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TMS320F28384D: VDDIO Decoupling Capacitor Configuration

Nick Zupancich
Prodigy 10 points
Part Number: TMS320F28384D
Other Parts Discussed in Thread: TMDSCNCD28388D

I'm designing a board that utilizes a TMS320F28384D and I have a question about decoupling capacitance on the VDDIO bank. 

The 'TMS320F28384x Real-Time Microcontrollers with Connectivity Manger June 2023' datasheet states in Table 6-1, Pg. 49 for VDDIO that for each "3.3V Digital I/O Power Pin, Place a minimum 0.1uF decoupling capacitor on each pin". There's essentially no way to place a 0.1uF capacitor per pin given there is 32 pins and some of them are embedded with the BGA and you no access to them. 

Our approach has been to place as 16 0.1uF near VDDIO pins as well as a 2.2uF capacitor bringing our total capacitance on that bank up to 3.8uF of capacitance, which is greater than the 3.2uF minimum called out in the datasheet. We designed this similar to the TMDSCNCD28388D demo board configuration. (Picture below). 

I just want to get clarification that this is acceptable, although it is contrary to the datasheet. 

  • 0
    TI__Mastermind 41905 points

    NIck,

    The goal of these caps are to decouple noise from the supply. They are required near each pin. Of course it is ideal to place them closer, though further away is still acceptable. The datasheet guidance is the letter of the law here, and will be my official recommendation.

    Unofficially the device will operate well with fewer caps as long as they are distributed well and the system noise on the respective power rails is not excessive. When doing this the potential for EMI and aggressor is increased, though practically it can be mitigated effectively with less caps. 

    These are decoupling caps, not bulk caps. Adding a 2.2uF cap is not necessary from a supply stability standpoint. It may not help with noise either, unless it decouples noise at appropriate frequencies for the noise in your system.

    Regards,
    Cody 

  • 0 in reply to Cody Watkins
    Prodigy 10 points

    Cody, thanks for the explanation!

    Could you give reason to why TI's own TMDSCNCD28388D demo board didn't adhere to the datasheet requirement on VDDIO decoupling capacitance?

    Thanks!

  • 0 in reply to Nick Zupancich
    TI__Mastermind 41905 points

    The cCARD is size constrained(not a good excuse) so there is a desire to keep it small. And more importantly my boss at the time said "eh, it'll be alright", and I chose to not contest.

    The correct course of action is to follow the datasheet and add the recommended 1cap/pin.

    Regards,
    Cody