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Thermal Pad to Ground?

Jeff Labute
Prodigy 130 points

Other Parts Discussed in Thread: MSP430G2231, MSP430F2370

Hi Everyone :-)

We are designing around the QFN16 package MSP430G2231. Should the thermal pad go to ground? Also, if we are running it at 1MHz and it sleeps 90% of the time... do we really care if there is a thermal pad? I noticed on some MSP430 spec sheets it mentions that it is recommended for the thermal pad to go to ground... but for the MSP430G2231 it doesn't say anything.

Thanks

 

Jeff

  • 0
    Prodigy 130 points

    QFN Pad - Pad NA QFN package pad connection to VSS recommended.

    It's ok, I found the answer... it is in the spec sheet! Page 6 under pin assignments... just called QFN Pad

    Thanks all.

    Jeff

  • 0 in reply to Jeff Labute
    Expert 2045 points

    Hello!

    By the way, could anybody explain why a thermal pad is used?

    By definition MSP 40 power dissipation is low, so what's the point?

    Or is there another advantage, for instance in terms of noise, in which case a more

    meaningful name may be worth?

    Thanks,

    Pascal

     

  • 0 in reply to Pascal4275
    Prodigy 130 points

    I wonder that myself... at least for the way I will be using the chip the pad will probably heat up by 0.002c... haha. I think perhaps under un-usual circumstances such as driving an LED on every IO while writing to flash could take a lot of current... could perhaps get slightly warm if you wanted it to.

    Of course... they don't actually call it a thermal pad in the msp430g2231 spec sheet... they just call it a QFN pad... maybe it is for noise?

     

    Jeff

  • 0 in reply to Pascal4275
    Guru 227245 points

    The MSP being low power desgign does not mean you cannot use it in a way that it exposes high power dissipation. E.g. try to draw 20mA from several port pins, the voltage drop on the pin drivers will produce a relatively high power dissipation. This induces self-heating that will change several internal parameters such as DCO frequency and voltage reference, but also render the internal temperature sensor pretty much useless.

    Using a thermal pad will support the MSP on keeping at ambient level, so the thermal sensor will show ambient temperature and the reference voltage and internal oscillators stay on the calibrated conditions.

  • 0 in reply to Jens-Michael Gross
    Expert 1385 points

    I don't think anyone has brought this up yet, so I thought I would throw it out there. My understanding is that the pad is required as a mechanical support. I think with QFN packages the pins aren't doing much to hold the part on the board. Having the pad soldered can really help hold it on the board when you've got some vibration.

  • 0 in reply to Jeff Labute
    Prodigy 10 points
    There seems to be no official statement by TI on the use of thermal pad. It is clear that in case of high load of output pins the die will warm up and the pad is useful.
    So it seems that the pad in case of very low power applications is useless and could be left open.
    In our experience an open thermal pad is extremely suscebtible for ESD influence. I think TI schould mention this in datasheets. We measured the capacitance on MSP430F2370 between all pins connected together to thermal pad and the value was 680 pF in one case. So it is rather probable that an unconnected pad is apossible reason for ESD problems.
  • 0 in reply to Julius Grabner
    Guru 227245 points
    From the 2010 MSP430F2370 device datasheet slas518c.pdf (Terminal functions table):
    "QFN Pad -- NA - NA - QFN package pad connection to D/AVSS recommended"

    I'd say this is a fairly enough official statement.
    It is not mandatory, as the device will properly work without. And depending on manufacturing method, soldering the thermal pad might be complicated (especially in prototyping with manual soldering).

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