I have been messing around with TLC5917 LED Constant Current Sink Drivers and I don't fully understand how they work.
Because of how I was taught how Voltage/Current works, to my understanding, a voltage of 5V should be able to be measured between an active pin on the driver and a 5V source going through an LED.
What I measure however appears the be the forward voltage of the LED.
It is my understanding that current is drawn and voltage is "applied".
Since the driver is constant current, the behavior makes sense but how exactly does the driver supply the correct voltage?
Or is my conceptual understanding of how voltage/current work misguided?
Best Answer
If you look on the datasheet you'll see that the external voltage applied to the LEDs is on the anode of the LED. The cathode of the LED is connected to the pin of the TLC device.
In this case the TLC is sinking current. To reiterate, the driver doesn't supply the voltage, the voltage is applied externally. The driver does, however, ensure the correct current flows through the LEDs.
It should also be noted that the driver takes up any difference between supply voltage and what is dropped across the LED. So you have to take care that you don't have a combination of external voltage and current drawn that would damage the chip. The higher VLED the higher the power dissipated by the the TLC.
The image below gives you a very basic internal schematic of the IC.
Remember that the LEDs' cathodes are connected to OUT0, OUT1 etc, with the anode connected to the external voltage. The little circles with the down pointing arrows are representing constant current sink. It is this that ensures the current drawn by the LEDs is the same and constant.
It might be that you're not familiar with the difference between a current source and a current sink, so it might be worth doing a little reading up of the difference between them if that is the case.