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Thread: LTC1775: Does anyone know this IC? 5V@20A, 3.3V@20A, 12V@4A all from a 6V-18V source

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    LTC1775: Does anyone know this IC? 5V@20A, 3.3V@20A, 12V@4A all from a 6V-18V source

    http://www.linear.com/pdf/1775f.pdf


    Typical applications: 5V@20A, 3.3V@20A, 12V@4A all from a 6V-18V source

    This is a hell of an IC. The datasheet has typical applications for the values I said, I'm not making this one up.

    The best thing is it does not need small value sense resistors, it can obtain the 12V from a source from 6V to 18V with a single inductor and the huge Amps for the 5 and 3.3V

    The not so good thing is the packaging:
    SMD S Package 16-Lead Plastic Small Outline (Narrow 0.150)
    and no free samples. Each one costs 6 USD.

    Miguel

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    FYI

    These schematics are from the datasheets:



    The 3.3V version can be obtained conecting the Vprog to SGND instead of IntVcc.



    add another simple solution for the -12V and you can have a powerfull ATX power supply (the -5V can be negleted because it's unused).

    Miguel

  3. #3
    Raw Wave Rob Withey's Avatar
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    Sorry, I guess I'm missing the point. This is a switch mode controller ICs. They need big fat mosfets to do the switching for them. why is this chip such a big deal? You can increase the current capacitor of most switcher ICs by adding fat transitors to them.


    Rob
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    New system at design/prototype stage on BeagleBoard.

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    You can? Are you sure?

    In that case why do most controller IC's have a maximum Iout that they can withstand? If you search in MAXIM website for controllers that can give you 5V@20A you end up with an empty search result. Their controllers have a maximum curret of 10A, and that's for the external MOSFET version (MAX797).

    I know that the current goes through the MOSFETS and not the controller, but if they have a Iout rating, that must mean something i guess.

    What am I missing here??

    Miguel

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    USE THESE, THEY ARE GOOD!!! they offer 97% at 20amps 5volts, this is good.... Do you have a better way to get more current @ 12volts???? that is small...

  6. #6
    Raw Wave Rob Withey's Avatar
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    Originally posted by mccc_mail_pt
    You can? Are you sure?

    Yes. Go here:

    http://www.us.st.com/stonline/books/toc/ds/93.htm

    Get the datsheet on the L296 (the switcher I'm using). One of the circuits is a 12v/10a circuit (even though the chip has a maximum current of 4a!!!). And guess what, the efficiency is up to 90%. The words "up to" are important, because at the higher currents you won't see that. I'm using this chip and getting around 60-70% efficiency. There's lies, damn lies, and statistics...!


    Regards

    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  7. #7
    FLAC
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    Originally posted by mccc_mail_pt
    You can? Are you sure?

    In that case why do most controller IC's have a maximum Iout that they can withstand? If you search in MAXIM website for controllers that can give you 5V@20A you end up with an empty search result. Their controllers have a maximum curret of 10A, and that's for the external MOSFET version (MAX797).

    I know that the current goes through the MOSFETS and not the controller, but if they have a Iout rating, that must mean something i guess.

    What am I missing here??

    Miguel
    Big external mosfet = more max Iout. Some spec a max because of the curren feedback loop starts to get really inefficient if you push more current. You can always modify the design to get more current out of it. But that requires that you really understand it.
    MPEGBOX - Plexiglass Computer
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  8. #8
    Raw Wave Rob Withey's Avatar
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    Also note with Mosfets that since they operate effectively as a resistor when switched on (Rds on), the power dissipated by the mosfet goes up as a function of the square of the current:

    P=IV
    V=IR
    P=IIR

    Getting a lower Rds on mosfet helps, but fundamentally you are pushing more current, and therefore will see more power dissipation and therefore lower efficiency. Bipolar transistors are a different kettle of fish. I'll leave the calcs on this one as an exercise to the reader.


    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

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    "On a BJT, the collector current depends not only on Vbe but also on Vce. The dependence on Vce can be modeled by assigning a finite output resistance Ro with the value Ro=Va/Ic, where Va is the Early voltage and Ic is the collector dc bias current."

    Microeletronic Circuits, third edition - Sedra/Smith

    On the other hand I could not find Va in this Power BJT datasheet.
    http://www.onsemi.com/pub/Collateral/2N3771-D.PDF

    Damn theory books

    The only calc I could do was

    R=Vcesat/Ic = 4/30 = 133 mOhm

    Miguel

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    Hey people.

    I am very much thinking about using this IC for a supply, and YES they can handle the current.

    I am currently using hte LTC1735-1, but I hate these small 0.01 Ohm sense resistors.

    Per the dissipation of the power FETS.

    There is more dissipation there than you have stated, the Rds(ON) is only the DC power dissipation.

    These switcher supplies operate with a ripple current amplitude set by the external inductor, and frequecny set with an external cap.

    Anyway, if you look in the LTC1775 datasheet they will show you the equations to calculate the AC power dissipation.

    May I suggest this other chip, which includes the two power FETS and the Schottky diode in one package!!

    International Rectifier, IRF7901D1

    Beware of schotkky noise though aroung 695kHz, should be no prob for audio though......

    Use a LP filter on the output.

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