BuckPuck LED Driver from iPod, 9V

Discussion in 'General Electronics Chat' started by BackyardBrains, Mar 20, 2011.

  1. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
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    I am trying to drive a bright 350mA blue LED using an iPhone as the signal generator. Please see the attached circuit. I am using a 350mA BuckPuck (datasheet attached) to drive the LED. I have the reference of the BuckPuck set to a regulated 5v supply. When the CNTRL voltage drops below 4.2v, the LED starts to turn on until it reaches it's max when CNTRL < ~1.5V.

    I have an iPhone app that can output square waves at +/- 1.3V (the max an iPhone can output). I want to amplify this to 5v and 0 (Full Off and On for the LED CNTRL signal respectively) so I used an op-amp (TLC2272, datasheet attached) to act as a simple comparator.

    When I test the circuit without the BuckPuck, I get a clean 0-5v square wave at TP2 (the output of the OPAMP). However, when I connect the BuckPuck, this square wave drops to 0-3.5v. The LED goes "on" at full brightness, but the "off" state is still rather bright.

    I disconnected the OpAmp, and tested the BuckPuck using a POT. The BuckPuck drives the LED until 4.2v, then turns off as the signal approaches 5v. So both systems seem to work independently, but not together. Any suggestions?

    Some specific questions:

    1) Am I using the OpAmp correctly?

    2) Should I be using a Voltage Regulator? I wanted to be sure to keep the REF voltage at 5V. When I used a voltage divider, but I notice the voltage dropped a lot when the LED was drawing current when it was turned on. Will the voltage regulator shorten the 9v battery life?

    Thanks,

    Greg
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A TLC2272 has a very low output current. When overloaded then the output high voltage drops.
    It is spec'd to have an output current of only 200uA to 1mA when its output voltage has dropped to a minimum of +4.25V but the control pin of the buck-thingy must be a minimum of +4.75V into 1.5k ohms (3.17mA).
     
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  3. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
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    Thanks, so much AudioGuru. I am still trying to figure out how to read datasheets correctly. I thought the 2272 was saying that it could drive up to 50mA! (page 5). What should I be looking for to see that it goes up to 1mA? I do see that Figure 17 on shows a significant drop off over 2.5mA (didn't see that before).

    Thanks for taking the time to answer my question. I'm going to try again with an LM358 opamp. From it's specs, it looks like it can provide much more current (again, if I am reading the specs correctly).
     
  4. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
    33
    0
    I swapped out the TLC2272 with the LM358. This chip has the same pinout, but appears to support higher output current. When I started up, I found that I am getting the same results.

    What should I be looking for in the Spec Sheets. I see where you found the input resistance (1.5k) from the BuckPuck and the 4.75V to get 3.17mA. But I it looks to me that at least the LM385 (datasheet attached) can supply the proper current.

    Thanks for your help. Your feedback (as always) cuts though the fog and finds the one thing I need to keep an eye out for. That is the skill I am trying to gain as I develop more and more circuits.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    That is the max allowed output current when it has a high supoply voltage and you short-circuit the output. the written spec's say, "VOH High-level Output Voltage is a minimum of 4.25V when the current is only 1mA."

    It has a supply voltage of plus and minus 5V which is double what you have so its output current is much higher.
    It is for a "typical" IC that you cannot buy. The one you buy might have minimum spec's. Maybe all the ones available have minimum spec's.

    It is worse. Its minimum output is only +2.0V when its load is 20mA. A graph shows its "typical" output voltage loss with different output currents called "Output Characteristics Current Sourcing" so its typical output voltage is only 3.7V (much too low) when its load is a few mA. It will be lower if the device is a weak but passable one.
     
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  6. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
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    Thanks. You were right, the LM358 was worse.

    I am still not having much luck tonight. I tried to tie the 2272 power to 9v instead of 5v to be able to increase the voltage of the control signal to the BuckPuck. It helped, but still isn't perfect.

    Is there a way I can use a simple NPN to switch on the 5v control signal without worrying about the output current of the 2272? I am heading over to the Semiconductor library now...

    Thanks so much for your help.

    Greg
     
    Last edited: Mar 21, 2011
  7. Audioguru

    New Member

    Dec 20, 2007
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    Nearly any opamp will work if it has the normal output current of 20mA and a power supply voltage high enough for its output to reach +4.75V when its load is 3.17mA.

    Or you can add an emitter-follower transistor to the output of the TLC2272 flea-power opamp and increase its supply voltage.
     
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