Segmented LED Management Design Approach - Any Advice?

Comments

• tonyp12 Posts: 1,951

  2011-04-20 09:13 edited 2011-04-20 09:13

  What kind of voltage do you have available?
  
  Most LED drivers sink currents.
  so just using the single one you mentions above
  may not work.
  
  So maybe getting 6 TPS61160
  look at the second half of this pdf, first half is more about 115v ac input drivers.
  http://www.ti.com/ww/en/analog/led/slyt394a.pdf
• bradharper Posts: 64

  2011-04-20 10:31 edited 2011-04-20 10:31

  @tonyp12 - Thanks, I'll check into that chip. And, I have 6-12V in.
• kwinn Posts: 8,697

  2011-04-20 12:59 edited 2011-04-20 12:59

  Seems to me some of your decisions are already made by the hardware setup. What is the constant current led driver part number?
  
  The first thought that comes to mind is that if you have 12V available you wire the 16 leds in each segment into 4 parallel columns of 4 leds in series. This minimizes the current draw. Exactly how you do the rest depends on the led driver IC.
• Searider Posts: 290

  2011-04-20 14:55 edited 2011-04-20 14:55

  You might want to check out the TLC5940. It is a LED driver that can manage 16 LEDs and and be stacked serially. I have project that has 12 chips running spread accross 4 boards (3 each for RGB) and all controlled by 5 or 6 pins of a Prop. There are a couple of drivers in the Prop Exchange.
• bradharper Posts: 64

  2011-04-21 07:17 edited 2011-04-21 07:17

  Thanks everyone. I'm going to look into each of the components and conventions mentioned.
• JasonDorie Posts: 1,930

  2011-04-22 22:55 edited 2011-04-22 22:55

  I've worked with the TLC5940 - it's what I used for my lamp project (and there's a PASM driver for it in that code). It'll sink up to 120ma per channel for 16 channels, and has a constant current driver built in. They're pretty straightforward to use, and the data rate for it can be as high as 30MHz - they were made for video displays.
• bradharper Posts: 64

  2011-04-25 12:12 edited 2011-04-25 12:12

  @kwinn - The driver is the STCS2.
• kwinn Posts: 8,697

  2011-04-26 11:32 edited 2011-04-26 11:32

  bradharper wrote: »

  @kwinn - The driver is the STCS2.

  
  Bradharper, here are block diagrams of three approaches.
  
  Circuit A is the simplest and most energy efficient one if you have 36 to 40 volts available. It requires one STCS2 per segment.
  
  Circuit B can be used when a voltage in the range of 10 – 15 volts is available. Not quite as simple and energy efficient as circuit a. It requires one STCS2 and 4 current balancing resistors per segment.
  
  Circuit C is the most complicated one, and requires 4 resistors and 1 transistor per segment, as well as a ULN2803. If a high side driver was used instead of transistors the ULN would not be needed, but both approaches require 6 drive pins.
  
  Circuit C does have the advantage of requiring only one STCS2 for all 6 segments.
• bradharper Posts: 64

  2011-04-27 09:01 edited 2011-04-27 09:01

  Thanks kwinn, this is a nice summary of my options using the STCS2. I have a few questions:
  
  1) I don't understand the circuit A requirement of 36-40V. My grasp of that circuit led me to assume that since the total current required for a segment is 320mA, I simply needed to use the correct current limiting resistor (.3125Ohm), and my input voltage of 6-12V would suffice for 6 parallel segment circuits.
  
  2) I definitely need independent PWM dimming control on each of the six segments, and I'm unclear how circuits B/C would provide that capability. I see how I could turn each segment on/off, but could I have each segment at a different brightness level using only one STCS2?
• kwinn Posts: 8,697

  2011-04-28 10:27 edited 2011-04-28 10:27

  bradharper wrote: »

  Thanks kwinn, this is a nice summary of my options using the STCS2. I have a few questions:
  
  1) I don't understand the circuit A requirement of 36-40V. My grasp of that circuit led me to assume that since the total current required for a segment is 320mA, I simply needed to use the correct current limiting resistor (.3125Ohm), and my input voltage of 6-12V would suffice for 6 parallel segment circuits.

  In circuit A all 16 leds are in series so the input voltage has to be high enough to overcome 16x2.1V plus the 0.5V required for the STCS2 to work. That comes to a total of 34.1V. Since component characteristics do tend to vary a bit from the spec sheet I would suggest a minimum of 36V for this circuit. Also, since all the leds are in series the same current runs through all of them and you only need to supply 20mA per string. Since you only have 6-12V available this circuit is not suitable.

  2) I definitely need independent PWM dimming control on each of the six segments, and I'm unclear how circuits B/C would provide that capability. I see how I could turn each segment on/off, but could I have each segment at a different brightness level using only one STCS2?

  Circuit B voltage requirement would be 2.1Vx4 plus the voltage required by the STCS2 and the series resistors in each string of 4 leds, so 8.4V + 0.5V + 0.02xR. If you used 27 ohm resistors for R and 20mA on current that would be 8.4 + 0.5 + 0.54 = 9.44V. As before you need to go a bit higher to compensate for component tolerances, so 12V would be a suggested minimum. The 6 STCS2's would be PWM using the 6 prop pins
  
  Circuit C is a multiplexed version of circuit B. Each string would be on for a maximum of 1/6th of the time and the current provided by the STCS2 would be set higher to compensate.The length of time each transistor is turned on would be PWM to control the brightness of it's led string.
  
  For your application Circuit B is probably the best way to go unless the STCS2 chips are very expensive. If you had slightly more than 12V available you could do the same thing using 6 transistors and 12 resistors.