Given: Cree XM-L LED.
Want: Up to 2A drive, PWM controled by PC via USB.
This can be two parts. ie actual LED drive and PC to LED drive interface. These may or may not be integrated.
A "very easy" approach is to
1. use an off the shelf USB to "output" device. "Output" may be analog level, PWM, 8 bit port etc to control ...
2. An off the shelf LED driver that uses analog or PWM input.
For example, the circuit below using a RT8482 requires an analog input level or PWM with a simple RC filter (to convert the PWM to analog). The analog could be provided by a USB to analog output I/O device (COTS) or by a USB to parallel port device (not a printer port per se) (COTS) with a simple R2R digital to analog converter (about 16 resistors plus maybe a cheap op-amp).
Many examples of R-2R ladders here - links live
Or a microcontroller with USB capability could have a relatively simple program written to provide PWM or analog output. A USB enabled Arduino or a Raspberry Pi would do this. (USB has to be slave not host mode).
LED drive:
(1) "Off the shelf" complete units that do the LED drive part of this job well are available at good prices from eg ebay, or Mouser and similar. Using such is a good default solution unless you have some reason to do otherwise.
(2) DIY LED driver.
Digikey LED drivers are found here. Alas the parametric search is poor in this case (which is unusual).
Searching using LED driver 2A gives better results.
There will be a nummber.
Example only: For $US1.52/1 in stock Digikey you get
1
Ricktek RT8482, buck or boost, LED driver.
Drives external MOSFET so LED current capability essentially unlimited.
Looks like a good start. 350 kHz for smallish inductors.
- High Voltage Capability : VIN Up to 36V, VOUT Up to
48V
Buck, Boost or Buck Boost Operation
C u r r e n t M o d e P W M w i t h 3 5 0 k H z S w i t c h i n g
Frequency
Easy Dimming : Analog, PWM Digital or PWM
Converting to Analog with One External Capacitor
Programmable Soft Start to Avoid Inrush Current
Programmable Over Voltage Protection
VIN Under Voltage Lockout and Thermal Shutdown
16-Lead WQFN and SOP Packages
RoHS Compliant and Halogen Free
A MOSFET suitable for use as M1 would be eg ONSEMI NTD4960 $US0.40/1 in stock Digikey, 30V, 9A, 9 milliohm on resistance nominal, logic gate - data sheet curves show good at 4V gate and say 4A.
ADDED:
Should I be looking at specific types of inductors for this sort of application
Inductors are very special for best results. If this is a one-off then off the shelf inductors from eg Digikey or similar are wise. We can give advice in this when final real spec is known.
I'm assuming all of the caps in this type of application would be ceramic?
Ceramic capacitors will work well for all capacitors shown. At least 10V rating. More or much more voltage OK.
D1 is Schottky and should have current rating equal or greater than LED max current.
Now I just need to figure out how to generate the PWM signal.
PWM is "easy" [tm] and may not be needed. Above LED controller example can use analog or PWM control.
USB to I/O
This USB to paraell FIFO I/O module](http://www.ftdichip.com/Support/Documents/DataSheets/DLP/usb245r-ds-v10.pdf) uses FTDI's FT245R USB-parallell FIFO interface IC - datasheet here .
Vast amounts of related FT245 information here
FT245 available from Digikey ~= $US4.50/1 from here
FT245 based module from Digikey for about $40/1 here
This page discusses a DIY USB printer port which, as you have complete control over the hardware and how it acts, could "easily" meet your need. Based on a PIC18F4550 microcontroller and not much else. All software PCB patterns, circuit etc free.
Typical commercial USB to analog device
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
There is already a solution to converting stereo audio to a digital optical signal and back again that's widely used and readily available: TOSLINK. It basically is a simple conversion from the electrical S/PDIF signal to an optical format. There are many commercial chips that can convert directly between analog audio and S/PDIF (in both directions).
The only difference is that TOSLINK normally uses inexpensive plastic fiber to couple the light between the transmitter and the receiver, and you want to do a "free space" transmission. Therefore, you are going to have to focus on the optical part of the link, dealing with highly-variable signal levels and lots of potential interference.
White phosphor-based LEDs tend to have poor frequency response, mainly because of the slow decay of the phosphor's glow, although if you read datasheets carefully enough, you can find some with fast phosphors.
But you could use the slower LEDs if your receiver includes an optical filter that passes the blue light directly from the LED and ignores the yellow light from the phosphor. This might be a good idea in terms of rejecting interference anyway.