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Bootstrap Cap Function?

Discussion in 'Electronic Design' started by Carey Fisher - NCS, Jan 18, 2004.

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  1. Can anyone tell me what the "Boostrap" capacitors do as shown in the
    application schematics for the TDA2005 stereo/BTL amp chip? The data sheet
    is here:
    http://us.st.com/stonline/books/pdf/docs/1451.pdf

    Thanks!

    Carey Fisher, K8VZ
    Chief Technical Officer
    New Communications Solutions, LLC
     
  2. James Meyer

    James Meyer Guest

    Without looking at the doc pdf.....

    When NPN transistors are used as the upper elements in a totem-pole,
    push-pull, output stage and in order to get the output to swing all the way to
    the upper supply voltage, you need to provide the upper transistors with a base
    current from a voltage supply higher than the supply rail. One simple way to do
    that is with a "bootstrap" capacitor.

    The bootstrap capacitor will (slowly) charge to the difference between
    the output and the upper supply. Then when the output swings high, the side of
    the capacitor not connected to the output will swing higher than the upper
    supply.

    For extra credit: What determines the minimum value for the bootstrap
    capacitor?

    Jim
     
  3. Jim Thompson

    Jim Thompson Guest

    They provide a drive bias higher than the high rail (lower than the
    low rail) so that the output can swing closer to the rail potentials.

    ...Jim Thompson
     
  4. Thanks Jim! Extra credit....well...uh... the minimum value for the cap is
    probably determined by the lower 3dB freq ?Carey
     
  5. Thanks Jim. I guess one of the constant current sources shown in the chip
    is used to charge the bootstrap cap.
    This explains why I can get more output power with the bootstrap cap in
    place.
    Carey
     
  6. Jim Thompson

    Jim Thompson Guest

    There's usually a disconnect diode there also, to keep from dumping
    the cap back into the rail.

    ...Jim Thompson
     
  7. James Meyer

    James Meyer Guest

    That's one factor. What's another?

    Jim
     
  8. Circuit Gain?
    Carey
     
  9. James Meyer

    James Meyer Guest

    The cap supplies drive current to the output stage. It's functionally
    part of the power supply. The current it supplies and the length of time it
    needs to supply it will determine its minimum size just like the other caps in
    the power supply.

    Jim
     
  10. Thanks for the explanation. I've tried the circuit with and without the
    bootstrap cap. It only adds 0.6 Vp-p to the output voltage swing (9.4 Vp-p
    without, 10.0 Vp-p with cap, all on a 12VDC supply). Almost doesn't seem
    worth it!
    Carey
     
  11. I read in sci.electronics.design that Carey Fisher - NCS
    Something else is limiting the voltage.
     
  12. NCSRadio

    NCSRadio Guest

    How much of an increase should I expect?
     
  13. Ben Bradley

    Ben Bradley Guest

    In sci.electronics.design, "Carey Fisher - NCS"
    Aww, you're just guessing. :) I'll butt in and give you a hint.

    The capacitor connects two points in the circuit, the output and
    something else. The output has effectively zero impedance (for the
    purpose of this calculation) - what is the impedance at the other end
    of the capacitor (once you have that, you can calculate the -3db
    frequency and/or the size cap you need for a certain low-frequency
    response)? Another oblique hint, this may be even trickier than you
    think...
     
  14. I read in sci.electronics.design that NCSRadio
    Post your schematic on a.b.s.e and tell us here that you have done it.
    Very roughly, and making several assumptions about the configuration, an
    increase of a quarter of the supply voltage might be expected. But in
    this case, the voltage you get with no bootstrap is unusually high.
     
  15. Michael

    Michael Guest

    You did this test with the amp loaded right?
     
  16. Ian Bell

    Ian Bell Guest

    The normal use of bootstrapping is to provide positive feedback in order to
    raise the collector impedance of the previous stage. This raises the stage
    gain and reduces distortion (with feedback).

    Ian
     
  17. Jim Thompson

    Jim Thompson Guest

    In this particular case it's a bias bootstrap.

    ...Jim Thompson
     
  18. Yes, with both 4 Ohm and 8 Ohm non-inductive resistors.
     
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