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TMS320F28020: Power Supply and Ground Connections

Part Number: TMS320F28020

Hi All,

I'm designing in a TMS320F28020 and have questions about the best way to connect power supply and ground pins.

Power Supply:
Software development is underway using a LaunchXL TMS320F2802x.  The LaunchXL documentation (link below), which is for a TMS320F28027PT, shows VDDA and VDDIO connected to +3V3 through separate ferrite beads.  Then VDD1 and VDD2 have 2.2uF caps to GND.

Translating that to the TMS320F28020, I believe the following should happen:
1. VDDA and VDDIO should be treated the same as it is on the LaunchXL.  Each of these pins go to +3V3 through individual ferrite beads.
2. VDD at pin number 1 and 11 should tie to GND through individual 2.2uF caps.

Grounds:
LaunchXL documentation shows VSS1, VSS2, and VSSA going directly to GND.

For the TMS320F28020, should it happen like this?
1. Run separate grounds for digital (DGND) and analog (AGND).
2. VSSA on pin 23 goes to AGND
3. VSS on pin 2 and VSS on pin 12 go to DGND

Here's the LaunchXL TMS320F280x documentation:

Thanks in advance for your valuable help.
robin

  • Power supply, VDDA and VDDIO need to ramp together, keeping them on the same source is a convenient way to do that. Both VDD and VDDIO require decoupling caps. The DS gives some guidance, but also mentions that you should ensure that your power regulator's spec is met.

    Grounds: can be kept together or separate. Separation has the ability to increase noise rejection if done correctly, if done incorrectly separation of grounds can greatly add noise. Please take care when separating out ground planes. All VSS pins need to eventually make it to ground, how you do that or separate them is up to you.

    Please also check 7.2 "signal descriptions" in the DS.

    Regards,
    Cody 

  • Hi Cody,

    Thanks for your response.

    I'm a little confused over documentation that doesn't seem to be in agreement with each other.

    The LaunchXL schematic and PCB layout show VDD1 (pin 32) and VDD2 (pin 43) not going directly to 3.3V.  Instead, they ONLY go through 2.2uF caps to GND.  Here's an image from spruhh2c.pdf:

    However, SPRS523P – NOVEMBER 2008 – REVISED FEBRUARY 2021, page 10, would seem to indicate that the VDD pins are best tied to 3.3V, and should come up before VDDIO.  Here's an excerpt:

    So, just to be clear, should I tie VDD pins directly to 3.3V WITH caps to GND, or ONLY through caps to GND?

    Thanks again,
    robin

  • OK, I think I see what's going on...

    When using the internal Voltage regulator (/VREGENZ) these pins will already have 1.8V on them and are to be used for 1.2uF or higher bypass caps.  When using an external 1.8V regulator, these pins connect to the external regulator.

    So, since I'm using the internal regulator, I only need the caps to ground.

    Can you confirm that's right?

    Thanks,
    robin

  • Robin, 

    Correct, the internal VREG is used on the LaunchPad. When using the internal VREG you do not need to provide 1.8V power externally. VDD should never be connected to 3.3V, this is a 1.8V supply. VDDIO and VDDA are the 3.3V supplies. Decoupling caps to ground are needed on VDD, but the value of 2.2uF is not specifically required.

    Regards,
    Cody 

  • Hi Cody,

    That's just what I needed to know.  This resolves my issue.

    Thanks for your help.
    robin