driverled
The specs of TI TLC5944 LED driver says "constant current sinking" capability is 60-mA.(LED Power supply voltage up to 15 Volts, and Vcc=3.0V to 5.5 V )
My understanding is if Sink current is low, then its better as it will not heat-up my LED driver chips.
1- I want to understand What is the importance of High (or Low) LED driver Sink current?
2- Why we do not talk about sourcing Current instead?
Any comments?
short version: current sources connect things to Vcc, current sinks connect them to ground.
longer version: The following is a practical explanation of current-sources/sinks as used in microcontrollers & TTL logic. For a more theoretical description, see the Wikipedia page on current source.
Some devices are very good at creating a connection to ground. (or whatever the lowest voltage is in the system, e.g. 0V) Other devices are very good at creating a connection to Vcc. (or whatever is the highest voltage in the system, e.g. +5V)
Those devices that are good connecting to ground are called current sinks; those good at connecting to Vcc are called current sources. Until recently (the last decade or so), it was unusual for integrated circuits to be good at being both. Most were good at being current sinks but were terrible at being current sources. So at lot of circuits were designed so all the chip had to do was connect to ground to make the circuit do its thing. Many chips still have an asymmetric current drive ability and function better switching to ground than switching to Vcc.
To me a good example of current source and current since are the standard "switch" configuration of a PNP and NPN transistor. A PNP is a good current source: you almost always connect its emitter to Vcc, and it switches it on/off. A NPN is a good current sink: its emitter is almost always connected to ground and it switches the ground connection on/off.
Why you choose one over the other often depends on the capabilities of the parts available to you. For instance, an RGB LED is often a "common-anode" type where the anode (positive lead) is connected on all three LED elements, so to turn on an element you need to connect its lead to ground. You can use three pins on a microcontroller to do this (or three NPN transistors) and they would be acting as current sinks.
You can build your own common-anode driver out of discretes like this:
A forward-biased diode at the zener (D2) cathode will let you use 1 zener and a bunch of standard silicon diodes for lower cost and easier availability.
Remember that discrete devices are available in arrays, which will greatly simplify breadboard or perfboard construction. However, this will reduce your available power significantly. If you're doing it on a PCB, it's perfectly fine to do it with discretes (as long as you have room.)
With that circuit R1, D2, and Q1 all dissipate minimal power, so can be SOT-23 and 0805/0603, or low power arrays.
Best Answer
The current value depends on the led requirement, you should drive the leds with the proper current to have the desired luminosity, the device you are talking about actually can change this current to have different luminosity levels. The 60mA of the device is the maximum the device can lead. They talk about sinking current because the led are connected to the VCC and the device "ground" the LED to switch it on, so the current flows inside the LED and "sink" into the device.