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TUSB1310a VBUS resistor and bulk capacitor question

Atsu K
Prodigy 100 points
Other Parts Discussed in Thread: TUSB1310A

Hi I'm looking at the TUSB1310A datasheet (http://www.ti.com/lit/ds/sllse32e/sllse32e.pdf) and have a few questions on the setup for this chip.

VBUS resistors

1) On p27 Section 5.1 It shows the VBUS pin of the TUSB chip connected to the voltage divider of the 90.9K and 10K resistors.  Is it correct that the power rating on the 90.9K resistors should be 5+ Watts because USB3.0 specifications say that VBUS is 5V and can carry 900mA of current. Since most of the voltage drop would be across the 90.9K resistor (5V * 90.9/(10+90.9) ), the power rating seems like it should be 5+ watts.

2) If #1 is correct, are there recommended resistors for this voltage divider? I could not find any surface mount resistors on DigiKey with that rating, and only one thru-hole resistor that met those specifications. It seems odd that this chip requires such a resistor.

Bulk capacitor

3) Section 3.4 (p4) of the Implementation Guide (http://www.ti.com/lit/an/sllu123/sllu123.pdf) recommends the use of bulk capacitors.  What are the suggested capacitance values for these?  And is it correct that I would want one for each supply rail placed close as possible to the voltage pins on the chip?

4) Would these bulk capacitors be in addition to the Pi filters for the analog supply rails?

Thank you for your help.

  • 0
    TI__Guru* 76130 points

    Hello,

    1) When VBUS=5V there will be a voltage drop of 4.5V on the 90.9K resistor, this means a current of 49uA.

    2) You can use a 1/16W resistor.

    3) 0.1uF, one for each power terminal.

    4) Yes

    Regards

  • 0 in reply to Elias Villegas M.
    Prodigy 100 points

    Hi Elias,

    Thanks so much for the response.  That makes a lot of sense.  

    As for #3, is the "bulk capacitor" of 0.1uF you recommend the same thing as the "bypass capacitor" of 0.1uF that's recommended in the earlier sections (3.1, 3.2) for the analog and digital power terminals?  Or do I actually need two 0.1uF caps for each power terminal? (one as the "bypass" and one as the "bulk"?)

    In section 3.4, it says 'bypass capacitors" should be X7R, but then goes on to say that "bulk capacitors" should be aluminium electrolytic caps, so it would make sense that those are two separate things.

    Thanks for the clarification.

  • 0 in reply to Atsu K
    TI__Intellectual 1805 points

    Hi Atsu, 

    Bypass capacitors are the ones that are connected directly to the power pin. Its value should be 0.1uF or 0.01uF.

    Bulk capacitors are the ones that are connected next to the ferrite bead in a noise filter or in VBUS at the inrush current limitator. Its value vary from 1uF, 22uF to 150uF depending on its purpose.

    Regards,

    Aldo

  • 0 in reply to Aldo Acero
    Prodigy 100 points

    Hi Aldo,

    Thank you for the explanation.  

    Right now I have both 0.1uF and 0.01uF bypass caps for each digital and analog power pin as recommended on the implementation guide.  I also have a pi filter (0.1uF cap, 220ohm 200mA ferrite bead, 0.3uF cap) for each analog rail.  

    What is this other noise filter you are recommending that uses the 1,22,o150uF bulk cap, and which pins/rails is it recommended for? 

    Thanks again.