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unfamiliar symbol

Discussion in 'Misc Electronics' started by gearhead, Jan 31, 2009.

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  1. gearhead

    gearhead Guest

    I had a couple of things I wanted to figure out about the SG3525 and
    got to looking at the schematics for the oscillator and the error
    amplifier at the bottom of page 6 of the datasheet. (I wanted to
    figure out how to use the sync pin, since the datasheet doesn't
    explain it. And how low the common mode of the error amplifier goes,
    since the datasheet only states that it includes Vref, or 5 volts.)
    http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
    What does the symbol that looks like a transistor with two collectors
    represent and if you can, help me figure out the answers to my two
    questions.
     
  2. Bob Eld

    Bob Eld Guest


    The two collector transistors are current mirrors. The area of the two
    collectors is in a precise ratio, usually one to one. Therefore the current
    through the transistor divides per that ratio. If it's one to one, the
    current divides 50%- 50%.

    In a current mirror, one collector current is defined by the appropriate
    voltage drops and the other collector follows or mirrors that current. This
    kind of structure is available within integrated circuits but not available
    in discrete devices. You can make discrete current mirrors but not with
    divided collector areas.

    Apparently the sync input must pulse high to restart the ramp. If you do not
    use it, the device will self oscillate asynchronously.

    The common mode range of the error amp is 1.5 volts to 5.2 volts to stay
    within the specified common mode rejection values. It's in the spec. on page
    4.
     
  3. Shaun

    Shaun Guest

    Hello,

    If you look on page 4 of the datasheet, the minimum voltage to operate the
    sync line + 1.2 volts. That is because you have to turn on the transistor
    (0.7 volts) called bias, and there is a voltage divider in front of it too.
    This pin "sync" is for an external oscillator, if you don't want to use the
    built in one plus Ct and Rt. You would have to supply it with a signal that
    is greater than + 1.2 volts on the peak or add a positive offset voltage to
    the signal.

    The dual collector trasistor is common for transistors in an IC, don't ask
    me why, I don't know what the advantage is.

    Page 4 also gives information for the error amplifier.

    Shaun
     
  4. gearhead

    gearhead Guest

    Okay, I know about current mirrors like Q12 and Q13, now I know that
    symbol means something similar.
    Then a high signal on the sync would have to turn on Q14 in order to
    discharge the timing capacitor;
    but when I try following the current mirrors around the circuit it
    looks like just the opposite happens.
    I can settle this by sticking a chip in the breadboard.
    Thanks!
     
  5. Bob Eld

    Bob Eld Guest

    Okay, I know about current mirrors like Q12 and Q13, now I know that
    symbol means something similar.
    Then a high signal on the sync would have to turn on Q14 in order to
    discharge the timing capacitor;
    but when I try following the current mirrors around the circuit it
    looks like just the opposite happens.
    I can settle this by sticking a chip in the breadboard.


    Yes I agree it does look weird. I checked the Texas instruments version of
    this part, UC3525 and they do NOT show a connection between the collector of
    Q13 and the base of Q14. I have not studied it enough to figure out the
    actual circuit or who is right. Obviously there is at least one error in at
    least one of the schematics. So who knows?

    Tell us how the breadboard reacts and then maybe we can make sense of the
    schematics.

    Keep in mind that the schematics are for reference and are abbreviated and
    stylized for simplicity. They are NOT the actual circuitry used with
    complete detail.





    Thanks!
     
  6. gearhead

    gearhead Guest

    The TI schematic is all wet.
    Q14 is floating there in space, doing nothing... the base doesn't
    connect to anything that can bias it on.
    Likewise with the whole discharge circuit: it has no connection to
    the components in the timing section.
    So the TI diagram is missing connections at the base and emitter of
    Q14 that the OnSemi circuit diagram shows.
    The OnSemi diagram starts to make sense as I look at it now. I had
    the mirrors turned around or something when I looked at it before.
    Without going too deep into it--
    The 7.4k and 14k resistors set a trip point for the timing ramp, not
    far from one timing constant.
    At that point the mirrors turn on Q14 (I had it backwards the first
    time Iooked). That discharges the timing cap.
    Same thing happens if you pull up the sync pin. You were right, Bob.
     
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