LM556 or LM324 for PWM

Discussion in 'General Electronics Chat' started by iONic, Jul 21, 2011.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    For motor control, which IC woild make the better PWM with variable Freq. and variable Duty Cycly? Perhaps the introduction of a dead band as well...?

    Requirements:
    Frequency between 20kHz and 40kHz
    Duty Cycle between 0% and 100%
    IC operating Voltage of 5V - 12V DC

    Are there advantages or disadvantages of one or the other?

    Or................neither, IC "x" would be better!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, neither. :)
    The LM324 is just too slow for 20kHz-40kHz when you're looking for a square wave output. It runs out of steam at just a few kHz.

    The 555 timer can still crank pretty well up to around 100kHz, but you mentioned deadband - so that's out as well.

    I'm thinking that a synchronous buck controller would be a mighty spiffy application for a motor controller. You'd practically eliminate the losses you would otherwise have in the diodes across the motor. Besides, you wouldn't be accused of trying to turn an Escort into a Lincoln. ;)

    A TI UC3638 is specifically made as a current mode PWM motor controller.
    http://focus.ti.com/docs/prod/folders/print/uc3638.html
     
    Last edited: Jul 21, 2011
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Lots of choices...Google: PWM controller IC

    Ken
     
  4. iONic

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    Nov 16, 2007
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    Just did a quick search on National's website. Their "Buck Syncronous Switching Controllers" starting freq. are at 50kHz. The driver board I was considering accepts up to 40kHz. I'll have to admit, the whole concept you have just presented is absolutely new to me.

    Any other Op-Amp IC that might be able to handle around 30KHz of the top of your head?

    By the way, how would I know what the switching frequency limit is on the LM324 or any other OpAmp for that matter?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet for a low power (which means very slow) National Semi (its inventor) LM324 quad or LM393 dual opamp shows a slew-rate graph where it drops its output into a triangle-wave above about 1kHz at full output. Most audio opamps work perfectly up to 100kHz.
     
  6. SgtWookie

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    Jul 17, 2007
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    Well, opamps are slow because they're compensated, and the way their outputs are configured. If you want something a lot faster, you can use a comparator, like an LM339 quad comparator.

    It will run circles around most opamps for bandwidth. However, it makes for a lousy opamp, as it isn't compensated and has an open-collector output.

    I'd thrown together an LM339 PWM circuit for some fellow months back.
     
  7. iONic

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    Nov 16, 2007
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    Ah Ha! just found the TL494CN from TI. It includes dead time control and an oscillating frequency between 1 and 300kHz.
     
  8. SgtWookie

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    Jul 17, 2007
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    Check out the SG3525A, UC3525, UC3825, UC28025 current mode PWM regulators. The TL494 was really designed for driving transistors. If you're planning on feeding a TTL signal to something, these will be easier to connect.
     
  9. iONic

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    Nov 16, 2007
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    Thanks, the SG3525A looks real good.
     
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