TLC59108/TLC59116

Giorgio,

The TLC59108/TLC59116 can sink up to 120mA per channel.  Overheating occurs when the Power is too high for the die / package.  Power on these LED drivers is calculated by adding the power on each output with the VCC Power.  To measure the power on each output pin, you need the current (27mA) and the voltage on that pin.  If you've got 5V for your LED power and a forward voltage of 3V then the voltage on the output pin is 5V-3V = 2V.  The power on this pin is then 2V*27mA = 54mW.  For 8 outputs, that would be 432mW.  The power dissipation capability of the LED Driver depends on the package and PCB mounting.

For what you describe, I expect the part is overheating.  I would check the OUT pin voltages to see what power each output is generating.  Reduce the LED voltage and I suspect you'll get the results you're expecting.

Regards,

Dick

Dick,

thanks for your answer.

I'm using tlc59108 in tssop-pw package and I have made a big ground plane for power dissipation, in case I can expand the ground plane under the chip or do you think that the best solution is use QFN-RHB package? (or simply can I insert an uln2803 or mosfet as driver for higher current?)

If I would use higher voltage on pin is valid the application note SLVA280 (for TLC5940) ?

Best regards

Giorgio

Giorgio,

The best option is reducing the power dissipated by the TLC59108.  If you have a higher LED voltage (VLED) than is needed based on the forward voltage of the LED (VF) plus the required output voltage of the OUT pin (based on the IOUT) then reducing the voltage across the TLC59108 is a good possibility.  Using the FET as in the SLVA280 application note is not exactly what's needed.  That isolates the off voltage from the device, but does not dissipate any power.  Adding a power series resistor to drop the excess voltage is a very good option.

An example would be something like this:

VLED = 12V, VF = 3V (3 LEDs), IOUT = 26mA.  

Figure 13 shows that 0.5V on VOUT will be enough for the 26mA current.  The "excess" voltage is (12V - 3V * 3 - 0.5V = 2.5V).  Using a resistor that drops 2.5V at 26mA (~96 Ohms) will improve the system performance.  This changes the dissipation in the TLC59108 from (3V * 26mA * 8 outputs = 0.624W) to (0.5V * 26mA * 8 = 0.104W).  The tolerances of the VF and resistors must also be taken into account.

Improved Power dissipation practices on the PCB are always beneficial.

Regards,

Dick 

Dick,

Thank you very much for your help, 
i will follow your advice.

Regards

Giorgio