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TPS62140 review for GSM supply

Sebbe
Prodigy 170 points
Other Parts Discussed in Thread: TPS62140, TPS62130

Hello Ti,

I have some questions regarding TPS62140, and I would appreciate if someone can take a brief look and comment on schematic and rough layout.

I have performed some basic tests on EVKIT for TPS62140.


Information regarding solution.

VSYS is 4.6 - 7.5V input voltage.

Output is 3.8V

Nominal load current on 3V8 is 50mA, with transients up to 2A. The 3.8V output voltage cannot under any circumstances drop below 3.3V

C200 is placeholder for feed forward capacitor.

R204 is placeholder for setting DEF pin high.

Schematic

Schematic questions and sanity checks.

  • How much will capacitors C209-C16, bulk capacitors for VGSM, affect the control loop of U200? Do I need to recalculate and choose new components for LC combination, new inductor etc based on application report SLVA463?
  • To sustain a solid output on 3.8V during fast 2A transients, the best solution is to add mode Cout?
  • DEF (pin 8) and FSW (pin 7) can be left floating when resistors are not mounted? Can i tie to output 3V8 for high level?

Layout

Bare in mind that this is just a rough layout to visual current paths and feedback loops.

Common ground on layer 2, this is a 4-layer board.

Point of load at the end of yellow path with vias down to bottom layer.

Layout questions

  • Are input capacitors positioned okay?
  • Bulk output capacitors positioned along path for load where space is available, positioning okay? Also related to schematic question regarding control loop stability.
  • Are there any layout related improvements that you can see?

I appreciate all feedback I can get.

Best regards,

Sebastian

  • 0
    TI__Guru**** 150730 points

    Yes, this is a good device for your application.  The TPS62140 has very good transient response, so I doubt Vout would drop below 3.3V even with just the 22uF Cout.  More output cap is the best way to reduce this drop.  You can test the EVM to see how much you need.

    SLVA463 provides a guide to stability but you should check it on the EVM and/or your PCB if you go too far away from it.  Cout with ESR like tantalums usually helps stability compared to the all ceramic caps used in the app note.

    You can leave FSW and DEF floating to generate a low, but we recommend that you just connect them how you need them especially since you won't be changing them during operation.  Yes, Vout is best for a high level and is required for FSW.

    Your layout follows the EVM one very closely.  Good job!

  • 0 in reply to Chris Glaser
    Prodigy 170 points

    Thank you for your quick response.

    Regarding FSW and DEF pins, I have chosen to do it this way since it gives me more versatility later, I can just no mount a resistor to select a specific setting.

    Regarding stability and output capacitances, you recommend tantalums instead of ceramics for bulk, have I understood you correctly?

    New question, there are concerns that in worst case scenarios we can have fast current transients up to 2.5A actually. Is TPS62140 still a good choice in this case or should I move on to TPS62130?

    What are the differences between TPS62140 and TPS62130 except current capacity? Can I just make BOM replacement to TPS62130 or am I forgetting something, I can hardly see any differences in the datasheets.


    Best regards,

    Sebastian

  • 0 in reply to Sebbe
    TI__Guru**** 150730 points

    Yes, if you need a higher current, the TPS62130 is a better device. Only difference is current rating.

    I recommend that you measure loop stability with a given output cap configuration, using SLVA463 as a guide.  If this is not sufficient, then tantalum caps would generally improve the stability, as would a smaller inductor value and Cff.

  • 0 in reply to Chris Glaser
    Prodigy 170 points

    Thank you for your support!