LMC7215 Damaged?

Discussion in 'The Projects Forum' started by tracecom, Nov 24, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    My first effort at soldering SMD was to attach an LMC7215 SOIC8 to a breakout board. (That is shown in another thread.) When I tried the breakout board in this circuit http://discovercircuits.com/PDF-FILES/AUDIODETECT1.pdf , I couldn't get the circuit to work. (I think I correctly transposed the pin numbers to my SOIC8.)

    Thinking that perhaps I damaged the op-amp with too much heat, I built the attached test circuit, but I don't really know if the circuit proves anything or not. The LED lights, but does that ensure that the LMC7215 is good?

    Thanks.
     
    Last edited: Nov 24, 2012
  2. tubeguy

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    Nov 3, 2012
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    The Audio Detector circuit output is a sink output.
    Not a +/- output as it is labeled. Your chip might be fine.
    Connect: 3V+ to 220 res -> LED -> Drain on FET. ( Led cathode to FET)
     
  3. tracecom

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    Now, I am even more baffled. In the test circuit schematic in post 1, the output is oscillating from rail to rail at 33.3 kHz. Is the op-amp fried?
     
  4. tubeguy

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    Edit: oops thought this was an opamp, its a comparator.

    The + input is biased slightly V+. It might be an oscillator circuit.
    Try changing your test circuit. Remove R3, remove R4, ground the - input.
    Led should be on.
    Then make R1 much larger to turn off the LED.
     
    Last edited: Nov 24, 2012
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  5. tracecom

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    Thanks for your help. As you can tell, I am lost.

    I removed R3 and R4 and attached pin 2 to the common ground. LED is still lit. Pin 6 is at about +2 V. Oscillation has stopped.

    ETA: Just reread your post. I will increase R1.

    ETA: Replaced R1 with a 1M pot, and was able to turn LED on and off with pot. So, I guess that means the IC is good?
     
    Last edited: Nov 24, 2012
  6. tubeguy

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    Yes, the IC seems good.

    The Audio Detector circuit should work if you connect the LED as suggested.
     
  7. tracecom

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    Thanks for your help.

    When I breadboarded the circuit, I didn't actually use a MOSFET. I was just looking for a positive voltage from pin 6 that would gate the MOSFET on. Are you saying that the LMC7215 will actually need to sink current to gate the MOSFET on?
     
  8. tubeguy

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    So, I'm new to surface mount. What did you use for soldering equipment ?
     
  9. tracecom

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    Just what I already had: a Hakko 936 with a 900-T-B tip and .032" solder. I needed smaller solder and some flux, and I might have done better with my 1.2mm spade tip. Of course, the SOIC8 is pretty easy as SMDs go.
     
  10. tubeguy

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    No, The IC would supply a positive voltage to the FET gate. This turns on the FET to sink current. Think of the FET as a switch in this circuit.

    You may need the FET or a transistor with a small base resistor to drive the LED. The IC may not have enough current capability at that supply voltage. Check the datasheet
     
  11. tracecom

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    That's what I thought. I must have wired the breadboard wrong; I'll rebuild it more carefully now that I know I have a good LMC7215.
     
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