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Discussion in 'Electronic Design' started by New User ^_^, Nov 19, 2004.

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  1. New User ^_^

    New User ^_^ Guest

    Hi All,

    First of all, I'd like to thank to those who answered my question
    before regarding boost converter IC. Actually, I'm not sure the
    difference between boost converter and boost switching regulator.

    For example, I want to boost the input voltage by 10V, i.e. Vout = Vin
    + 10V. Shall I use boost converter or boost switching regulator??

    What does it mean by the following terms?
    Fixed 3.3-V High-Efficiency Boost Converter
    Adjustable 3.3-V High-Efficiency Boost Converter
    Fixed output voltage?? Adjustable output voltage??

    For my case, shall I use UC2577-ADJ or LM3478 if I just want to boost
    Vin by 10V for 3V<=Vin<=10 ??

    Thanks for your help in advance.

    Regards,
    Will
     
  2. Anders F

    Anders F Guest

    Same! The first is just short for the latter...
    Set by the device (controller)
    Set by you (usually by a resistor divider)
    Most designs boost to a stable voltage. You might do some design to add the
    input voltage to the feedback. Bust most probably you will be satisfied with
    ie. 20V out from 3-10V input?

    /A
     
  3. Fred Bloggs

    Fred Bloggs Guest

    In order to obtain Vout= Vin + 10V which is equivaleNt to
    (Vout-Vin)=10V=~8*Vref of the UC2577, you will have to do something like
    this:

    View in a fixed-width font such as Courier.


    ..
    ..
    ..
    .. L1
    .. Vin >---+---UUUUU-----+
    .. | ===== |
    .. | |
    .. +------------+ |
    .. | Vin | | D1
    .. | sw +----+-----|>|--+---+---------+------> Vin + 10
    .. | | | | |
    .. | | === | /
    .. | | | | 20K OA=LT1013
    .. | UC2577 | GND | /
    .. | | | \ or other
    .. | | OA |/| |
    .. | | 6.0K /+|-------+ single-
    .. | (2)+----+---/\/\--+---< | |
    .. | FB | | | \-|--+ / supply
    .. | | / | |\| | 10K
    .. | | 2,0K | | | / 1MHz GBW
    .. | | / | gnd | \
    .. | | \ | | |
    .. | GND | | | | gnd
    .. +-----+------+ gnd | 10K |
    .. | +---/\/\--+
    .. +-----gnd |
    .. | 20K |
    .. | Vin >---/\/\--+
    .. |
    .. -------+--------------------------------------------> GND
    ..
    ..
     
  4. Mac

    Mac Guest

    It is VERY unclear what you are trying to do.

    I'm going to ask some questions, and I suggest you take the time to answer
    them. If you don't know the answers to some of them, then you can just
    say that you don't know. If you don't know the answers to ANY of them,
    then you are not ready to select an IC.

    1) What range might Vin be?

    2) How much current do you need to supply from Vout?

    3) How does Vin vary with time? Is it a non-rechargeable battery which
    drains slowly over time? Is it a rechargeable battery which slowly
    drains but might experience a rapid rise when the charger comes on-line?
    Is it a music signal? Voice? What?

    4) What if, instead of producing Vout = Vin + 10, we could just produce
    some nice fixed voltage such as 20 volts or 48 volts or something? Would
    that meet your needs? It might be easier to do this than to create Vin +
    10 volts.

    5) What, if any stable voltage supplies do you have available to you?

    It is important that you answer these questions, because a variety of
    people are probably going to give up trying to help you if you don't start
    telling us a little bit more about your application.

    [snip]
    You can best show your appreciation by supplying answers to the questions
    that I and others have asked you. ;-)
    --Mac
     
  5. kell

    kell Guest

    I am wondering about your post. I can't think why you would
    want your otuput voltage to wander up and down.
    Because your input voltage will probably vary with load.
    Do you really want a converter with an output that is not regulated?

    If the answer to that one is yes, one way is to use a
    converter with an isolated output, like
    a push-pull converter. That way you can just take the output
    and stack it on top of your input power source.

    Push-pull converters differ from the boost
    converter topology in that they use a transformer instead
    of a single inductor. Push-pulls will put out more power.

    What is the application?
     
  6. New User ^_^

    New User ^_^ Guest

    Hi Mac,

    Sorry for making trouble. I should have made it clear about my
    question.

    Actually, I'd like to step up Vin by 10V to Vout which is the supply
    voltage of a AND logic gate. This AND gate drives the gate(Vg) of a
    N-channel power MOSFET. But at the same time, the value of Vs is more
    or less the same as Vin. That's why I want to step up Vin by 10V in
    order to turn on the MOSFET completely.

    Hahaha, one thing I should have verified is that Vin is FIXED. The Vin
    given before is just a range of possible voltages.

    What I really want to know is: For Vin=3V, I make a boost converter
    (boost converter IC + different values of components, e.g. L, R, C )
    to step up Vout to 13V. Up til now, I think everything should be
    correct. But, then, I change Vin=10V. MY QUESTION is: Is it possible
    to step up Vout to 20V without changing the values of the components??
    Vin could be 3V-10V or higher
    About 100mA
    Vin is FIXED. The range is mentioned above.

    It would be nice if I could make Vout = Vin + 10V as I explained
    above.
    I have stable voltage supplies, 3V or whatever stable voltage source.

    Thanks for your help in advance.

    Best regards,
    Will ^_^
     
  7. Fred Bartoli

    Fred Bartoli Guest

    <Snipped diagram>


    Or if the OP doesn't mind a -2mV/K temperature dependency, here's the
    simpler :


    .------------------------------.
    | |
    . | L1 |
    . Vin >---o---UUUUU-----+ |
    . | ===== | |
    . | | |
    . +------------+ | |
    . | Vin | | D1 |
    . | sw +----+-----|>|--+-----)-------+------> Vin + 10
    . | | | | |
    . | | === +---. |
    . | | | | | .-.
    . | UC2577 | GND | | | |20K
    . | | | V | |
    . | | | - '-'
    . | | --- | |
    . | (2)+-----------+--------/ ^--+---'
    . | FB | |
    . | | .-. Small sig PNP
    . | | | |
    . | | | |2K7
    . | | '-'
    . | GND | |
    . +-----+------+ |
    . | ===
    . +-----gnd GND
    . |
    . |
    . |
    . -------+--------------------------------------------> GND
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
     
  8. Fred Bloggs

    Fred Bloggs Guest

    Very nice!- and good enough for high side MOSFET(s) gate drive bias.
     
  9. Fred Bartoli

    Fred Bartoli Guest

    Ah, I missed that. Yes indeed.
     
  10. Ken Smith

    Ken Smith Guest

    You can compensate for the -2mV/K like this:
    Editted diagram:

    R1
    -------- = Vfb/10V (about)
    R1 + R2
     
  11. Fred Bloggs

    Fred Bloggs Guest

    This is almost precision:

    View in a fixed-width font such as Courier.



    .------------------------------.
    | |
    . | L1 |
    . Vin >---o---UUUUU-----+ |
    . | ===== | |
    . | | |
    . +------------+ | |
    . | Vin | | D1 |
    . | sw +----+-----|>|--+-----)-------+------> Vin + 10
    . | | | | |
    . | | === | V D
    . | | | | -
    . | UC2577 | GND | |
    . | | .-. .-.
    . | | | |20K | |20K
    . | | | | | |
    . | | '-' '-'
    . | | | |
    . | (2)+------+--------------|-------+
    . | FB | | | |
    . | | | | |
    . | | .-. +---+---+ |
    . | GND | | | | | | |
    . +-----+------+ | |2K7 | c | c
    . | '-' | |/ | |/
    . +-----gnd | +-| T1 +-| T2
    . | | |\ |\
    . | === e e
    . | GND | |
    . -------+----------------------------+-------+-------> GND
     
  12. Fred Bloggs wrote...

    Try this way, same part count, but simplified concept and math...

    ..
    .. Vin >--+-------------+--------------------------,
    .. | | | gate-drive supply for
    .. +------------+ L1 | high-side FET switches
    .. | Vin | | D1 |
    .. | sw |----+--|>|--+----------------- | ----+---> Vout
    .. | | | Vin D2 | _|_+ Vin + 10
    .. | | '-- R2 --+---|<|---+ ---
    .. | | | | |
    .. | UC2577-ADJ | e\| |/e gnd
    .. | etc | |--+--|
    .. | | /| | |\
    .. | | 1.23V | | | Vo = Vin + Vfb R2/R1
    .. | fb |------+--------------' '----+
    .. | gnd | | pnp pair | Vo = Vin + 10 for
    .. +------------+ R1 bcm857bs 10k R2/R1 = 8.13
    .. | | bc857bs |
    .. -------+-------------+------------------------+-----> GND

    D2 protects the pnp transistor when Vout is more than 5V below Vin.
     
  13. Ken Smith

    Ken Smith Guest

    [.. diagram snipped ...]


    Linear makes an "isolated flyback" converter that contains much the same
    function as the PNP transistor. If the OP can stand the inaccuracy it
    has, it would be worthwhile looking at it.


    Unfortunately the Linear web site is still all screwed up so I can't find
    it and post the number. Its something like LT1723 or there abouts.
     
  14. Mac

    Mac Guest

    Have you considered using a P-Channel MOSFET with its gate pulled up to
    VCC (or VDD)? Then it would be easy to turn it on, provided it is a logic
    level MOSFET.
    There is probably some way to do this, but it would not be a standard
    application of the IC's that I am familiar with. They are usually designed
    to regulate to a particular voltage, and are usually NOT designed to be
    put in series with their V_IN.
    I see that you specified that in the post I replied to, so my apologies.

    Why do you need 100mA to turn on a MOSFET? Is it because you want to turn
    it on quickly, and switch it at a (relatively) high rate?
    Now that I know you are trying to turn on a MOSFET I have a better
    understanding of your needs.
    There is a fairly standard technique for turning on high-side MOSFET's in
    switching regulator applications which may be applicable to your situation
    if you are switching the MOSFET rapidly. It won't get you to Vin + 10, but
    it could get you nearly double Vin, which might be enough for you if you
    choose the right MOSFET.

    The idea is to use a diode and a capacitor. The diode is connected between
    VCC (or Vin) and the power supply for the chip driving the MOSFET gate.
    The capacitor is connected between the MOSFET source and the power supply
    for the chip driving the MOSFET gate. See the following ASCII art
    schematic (use courier or similar font). I hope I've got this right:


    VCC
    |
    +----------------------------+
    | |
    ----- |
    \ / Diode |
    --|-- |
    +---------------+ |
    | |\| | D
    ----- Cap | \ ||--+
    ----- _______| \_______|| N-channel
    | + | / G || MOSFET
    | | | / ||--+
    | Gate |/| | S
    | Driver | |
    | | |
    | GND |
    | |
    | |
    +----------------------------+
    |
    -----
    | L |
    | O |
    | A |
    | D |
    -----
    |
    GND

    The gate driver has to be able to withstand VCC * 2. The MOSFET has to be
    of a type that can be turned on by VCC, and there has to be frequent
    enough switching to guarantee that the capacitor will always be charged up
    enough to drive the gate.

    --Mac
     
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