Hi,
I want to drive ledstrips using the TLC5940, but the power dissipation of that chip is quite limited. Therefore, I have been googling for few hours and this is what I found:
-you can use the output of the TLC5940 to connect to an NMOSgate (with pullup resistor to 5V). Source of NMOS is connected to ground, while drain is connected to cathode of LED and anode of LED is connected to 12V.
-the problem with this setup is that the leds don't fully shut off which is quite noticeable
-you can use pnp to avoid this issue, but as far as I understand this only works with common cathode LED, correct?
-since I have common anode, I would need to use the NMOSFET, but then invert the signal between TLC5940 and gate of the NMOSFET? Then I need to switch the code to drive the reverse signal from the TLC. Is this correct?
Has anybody done something similar?
Which components did you use?
Thanks for the help!!
If you are using a constant current driver to power a MOSFET gate with a pull-up, make sure the pull-up is large enough that the driver's output pulls fully down - from the curves for the TLC5940 it looks like using a set output level of 20mA and using a 4k7 pull-up would provide good pull-down. Using a set current of 120mA won't be as good whatever the pull-up resistor. Assuming you can trust that graph in the datasheet.
A far more direct way is to use a serial-in/parallel-out shift register to drive the MOSFET (the 74HC595 being the classic example) which will pull down to the rail properly and need no pull-ups. You could also use Darlington arrays to power the LEDs but they have a large Vsat and won't handle very high currents.
I was planning to use a 10k pull up resistor. At this moment I have constant current sink of 33mA. You think this could work? So you say that I should be able to pull down good enough with the proper selection of pull up and current sink? No need for inverters then?