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TPS22994: Input capacitors vs upstream output capacitors

EnPa
Prodigy 40 points
Part Number: TPS22994

Tool/software:

Dear forum

I want to use a TPS22994 to switch the outputs of four TPS6223* converters.

The TPS6223* seems to be optimised for a 4.7 uF output capacitor, even tough 2 - 15 uF should be ok (datasheet, page 13, section 9.2.2.1).

On the other hand, the TPS22994 calls for an input capacitor of 1 uF and an output capacitor of one tenth of this value (datasheet, page 35, section 11.1).

What is the better strategy in this case?

  • Try to place the buck converters as close as possible to the load switch and use a single 4.7 uF capacitor. Use 470 nF as output capacitor for the load switch.
  • Use 4.7 uF at the buck converter output. Use an additional 1 uF at the load switch input. Use 100 nF at the load switch output.
  • Use 4.7 uF at the buck converter output. Use an additional 1 uF at the load switch input. Use 560 nF at the load switch output.
  • Use 3.3 uF at the buck converter output. Use an additional 1.5 uF at the load switch input (totalling 4.8 uF). Use 150 nF at the load switch output.
  • Use 3.3 uF at the buck converter output. Use an additional 1.5 uF at the load switch input (totalling 4.8 uF). Use 470 nF at the load switch output.
  • Others?

Thank you in advance, Enpa

  • +1
    TI__Mastermind 39045 points

    Hi Enpa,

    EnPa said:
    TPS22994 calls for an input capacitor of 1 uF and an output capacitor of one tenth of this value (datasheet, page 35, section 11.1).

    We recommend keeping atleast 1uF cap at input to help prevent any voltage drop. Similarly if Cout is greater than Cin, Vout can become more than Vin when these caps are discharging and this can result in current flowing through the body diode of device. To prevent this, we have this recommendation.

    Although all of them should be functionally fine. You can use following. 

    EnPa said:
    Try to place the buck converters as close as possible to the load switch and use a single 4.7 uF capacitor. Use 470 nF as output capacitor for the load switch.

    or if spacing issue is present, use this

    EnPa said:
    Use 4.7 uF at the buck converter output. Use an additional 1 uF at the load switch input. Use 100 nF at the load switch output.

    Best Regards,
    Arush

  • 0 in reply to Arush Gupta
    Prodigy 40 points

    Thank you Arush!