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Low drop regulators

Discussion in 'Electronic Basics' started by vic, Jun 26, 2006.

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  1. vic

    vic Guest


    I'm wondering what are the principles used in low drop regulators.

    I 'm trying to make one using standard components, I started by using a
    zener and a resistance, the closer I could think to a low drop
    regulator. However there is a constant current draw independant of the load.

    I tried then to use a transistor with a fixed voltage at the base
    (provided by a LED), and then using the fact that VBE is constant =
    0.6V. However there is still a voltage drop across the transistor.

    So, can one achieve almost 0V drop in some regulators ?

  2. John Fields

    John Fields Guest

    Use a PNP for your series pass element (with the emitter going to
    your raw +DC and the collector going to your load) and you'll be
    able to get the voltage drop across the transistor to Vce(sat),
    which is a lot better than than you can get with the transistor
    wired as an emitter follower.
  3. The low drop regulators all use a pass element (either bipolar
    junction transistor or mosfet) operating in inverting mode. For a
    positive regulator, this implies either a PNP transistor or a P
    channel mosfet. This also means that the pass element is acting, not
    as a follower (with its inherent fast negative feedback) but as a
    voltage amplifier that needs external voltage feedback to control the
    output voltage. It is also a lot harder to stabilize over a wide
    range of output current, and in the case of the PNP pass element, adds
    the base current requirement to the load current requirement to come
    up with the total input current.

    You might look at the equivalent schematics for some of the integrated
    low drop regulators to see what kind of circuit performs this function.
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