If you were running the LEDs in series then yes, you would need a higher voltage. You aren't though - otherwise they'd all be on or all be off. You are running them in parallel.
From what I understand of your description you have the one output of the '595 going into the anode of the all the LEDs of one column. The cathodes of these LEDs then go into separate inputs of the TLC for PWM.
And, as you say, the '595 can supply 35mA of current per output. That is enough for to light one LED reliably.
You will need to supply 6 times that current for 6 LEDs.
The simplest way would be to use a single transistor and resistor per column. For example, the '595 output connects to the base of an PNP transistor through a 1KΩ (for example) resistor. The emitter connects to Vcc, and the collector connects to the LEDs in the same way the output of the '595 used to. When the '595 sets an output low it turns on the transistor which then allows the current to flow from Vcc to the LEDs.
- I don't know what the TLC5940 does in the way of current limiting - I haven't shown any current limiting resistors that may be required for the LEDs if the TLC5940 doesn't do that for you.
You can't use the TLC with, for example. a ULN2803 as they are both current sinks. You need something which can be a current source, which a transistor can be.
To answer the TLC5940 side of the question:
First of all, bear in mind that when using TLC5940 your intensity need not be 12-bit values (4096 values): you can use the TLC5940 using with intensities of any value 12 bits or less. For instance, 8-bit intensities (256 values) do provide a very satisfying result. More on this latter.
Assuming 12-bit intensities, here's how GSCLK
and BLANK
work: TLC5940 doesn't have its own clock. So GSCLK
is used to figure out when to turn on and off each LED. At the beginning of a cycle, all LEDs are on. Each time positive-going edge on GSCLK
is received an internal counter is incremented on TLC5940. Each LED whose intensity value is lower than the counter is turned off. So LEDs with intensity 1
are turned off after the first cycle, LEDs with intensity 2
are turned off after the second cycle, and LEDs with intensity 4096 are not turned off at all. At the end of the cycle the chip does not reset itself, rather it expects a positive-going edge on BLANK
to reset it, and after this the cycle begins again.
Here's what this means for driving the TLC5940: you need two PWM outputs; one for GSCLK
and one for BLANK
, and the one for BLANK
needs to happen every 4096 cycles of GSCLK
. Now notice that we are talking about the frequncy here, and not the duty cycle, whereas it is the duty cycle that analogWrite()
controls. To drive the TLC5940, you could use a library written for driving TLC5490, or you can do the lower-level driving of TLC5940 yourself, which can use one of the following approaches (assuming you are using an ATmega-based Arduino, and in scale of increasing difficulty):
- Program the two timers yourself such that they use different prescalers such that the
BLANK
line is driven at 1/4096th the frequency of the GSCLK
- Program the
CKOUT
fuse on the ATmega, causing it to output the clock signal on one of its output pins. Use this for GSCLK
. Then use a timer to generate a BLANK
pulse at 1/4096th of clock frequency.
- Clock the ATmega externally, and use the same clock for
GSCLK
. Have an ATmega timer generate the BLANK
pulse at 1/4096th of clock frequency.
Now to the question of frequency relationship between the TLC5940 clocking and the PWM. The BLANK
line will have a duty cycle of 1/4096 (or whatever the maximum intensity value you are using), so that probably will not work for your servos. The GSCLK
is usually 50/50 duty cycle but need not be. Lets assume that you want your LEDs to appear to be steady, and lets take the flicker theshhold to be 50Hz. This would mean that you need your intensity 1
LED to be flickering at 50Hz or above, meaning that a 4096-clock long cycle should complete in 20 milliseconds, meaning that your GSCLK
clock should be at least 204kHz. At 204kHz the clock pulses are about 5uS long. So while in theory you could use the same clock for your servos and the TLC5940 (I think that's what you are asking): if you maintain the clock frequency (at 204kHz) and change the duty cycle you could control your servos and clock the TLC5940. However, if you use 12-bit intensities, then the greyscale clock needed by TLC5940 is going to be too fast for the servos.
But, if 4096 intensity values is too much to handle, consider using 8-bit intensity values. You will still have to send them as 12-bit values (that's what the TLC5940 interface expects), however, the is no law that says that your BLANK
pulse must occur every 4096 GSCLK
clocks. If it occurs every 256 clocks, you have yourself 8-bit intensity. So your 8-bit intensities should be sent as valid 12-bit values (with the high four bits being zero), and you'll restart the clocking cycle every 256 clocks. You can use any other number of intensity bits, as long as it is 12 or less, in the same manner. If you are using 256 intensity (=greyscale) values, then your minimum clock is 12.8kHz, and the clock duration is 78uS. Closer the 2400uS +90 pulse, but still quite far away. If we assume that +90 pulse is 90/10 duty cycle, then we calculate the clock cycle length to be 2.6mS, which translates into 375Hz clock. At this clocking, the maximum intensity value that will yield no flickering is 8 values (3 bits) at 50Hz persistence theshhold, and 16 values (4 bits) at 25Hz. You can decide whether that is good enough for your purposes.
Best Answer
You connect the base of any small-signal PNP transistor (for example: BC327, BC807, BC556/557/558, 2N3906) to the output of the TLC via a series resistor (500-10k ohms). The emitter of the transistor goes to Vdd, the collector goes to the input of the CAT. Another parallel resistor (1k-50k ohms) should be connected between the input of the CAT and GND.
You should configure the TLC to generate only a small (<1mA) current, by selecting the maximum reference current resistor value supported (see page 14 of the TLC data sheet), and programming the dot correction to lower the current.
simulate this circuit – Schematic created using CircuitLab