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Dumb question regarding SMPS

Discussion in 'Electronic Design' started by spamtrap1888, Oct 8, 2012.

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  1. spamtrap1888

    spamtrap1888 Guest

    If someone has a free moment, I'd like to know:

    I'm on my third laptop right now. Every time I plugged my Dell's "fat
    snake" into the wall, I drew quite an arc. The Lenovo's arc was not
    noticeable, but now I get a noticeable arc with my new HP -- not as
    big as the Dell's, however.

    I know FA about switch mode power supplies, obviously, so I wonder

    1. What produces the arc?
    2. Why would different power supplies produce different arcs (does it
    just depend on output power capability)?
    3. Why is there no arc when I pull the plug from the outlet?
  2. Jamie

    Jamie Guest

    That verifies it.

  3. I like the ucc28019a. works like a champ. Undervoltage lockout too ;)

  4. Joerg

    Joerg Guest

    Might be the new normal. I want to design in a National video driver,
    LMH6722. Has a thermal pad under its belly. In the datasheet they forgot
    to mention where its s'posed to be connected to. Probably V- but I'd
    rather make sure. Filed a support ticket with the new owner TI on 10/2.
    Got a service request number.

    Today is 10/8 and (finally! ... or so I thought) there was a message in
    the inbox this morning. A form letter, merely saying that, tada, a
    service request number has been issued. New number: Same as the old number.

    So I responded politely as to when I might be expecting an answer. No
    response all day.

  5. Joerg

    Joerg Guest

    No kidding, that may be the only way :-(

    Just made a CAD model for a 100-TQFP processor. Now I know why I chose
    to become an analog guy and not a digital one.
  6. spamtrap1888

    spamtrap1888 Guest

    They're either overworked or lazy, so they punted. Probably the
    responders are graded based on how fast they turn around responses,
    and on yours they already hit infinity.
  7. MrTallyman

    MrTallyman Guest

    An idiot who can't even do simple connect-the-dots layouts.

    Yep, you just reinforced anyone's faith in your abilities. Not.
  8. Joerg

    Joerg Guest

    Lo and behold, just as I wanted to order samples I finally had a "You've
    got mail" event. TI support said the pad is not connected to anything.
    It can be left floating (which I'd never do, of course), connected to
    V-, or connected to GND (which I'll do).
  9. Guest

    I was curious so I looked at the pdf with the pcb for the evaluation
    it doesn't even have a pad on the pcb for the thermal pad
    V+ and V- goes under theat part from each end to the center pins

  10. Joerg

    Joerg Guest

    The challenge is always with switchers. The upper FETs of a bridge or
    sync-buck are easy, I just use a V+ plane. The lower ones hang on the
    switched node with their heat-carrying drain tabs. That is a real pain
    because normally you don't want to make that very capacitive.

    Modern datasheets are notoriously incomplete. Even uC with their
    hundreds of pages. They discuss the logic stuff ad nauseam and then, if
    you are lucky, you find one or too sparsely populated pages on the ADC.
  11. Guest

    we have everything modeled in inventor, pcb with components,
    connectors etc.

    so we know it will fit in the box, there's room for the connectors

  12. Joerg

    Joerg Guest

    No, just the schematic library part and footprint. A hundred pins, most
    of which have names like this:


    One typo and all hell can break loose because the routing resources in
    those uCs are sparse and can be unforgiving. Just had a major
    re-shuffling in one of them on another project, not because of an error
    but for a feature change. When those get maxed out in port pins the
    design can slow down as much as Van Ness at rush hour, mainly because of
    routing compromises.
  13. Joerg

    Joerg Guest


    Don't know about this one but sometimes with eval boards I have the
    feeling that they aren't always super great. I remember one where I
    fired it up, ran it with standard load and then caught a whiff of an
    "amperage smell". Followed by smoke. That was the snubber resistors
    turning themselves into charcoal. Another board ... one minute, two
    minutes, three ... *PHHHHUT* ... a diode had left its workplace without
    prior authorization to do so. A quick calc revealed a 3x or so overload.
    Luckily I found its pieces on the floor before a guide dog puppy we had
    for a week.
  14. Guest

    looks like they just more or less copied the layout from the packages
    that don't have a thermal pad
    strange how eval board are often like that, would think they wanted to
    their part in the best possible way

    can only guess they assume those who need the performance will build
    their own
    board anyway and those who buy the eval board generally won't notice

  15. Joerg

    Joerg Guest

    Wot reference? Sometimes they think that VCC _is_ a good-enough
    reference. Sometimes we've had to do ratiometric conversion just because
    there wasn't even a pin to pipe in your own reference.
  16. Joerg

    Joerg Guest

    Actually some are not bad at all. Otherwise they would not pass muster
    with agencies when they are placed in metering applications. Often it is
    important to halt processor activity during a measurement, that can make
    a huge difference. This is what we'll likely do for the project I am
    working on right now.
  17. Guest

    You *should* be able to either grab the names from a spreadsheet or
    cut-n-paste from a datasheet. The vendors often have models already built
    that can be used for a starting place, too. OTOH, our CAD people demand that
    chips look on the schematic like they do on the board - no functional
    partitioning (except BGAs, for some reason).
  18. Guest

    Or a hundred pages on register settings and one or two pages on the hardware
    itself; worse, the hundred pages isn't even complete.
  19. Joerg

    Joerg Guest

    Only for some CAD packages, if at all. My CAD has a lot of the Atmels,
    just not this big one.

    I insist on the same, I really hate netlist-style schematics where the
    front axle is on page 17 which the left front wheel it on page 32.
    Exceptions are logic gate and opamp multi-packs, of course. And I never
    use large BGAs, those can spell doom in a hi-rel environment.
  20. Joerg

    Joerg Guest

    Yup :-(

    Very classic omission: Do the ports have input hysteresis or not? And if
    yes, how much? Once the answer from the tech support engineer, after
    long head-scratching, was: "Good question! I'll have to inquire about
    that at the factory". Oh man ...
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