resistors needed.

Got that! But if I want to build strip myself… all of above I have to consider. And we are looking into how exactly, right?

But if I want to build strip myself.

Then you would be best just putting three LEDs in series with no current limiting resistor and running the chip as constant current with your low ohm resistor values to ground from the chip.

I have to stick with constant current. I first need to calculate resistors on feedback pins to do that. If I assume, I do not need more then 50mA on LED output pins, I can connect 12ohm resistors to feedback pins. Correct?

Yes, also make sure WS2801 voltage drop at 1~1.5 V.

I have to use transistor if I want more LEDs in series.

Yes

Anyway, I have found piece of 12V LPD6803 strip in my components box. With multimeter, I have found out that connection of LEDs is like on schematic below. I also tested strip and it works fine. Could I connect LEDs in the same way to WS2801?

No, apple is apple, and apple is not orange. :P

fungus:

yarko:
Cheers,
I was indeed planning on supplying it with a battery pack.

Are tightly grouped LED strips available? I’ve been looking but the closest I’ve found is a 32 LED strip that’s 1m long. I would need 32 LEDs in 33cm.

How about this?

http://www.ebay.com/itm/300912911837

WS2811

apple is macintosh.

Ws2811 seems perfect(and simpler of ws2801). Thanks.