Looking for RGB LED PLCC4 (Common Cathode)

[2camjohn]

I am looking for an RGB LED in PLCC-4 SMT package.

Common anode ones are pretty common but I now require a common cathode one in order to multiplex them.

I have tried all my favorite places, Farnell, RS etc but they only stock common anode ones.

So do common cathode versions exist?
If so, where could I order them from (im in the UK)?


Cheers
John

 

[Nigel Goodwin]

I am looking for an RGB LED in PLCC-4 SMT package.

Common anode ones are pretty common but I now require a common cathode one in order to multiplex them.

Why don't you multiplex the common anode ones?.

 

[2camjohn]

I want to control two RGB LEDs with only 4 pins.
I have attached a schematic of what I had in mind.



RED, GREEN, BLUE and CONTROL come from the microcontroller.

When CONTROL is low, the common cathode LED will display its colour depending on the state of RED GREEN and BLUE pins.

When control is high, the common anode LED will display its colour depending on the states of the other three pins, but this time inverted.

With this circuit I can easily switch between both LEDs so fast that they appear to be controlled independently.
(I am also aware that there will be 3 LEDs worth of current going through the control pin and when I move up to a higher duty cycle I will need transistors or an opamp to beef up the power).

That is my understanding of it...




Nigel you now have me excited, Im fully expecting you to reply with a much simpler circuit that allows me to use 2 common anode LEDs.

I await your response...

 

[Nigel Goodwin]

Nigel you now have me excited, Im fully expecting you to reply with a much simpler circuit that allows me to use 2 common anode LEDs.

Sorry, no, I didn't realise you wanted BOTH common anode and common cathode.

 

[justDIY]

looks like common anode is becoming the 'standard', probably from all the low side rgb LED drivers that are coming out of the wood work. here's another source to check: http://www.sunled.com ... and another: **broken link removed**

how about using discretes in a small package, you can wire 3 with a common cathode and 3 with a common anode.

or, if you have room for another chip, use three of your four control lines to drive a serial shift register, then you'll have six lines to run a pair of common anode LEDs.

 

[2camjohn]

JustDIY,

Thanks for your help.

I do have some discrete LEDs that would fit. But they come in at twice the size and three times the price.



Shift registers... Wow! Just googled them quickly and I think they will solve alot of problems for me..

I just hope they are fast enough as I am running PWM on all the channels of my LEDs.

 

[justDIY]

the 'standard' 74HC595 is rated at 50 mHz typ. clock at 5v, and if thats not enough, the 74VHC595 is rated at 180 mHz type at 5v ... of course, that would need some seriously efficient bit-banging on the microcontroller side to achieve that kind of speed.

the '595 is my favorite low-cost output expander for microcontrollers ... the daisy chain feature allows you to drive a large number of them off only three control lines.

they can direct-drive modest loads as well, sourcing up to 35mA and sinking up to 70mA