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TPS54388C-Q1: What the recommendation inductor and capacitor values?

Zirui Su
Expert 5340 points
Part Number: TPS54388C-Q1

Hi team 

Could you please help customer to review the TPS54338CQ1 schematic and recommend the proper inductor and capacitor values?

The background story is: Due to the shortage of TPS62811Q1, customer uses TPS54388CQ1 to replace it and they used 470nF before. So they would like to need your confirmation and recommendation in their schematic about the inductor and capacitor pair value. Does 4.7uH is good or a little bit big value? Their application condition is:

Vin=5V, Vout=3.3V Output current=0.5A; Fsw=2.1Mhz, please verify the inductor and capacitor values asap, thank you!

  • 0
    TI__Expert 5340 points

    Hi team

    Any feedback? Since it is urgent please help me asap and if needed you could contact me via e-mail, thank you very much!

  • 0 in reply to Zirui Su
    TI__Genius 17401 points

    Hi,

    I would suggest to take look also on TPS62850x-q1 device. It has internal soft startup of 1ms but the external components are are similar like TPS6281x-Q1 and doesn't require much changes on application circuit.

    TPS54388C-Q1 schematic soft start up is set for 1.3ms because the start charging current is 2uA. This device has max frequency of 2MHz.

    Inductor value 4.7uH is okay and effective minimum output capacitance should be around 20uF will work for normal regulation. Of course, it can be set higher according to their output ripple requirements as far as ESR is below 55mOhm as suggested in datasheet.

    Best regards,
    Tanvee

  • 0 in reply to Tanvee
    TI__Expert 5340 points

    Hi Tanvee

    Since customer has used the TPS54388CQ1 on their PCB, so maybe we should focus on their schematic now. Also, 4.7uH+20uF maybe large for their PCB, do you have any other recommendation to the customer schematic and requirement design? Thank you!

  • 0 in reply to Tanvee
    TI__Expert 5340 points

    Now customer would like to use 1uH or 2.2uH to replace 4.7uH since the 4.7uH inductor is quite big, so could you please recommend whether 1uH or 2.2uH is OK without potential risk? Need your guidance and recommendation, thank you.

  • 0 in reply to Zirui Su
    TI__Genius 17401 points

    Hi Zirui,

    According to datasheet equation, device will get higher output ripple at light load conditions and it is not recommended.  

    I don't understand your comment here because according to your description they want to replace TPS6281x-Q1 device on their application then how they are already using TPS54388c-Q1.

    Best regards,

    Tanvee

  • 0 in reply to Tanvee
    TI__Expert 5340 points

    Hi Tanvee

    Because they searched TPS6281X series and all in shortage issue so they need to switch to TPS54388C-Q1 and due to the time period they may have not enough time to evaluate TPS62850x-Q1. In addition, in TPS62811, they calculated that the inductor only for 470nF, different far from 4.7uH so they need to change the 4.7uH to be lower. What inductor values do you recommend below 4.7uH? Thank you very much for excusing my ignorance. 

  • 0 in reply to Zirui Su
    TI__Genius 17401 points

    Hi Zirui,

    4.7uH would work here. Application load quite light and with lower inductor output ripple would be higher that will also affect the transient and device accuracy.

    Best regards,

    Tanvee

  • 0 in reply to Tanvee
    TI__Expert 5340 points

    Hi Tanvee,

    Sure I think I got your ideas. The load 0.5A we should use 4.7uH+20uF because of the transient and accuracy. I will let customer to know about the fixed inductor value and consider the PCB area, thank you.