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Others have four legs, which makes sense to me, as ground can be shared.

One of many examples of a six legged RGB LED is the Kingbright KAF-5060PBESEEVGC.

feklee
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5 Answers5

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According to the datasheet, it has six legs because it contains 3 LEDs, with nothing common:

schematic from datasheet

Many RGB LEDs have a common cathode or common anode, so need only 4 leads. For some applications that may not be acceptable. For example, it wouldn't be possible to control two RGB LEDs with common cathodes or anodes in series, or to arrange them for charlieplexing.

Further, a part like this is at least more flexible. It can be made common anode, or common cathode. Or, a pair of the LEDs may be arranged in anti-parallel. It may make more sense in some situations to stock just one part than to stock several different parts to cover different cases.

Phil Frost
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  • Yes, I've seen on the data sheet that the three LEDs have nothing in common. I just wonder *why?* Could you name an application where it is not acceptable to have a common anode? – feklee Jul 16 '13 at 12:47
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    Could be nothing more than market positioning. If you make a common anode device, people who design for common cathode can't use it. – Scott Seidman Jul 16 '13 at 12:56
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    @feklee: How about if one wants to control two or more RGB LEDs in series? – supercat Jul 16 '13 at 12:57
  • I accepted this answer, because the presented use cases are plausible to me. But also [the answer](http://electronics.stackexchange.com/a/76064/21817) by @OlinLathrop and [the answer](http://electronics.stackexchange.com/a/76069/21817) by pjc50 provide very good reasons for having six legs on an RGB diode, at least as far as I can judge. – feklee Jul 19 '13 at 11:36
8

These RGB LEDs have six pins because all the connections are brought out individually.

You didn't ask, but others may wonder why manufacturers do this. The advantages of bringing out all connections individually over tying some together in the package include:

  • One part works in both common anode and common cathode configurations. If the package needs to be big enough for thermal or other reasons anway, then there is little advantage in economizing on pins. There can be bigger savings in only having to produce a single part for different applications than in reducing pin count.

  • There are other topologies beyond common cathode or common anode that such 4-pin packages don't suite at all. For example, it might be useful to drive a string of each color to get higher voltage, which may allow for more efficiency in the power supply. Or, you might want a different power supply voltage per color or at least one for blue and another for red and green. That can be done with common anode or cathode, but is simpler with individual connections. The fixed power voltage could go on the anode, then a low side switch on the cathode.

Olin Lathrop
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Before you linked the datasheet and explained that it wasn't common anode, I commented that it might be mechanical or thermal relief.

For both of those purposes, you want larger pins or more pins. Large pins take longer to solder, so if more pins fit easily within a rectangle surrounding the device you might as well choose more pins.

This device claims a total power of 350mW, which is starting to get hot to the touch, so thermal design is definitely a consideration.

Single-colour LEDs often have a "pad" underneath, but in this case with three disjoint LEDs there would be the question of which signal to connect it to.

An actual reason for the non-common anode, according to the datasheet's "Description": the three sub-LEDs are manufactured on different substrates with different processes (may not be true for all RGB LEDs). Therefore there are three small dies in the package each of which has two bond wires. It's probably easier to bond them out to different pins.

Final note: the orange LED is much brighter than the other two in this package, for the same current.

pjc50
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  • I'm curious: Am I missing some significance of that "final note", other than this is so? – Anindo Ghosh Jul 16 '13 at 13:45
  • It's not relevant to the legs, just an observation from the datasheet. – pjc50 Jul 16 '13 at 14:14
  • Thanks for pointing out that the particular model is probably hot to the touch. I'm such a noob, didn't pay attention to power and brightness. All I want is an indicator LED, so either I have to look for another one, or dim it, which seems non trivial. The form factor, however, is just perfect. – feklee Jul 16 '13 at 16:46
  • Fixed dimming is is trivial: set the current limiting resistors for If = 5ma or less. – pjc50 Jul 16 '13 at 17:17
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For the specific led you mentioned, it is a standard 5050 PLCC-6 smd led package (It says 5060, but 5mm x 5mm is nominal sizing, 5.5 x 5.5mm typical). It's a pretty standard 6 pin leaded package.

enter image description here

enter image description here

The same package is used to allow multiple colored diodes (RGB, or RGB+White or RGB+Warm White+Cold White), or even entire micro-controllers like the WS2812 RGB led controller:

enter image description here

Passerby
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The answer is so that you can have multiple LEDs in series. You can't do that if they have a common anode/cathode unless you don't mind all colours having the same intensity which defeats the object of RGB.

user20221
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