led driver circuit help

Discussion in 'General Electronics Chat' started by skk, Oct 20, 2011.

  1. skk

    Thread Starter Member

    Mar 15, 2011
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    I am trying to drive the rows and columns of an array of LEDs. I will be multiplexing so I have 1/7th duty cycle thus 7 times the steady state current for the same brightness. 3 of the columns are double led 7-segment displays, so 20mA per segment, 140mA total or 980mA peak if I want 100% brightness. For the other 4 they are 10mA leds or 490mA for full brightness at 1/7 duty.

    On the low side ( rows ) I need up to 20 * 7 or 140mA.

    I was planning on driving both high and low sides off 74595 shift registers and transistors, but I forgot about having to multiply the current by 1/duty cycle to get the same brightness.

    Can you recommend low cost low density transistor(s), driver array, replacement for the shift registers, or whatever what would make this work? What if I can live with 500mA max current pulse?

    p.s. I only have 5v.

    Thanks.
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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  3. elec_mech

    Senior Member

    Nov 12, 2008
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    Please post a schematic of what you have. I'm confused by having a matrix then stating you have 7-segment displays.

    As far as controlling the LEDs, are you using a microcontroller? If yes, then there are some ICs that can do this through serial communication, SPI, or similar.

    I haven't used the MAX7219 that John P recommended, but I've come across it. Unfortunately it's a little pricey: $12+. I've successfully used the STP16DP05 which is really nice and appears to run under $3, but I'm not quickly finding any thru-hole versions, so if you can live with a SMD version, you're set.

    If you prefer a thru hole, Digikey has the TLC5940 for around $4. I think I've purchased these before, but I haven't used one yet.

    Again, if you can post a schematic, we can offer more help.
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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    The MAX7219 is $10 at Sparkfun, though it's still an expensive little thing. But you have to consider what it saves you in terms of component count, and programming time (I assume there is a processor involved). You don't even need current-limiting resistors, just one for the entire array.

    The STP16DP05 is just a serial-input sink driver, not much use for a matrix scheme.
     
  5. skk

    Thread Starter Member

    Mar 15, 2011
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    The LED driving part of this schematic consists of at attiny88 controlling 2 8 bit shift registers. One shift register controls the rows and one controls the columns. I have the column register sourcing current and the row register syncing current. Don't be confused by the 7 segments displays, these are wired up like a bank of 8 leds. They are 'double led segments' so they draw 20mA each segment instead of 10.

    I wanted to use high power shift registers but I cannot draw enough current with them. I am leaning towards putting transistors between all 16 outputs of the 2 shift registers and the LEDs. 8 are syncing and 8 are sourcing. What transistors can I use?
     
  6. nickelflipper

    Active Member

    Jun 2, 2010
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    I've used some dual pfet irf7304's on the high side, with tlc59401's on the low side for a multiplexed (1/8th duty) 8x8 rgb matrix. So with all three colors on (i.e. white) the irf7304's were sourcing aprrox. 480ma.
     
  7. skk

    Thread Starter Member

    Mar 15, 2011
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    I am liking the irf7304,what transistors should I use on the high side?. Also do you need additional resistors for the inf7304, how do you hook it up?
     
  8. #12

    Expert

    Nov 30, 2010
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    We are definitely gettinto the "post a schematic" region.
    Can you please post a schematic of what you have so far?
    It makes life so much easier on this end.
     
  9. elec_mech

    Senior Member

    Nov 12, 2008
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    Really need a schematic. In the meantime, if all you need to do is add transistors between your shift registers and LEDs, look at the ULN2804 for the cathode side and the UDN2981/2 for the anode side. These are ICs which will each handle up to 50V and 500mA per pin.
     
  10. skk

    Thread Starter Member

    Mar 15, 2011
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    I have uploaded the schematic. The sincing component is LSYNC a 74573 latch and the sourcing component is SR1G a 74595 shift register. In the schematic I am using 2N2907 transistors between the 74595 and the LEDs, but these guys only give me 500mA, I need like 980mA.

    I don't want to use the ULN and UDN parts because I am trying to get the price per unit as low as possible. I want transistors, and I need 980mA on the source side. On the sinc side I need 140mA, so I need some kind of transistors there as well, or a latch ( or a shift register ) that will sync 140mA.
     
    Last edited: Oct 25, 2011
  11. elec_mech

    Senior Member

    Nov 12, 2008
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    Can you post a PDF or JPEG? I don't have software that will open a .sch file.
     
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