michinyon:
With the TLC5940, what is the voltage drop of the device when it is "on" ?
The chip is a current driver so the drop is whatever is needed
for the chosen current. If the LED needs 3V and the power
supply is five volts, the drop will be 2V.
michinyon:
Suppose firstly, that i want to connect a 2V, 20 mA red LED and drive it with 5 voltage directly.
I can then determine that I want a resistor which will drop 3V at 20 mA, therefore 150 ohm.Suppose now, I want to turn the LED on and off using the TLC5940 between the LED and ground.
I still need a resistor, apparently, because otherwise the chip would generate too much heat.
You don't need resistors if the total power dissipated by the chip is less than about 1.5W.
For 5 or 6 LEDs at 20mA that isn't the case so you don't need resistors.