driveropen-drainpwmshift-register
I am need of a 16 channel PWM Serial based LED driver, or serial in – parallel out shiftregister. I had been looking at TLC5940 and TLC59025 respectively.
But for my current set up the ouptuts cannot be open drain, the output of each channel needs to be +5v 10mA, to switch a transistor. The TLC5940 approach would probably be better than the soft pwm approach with the TLC59025.
Does anything like this exist? if not what other approach would I take?
The individual LED driver circuits shown below:
You can multiplex it to save drivers, up to about 8:1, but need a fair amount of speed to update sufficiently fast as you need to reload the drivers for each row, and row update needs to be fast enough to avoid flicker - something like 4ms for 12 bit PWM at 8:1 wih 1MHz PWM clock - you're probably going to need something with more grunt than an 8 bit MCU.
It appears that the reading of the datasheet whereby TLC5940 can sink only 120mA/chip is incorrect:
Rather, it seems that the correct reading of the "\$I_O\$ Output Current (dc)" value on page 2 of the datasheet is as maximum current per channel, which is also the interpretation arrived at by Sparkfun discussion to this driver. The ambiguity of the phrasing Output Current (dc) is resolved in favor of per channel reading owing to:
simulate this circuit – Schematic created using CircuitLab
To clarify the above point: In the schematic above, the voltage that the TLC5940 sees, i.e. LED Vcc (or \$Vcc_{LED}\$ but be above certain threshold, cal that threshold \$V_{HEADROOM/TLC5940}\$, which is around 1.25V for \$I_{output}\$ 120mA, and is .35V or so for \$I_{output}\$ 20mA. Any voltage at \$V_{OUT/TLC5940}\$ over \$V_{HEADROOM/TLC5940}\$ must be dissipated by the device as heat. For maximum possible current in the TLC5940, you would want to match \$Vcc_{LED}\$ to \$V_{forward}\$ of that LED, so that the TLC5940 IC dissipates the minimum power necessary, since power dissipation rating is a limiting factor on how much current the device can sink at each channel.
(and back to the list)
Best Answer
Well, you could add pull-up resistors. If you need a lot, they are available in a SIL package which will reduce your part count:
But here's even less parts, if you can use a PNP transistor to switch the LED:
And, you can still use a SIL resistor package for R1. Also, Q1 does not saturate, which may make your PWM control easier, if your PWM frequency is high!