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MSP430FR5969

WhiteHorse
Genius 4430 points
Other Parts Discussed in Thread: MSP430FR5969

Hi e2e,

We have a customer using the MSP430FR5969.  They have the following questions:

     If the internal ADC is not used (they are usig the clock multiplier) can AVSS be connected directly to DVSS without any filtering that would have been required if the ADC were?  The concern is that even if the ADC isnot being used that there may be other blocks such as the clock mulitplier that are being used that would require a cleaner power supply than the DVSS would be.

     On page 7 of hte data sheet it states "QFN package pad connection to VSS recommended.".  In the OrCad component geometry provided by TI there are about 20 vias to the package pad connection.  Is that many vias required or can some be removed?  What is the minimum recommended number of vias?

Thanks for your help!

Regards,

John Wiemeyer

 

 

 

  • 0
    TI__Guru 56650 points

    John,

    AVCC shall always be connected to DVCC. Please refer to the User's Guide: http://www.ti.com/lit/ug/slau367d/slau367d.pdf chapter 1.6 "Connection of Unused Pins".

    Regarding the vias, I think the important thing is that you need to make sure that the VSS/GND pad is well connected to the rest of GND plane. There is a general PCB layout guideline which can be found in the following application report:

    http://www.ti.com.cn/cn/lit/an/slaa530/slaa530.pdf

  • 0 in reply to Leo Hendrawan
    Genius 4430 points

    Hi Leo,

    A seperate AVCC is provided on the MSP430 to proivde clean power to the analog circuitry when the analog circuitry (ADC) is used.  In this case some filtering would be done between the connection of DVSS and AVDSS.  On page 52 of the User's Guide you referred to it shows table 1-4 which is Connection of un-used pins.  It indicates AVSS is connected to DVCC when analog circuitry is unused.  The customer is not planning to use the ADC, but will use the clock multiplier.  In this case should the customer connect AVSSS directly to DVSS or should their be some filtering due to the clock multiplier being used in the MSP430.

    Regards.

    John Wiemeyer

     

     

  • 0 in reply to WhiteHorse
    Guru 227245 points

    John, some confusion with SS and CC in your last post. :)

     Well, AVCC and DVCC, as well as AVSS and DVSS are internally connected through clamp diodes.
    If you leave the analog pins unconnected this will do no harm if the analog parts are unused. The remaining required current will be sucked through the clamp diodes, which will have only a few mV forward voltage at such a low current (and up to 0.3V@2mA)
    If the analog parts are used, it is recommended to connect DVCC and AVCC with a filter (series resistor and filter capacitor). This will keep the digital ripple form the analog supply, as long as it does not exceed the clamp diode voltage.
    And DVSSs and AVSS should be routed separately to the supply, so the currents on the DVSS line won’t cause an offset voltage for the AVSS GND reference.

    What is the clock multiplier you are talking about (can’t find a reference in datasheet or users guide, but maybe my versions are just outdated) and why should it run from analog supply? ‘clock’ is a digital thing and I would expect it (whatever it may be) to be supplied by DVCC anyway.

  • 0 in reply to Jens-Michael Gross
    Genius 4430 points

    Hi Jens-Michael,

    The customer is not planning to leave AVCC unconnected.

    Customer needs to know what analog components inside the MSP430FR5969 are powered from the AVSS pins.  They need this information so they can determine if their appication uses any "analog components" in the MSP430FR5969 and if so add the necessary filter components to the connection between AVCC and DVCC to filter the noise and proivde clean power to AVCC.  If the customer is not using any analog components then they would like to not include the additional filtering on the AVSS power pins.

    Regards,

    John Wiemeyer

     

  • 0 in reply to WhiteHorse
    TI__Guru 56685 points

    John,

    An update for you here. For this particular chip, if you are not using the ADC you should be ok without the filtering components between AVCC and DVCC. AVCC is connected to a few other circuits in chip, but these are not as susceptible to the digital ripple that can be introduced by DVCC.

    Regards,

    JH

  • 0 in reply to WhiteHorse
    Guru 227245 points

    AFAIK, the only components powered by AVCC are ADC, DAC, comparator and reference voltages (if present at all). Everyting else, including the oscillator circuits (even though they are analogue circuits, strictly speaking) are powered by DVCC.
    And only those four components will profit from filtering.
    However, filtering is one thing, but doesn’t give too much benefit if AVSS isn’t separated from DVSS too, to avoid GND currents that shift the analog GND reference. Here, no filtering is needed, but rather a separate routing to the supply GND point. Well, a connection to a GND supply layer rather than to a GND trace, will usually do too.

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