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ADN8831 cooling mode

Discussion in 'Electronic Design' started by [email protected], Mar 22, 2007.

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  1. Guest

    I am working on a new hardware design utilizing the Analog Devices
    ADN8831 TEC controller.

    We are having a problem with the ADN8831 going into cooling mode. We
    are monitoring the VTEC output of the device and out2. When we
    transition from 88.0 C to 44.0 C degrees, the VTEC pin does not go
    above 1.25 volts (VTEC above 1.25 indicates the ADN8831 is in cooling
    mode) but out2 goes less than 1.5. According to the figure 21 of
    ADN8831 data sheet, when out2 goes below 1.5 volts, its should go into
    cooling. We never reach the 44.0 C temperature.

    In our design, PVDD = AVDD = 3.3 volts.

    Heating seems to work as we expect.

    Has anyone powered this device at 3.3 volts, the datasheet indicates
    it should work down to 3.0 volts?

    Any ideas why it will not cool?

    Thanks,
    Alex
     
  2. Have you tried contacting ADI's Applications Group about this?
    I know a few of these guys (they work upstairs from me) and
    they're usually pretty knowledgeable.
     
  3. Guest

    The amount of heat (in watts) transferred through a Peltier junction
    per unit current (in amps) is a great deal lower in cooling mode than
    in heating mode. In my application, a 40 degree Kelvin temperature
    change in the temperature to be controlled meant a seven-fold decrease
    in heat transferred per unit current.

    The Analog Devices data sheet for the AD8830 didn't give the formula
    that allows you to work this out, and the Linear Technology data sheet
    for their similar part is no better.

    You might not be putting enough current through your junction to get
    the cooling you need, and/or you might not have a good enough heat
    sink on the exhaust side of your junction to deal with the much
    increased output heat flux you get when cooling.

    I wrote up our project and published it

    "A microcontroller-based driver to stabilize the
    temperature of an optical stage to within 1mK in the range 4-38C,
    using a
    Peltier heat pump and a thermistor sensor" by A W Sloman, Paul Buggs,
    James
    Molloy and Douglas Stewart, published in Meas.Sci.Technol. volume 7,
    pages
    1653-64 (1996).

    E-mail me (at bill dot sloman at ieee dot org) if you want a reprint.
    My paper does include a formula for calculating heat transfer as a
    function of drive current - if you know (or can estimate) the thermal
    resistances around your junction.

    This comes up here every few years.
     
  4. Joerg

    Joerg Guest

    Since you wrote that it doesn't cool at all, could one of the output
    bridge FETs be busted? Or maybe one of them lost a pin connection? Watch
    the voltage levels on the actual TEC pins. If one does full swing but
    the other floats about in cooling than that's the cause.

    I have only used it at 5V and it works like a champ. You might want to
    feed it a little higher VDD and see what happens. But if a FET blew
    that's not going to change anything so I'd check those first.

    Anyhow, Just in case I'll email you the name and email address of the
    engineer at AD who knows about this chip.

    One word of caution: Some of the formulas in the app note for the
    ADN8831 are, ahem, not quite right. They told me that it'll be
    re-written (but not when).
     
  5. Guest

    Some additional information.

    We noticed that when the linear amplifier tries to turn on to cool the
    TEC, the soft start / standby pin goes low. When this pin goes low,
    it turns off the driver section off including the linear amplifier.
    After the soft start delay, the linear amplifier again tries to turn
    on to cool the TEC and again SS/SB goes low.

    Every time the linear amplifier turn on, it gets turn off by SS/SB
    going low.

    Any ideas as to why SS/SB would be going low on the linear amplifier
    turns on? We have a .1uf cap on ss/sb, the same as the evaluation
    board.

    Regards,
    Alex
     
  6. Joerg

    Joerg Guest

    That's an input so this is a bit puzzling. Page 14 (under stanby mode)
    says a logic high can be applied. You could try that and see if it cools
    then.

    Failing that I guess it's time to swap out the chip. I know this isn't
    going to be fun at all. Hopefully you guys have a large via below so you
    can heat up the large center pad with a big iron.
     
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