Schematic/PCB Help for LED Driver

Discussion in 'The Projects Forum' started by bigclick_dean, Aug 4, 2010.

  1. bigclick_dean

    Thread Starter Member

    Jan 24, 2010
    38
    0
    Hi Everyone,

    I am just in the process of designing a circuit to run some high power LEDs and I am looking for some advice and feedback.

    I have attached a schematic for one of the individual channels and the top/bottom layer of a 4 channel board.

    For the schematic I have the following BOM (all from mouser):

    D1 - CD1206-S01575
    R1 - RK73H2BTTDD1000F (100ohm 1206 SMD)
    C1 - VJ1206Y104KXAMC (0.1uf 1206 SMD)
    C2 - C3216Y5V1H475Z (4.7uf 1206 SMD, not actually on the schematic, it sits between DRAIN and GND)
    U4 - STCS1PHR (Power SO8)

    Currently R2 will depend on the current output I want, I am looking at 950ma and 1450ma output so I will need to calculate them (I think the equation is 100/OUTPUT to get the resistor value).

    I have built this schematic from the STCS1PHR datasheet and another sample I found on google, I am unsure if the components I have selected are correct or if I am way off.

    With the PCB's I have combined 4 channels and laid out them out on a 7cm x 7cm board, the RJ45 socket will be used for PWM control from a microcontroller. This is my first PCB design so I am sure there will be some rookie mistakes there!

    I am a little unsure with the PWM input though, from the datasheet it looks like the input needs to vary between 0 and the value to vcc (in my case vcc will be around 30v). I am used to PWM being between 0v and 5v and have used this on another setup I played with (using an OnSemi 1A chip, CAT4101).

    For the PWM do I need to run a 0-5v input through a transistor or something to amplify it?

    Any input would be greatly appreciated as I am pretty new to the design side of electronics but have played around alot in the past.

    Cheers,
    Dean
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    I can't see any reason why it wouldn't work schematic wise BUT you may have trouble with the PCB. You have huge tracks leading the the diodes but only small ones going to the driver. Those small ones are sometimes called fuses ;-). Try making them a little thicker. This will probably only be a problem if you are running them flat out.
     
  3. bigclick_dean

    Thread Starter Member

    Jan 24, 2010
    38
    0
    Hi windoze killa,

    Thanks for the reply!

    I was thinking about those last night, would increasing them to the same width as the SO8 pin be the way to go? I would also have to increase the DRAIN pin as well I think as that is where it leads to the screw terminals to complete the circuit.

    Do you have any idea with the PWM for the driver? If I need to drive it between 0 and VCC what transistor would be good to boost he 0-5v from a regular PWM output?

    Cheers,
    Dean
     
  4. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
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    Almost any small signal NPN should do it. 2n2222 pop to mind
     
  5. bigclick_dean

    Thread Starter Member

    Jan 24, 2010
    38
    0
  6. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    From table 5 of the datasheet logic high for the pwm pin is 1.2 volts and above.
    You were probably looking at the maximum device specification.
    The output from the micro will be fine.
     
  7. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    That one will do fine. 1A is way more than needed.
     
  8. bigclick_dean

    Thread Starter Member

    Jan 24, 2010
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    0
    sweet, I will update my schematic and PCB and stick it up for some final feedback.
     
  9. bigclick_dean

    Thread Starter Member

    Jan 24, 2010
    38
    0
    Indeed I was looking at the absolute maximums (-0.3 + Vcc + 0.3) instead of the 0.4 for low and 1.2 for max.

    Will this mean that I need to find a source of 1.2v to feed into my micro as a reference voltage? Something along the lines of http://www.analog.com/static/imported-files/data_sheets/AD589.pdf

    So I shouldn't need to put a transistor control the PWM which is great.

    The only other concern that I have is connecting the GND of the micro to the GND of the driver. Is there anything I can do to protect the pwm lines? I was thinking of using an optocoupler but then I would have a lot of extra components to add to each board.

    Cheers,
    Dean
     
  10. bigclick_dean

    Thread Starter Member

    Jan 24, 2010
    38
    0
    On a semi related note, I have a couple 24 channel LED pwm drivers (http://focus.ti.com/lit/ds/symlink/tlc5947.pdf) laying around that I was hoping to connect up to my micro and extend the number of PWM outputs available.

    For the LED array I don't need a high refresh rate or anything but I am just looking for something simple.

    Would this chip be able to feed the PWM on the above driver circuit? I had a look through the datasheet and I am a little confused. It says V0 (OUT0-OUT23) has a range of -0.3 to +33 but then somewhere else I see it has a minimum vcc of 3.0v, if I was to use the chip would I just use the lower steps (4096 total steps) to achieve the 0.4v-1.2v?

    Cheers,
    Dean
     
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