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TPSM82916: TPSM82916 Ferrite location not specified

Mark Whittington
Intellectual 490 points

Part Number: TPSM82916
Other Parts Discussed in Thread: TPS62916, , TPSM82913

Hello, 

We plan to use the TPSM82916VCER with the ferrite filter in the loop. In the datasheet layout guidelines (section 7.4.1) there is a note that says:

"Avoid placing the ferrite bead in the keep out region as shown in Figure 7-21 ". However, there is no keepout information shown in figure 7-21.

Can you send us the corrected figure showing the keepout? Or provide the relevant info so we can lay this out correctly?

Thanks.
Mark

  • 0
    TI__Genius 10466 points

    Hi Mark, 

    The keep out region refers to the region in between the first bank of output capacitors and the inductor as noted below Figure 7--21

    Best regards,

    Britton

  • 0 in reply to Britton Jones
    Intellectual 490 points

    Hello Britton, 

    The note you referenced talks about the ferrite being closer or further away and its impact on the filtering of the ferrite.

    • The way that note reads seems to imply that the keepout region extends beyond just the output capacitors.
    • There is a large and obvious gap between output caps and the ferrite shown in figure 7-21
    • The note goes on to explain that you can be closer than the keepout as long as you don't get between the inductors and the output caps.
    • The layout guidelines explicitly note a keepout as shown in the figure.

    Additionally, there is another datasheet for PN TPSM8291X that is the same same series (integrated ferrite) and it has a note that uses very similar language (part can be placed closer, further avoids electromagnetic coupling, if closer filtering greatly reduced, etc).

    But in that note there is an explicit distance of 8mm to avoid the loss of filtering.

    All this points to a missing keepout in figure 7-21. Are you certain there isn't a keepout or constraints beyond just "not between inductor and output caps"?

    Thanks.
    Mark

  • 0 in reply to Mark Whittington
    TI__Genius 10466 points

    Hi Mark, 

    The example layout you see is from our EVM. Our EVM was originally designed for the TPS62916 converter which has an external inductor and capacitor bank. The ferrite bead was placed directly after this. When the TPSM82916 was developed, the position of the ferrite bead was kept the same for convenience and the first bank of output capacitors was shifted inward, creating the noticeable gap. 

    In your case, you will be placing a bank of 6x-8x 22uF output capacitors after the device, and then placing the ferrite bead after this. You will find that this ferrite bead position is already 8mm from the device meeting the same condition that is specified in the TPSM82913 datasheet. (4x 22uF caps placed directly next to each other is almost exactly 8mm) The TPSM82913 used a smaller output cap bank of only 3x 22uF caps so the restriction needed to be documented with more specificity in that datasheet. You can use the same 8mm specification, but if you place the ferrite bead after the recommended number of output capacitors then this rule of thumb will also be sufficient. 

    Best regards,

    Britton

  • 0 in reply to Britton Jones
    Intellectual 490 points

    Hello Britton,

    Thank you very much for the detailed response. 

    Mark