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TMS320F280025-Q1: Shall the application connect all VDD pins together or this is just recommendation?

Will Gao
Intellectual 2335 points
Part Number: TMS320F280025-Q1
Other Parts Discussed in Thread: TMS320F280039C

Hi BU, 

In F28002x datasheet, it said that: 

Customer has some designs that do not meet this recommendation, and I'd like to check with you if this is ok or shall change the design to connect all VDD pins together. Thanks. 

Regards, 

Will 

  • +1
    TI__Guru* 95315 points

    Will,

        For proper/reliable operation of the device, all power pins must be connected. Extremely difficult to predict exactly what would happen (and when) when this requirement is not followed.

  • 0 in reply to Hareesh Janakiraman
    TI__Intellectual 2335 points

    Hi Hareesh, 

    Thanks for the guidance. 

  • +1 in reply to Will Gao
    TI__Guru 73590 points

    Will,

    While what Hareesh has mentioned is valid; I think there is a special case for the F28002x since the internal VREG is always on. 

    As such the voltage is internally generated by the device, and distributed to each VDD pin via an internal passgate transistor.  In order to keep the voltage constant during the demands of the device we give a specification for bulk capacitance on each pin.

    In this scenario, as long as customer has distributed the required capacitance evenly, and has a cap local to each pin, then the requirement to tie all the VDD pins together externally can be waived.  We will be adding this as an option in the next datasheet revision.  You can look at the TMS320F280039C datasheet for an example of the above.

    If the customer wishes to implement a larger bulk cap and smaller local caps to meet the requirement, then the VDD pins would need to be tied together so that the large cap is seen on the same net.

    Best,

    Matthew