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Need a help for IC selection of a buck converter

Babak Manou
Intellectual 420 points
Other Parts Discussed in Thread: TPS56121, TPS56221, TPS56121EVM-601

Hello there,

I am looking for an IC and it's circuit (schematic) able to convert 12V DC (battery input) to constant 8V DC. The output current range is around 10 to 15A. This PCB and its IC will run a bunch of servos.

Thanks for your help,

  • 0
    TI__Guru**** 191445 points
    Babak,

    You can look at TPS56121: www.ti.com/.../TPS56121
    If you need more current, look at TPS56221: www.ti.com/.../TPS56221
  • 0 in reply to JohnTucker
    Intellectual 420 points

    Thank for your reply John. I had a look at TPS56121 manual before but couldn't figure out what components I need to change to get 8V output. Would you please have look at it data sheet (www.ti.com/.../slvu445b.pdf) in fugure1 and let me know for what components do I need to change for 8V output?

    Thanks again,

  • 0 in reply to Babak Manou
    TI__Guru**** 191445 points

    Babak,

    For small changes, you can get only change the lower FB resistor, R7. But you want to change from 1 V output to 8 V output, so R7 = 1.66k (or whatever the closest std value is). That will make the inductor ripple current huge, over 12 A. So you will need to change the inductor. I would use 1.2 or 1.5 uH. That change will require you to re-compensate the circuit. The compensation tool is located here:

    www.ti.com/.../sluc263

    Also, there is a complete design example in the datasheet you can follow. TP5 and TP6 are the EN and PWRGD test points. You do not need to connect them for operation.

  • 0 in reply to JohnTucker
    Intellectual 420 points

    John,

    Thank you very much for your reply and clear explanation. I really appreciate your patience with me. I have a couple more questions:

    1- I used the spreadsheet and re-compensate the circuit as you advise but I am not sure about the place of new components in my circuit. Would you please let me know if my assumption is right?

    Spreadsheet values                Datasheet values (Figure 1. TPS56121EVM-601 Schematic)

    R1      -------------------------------> R4

    R2     --------------------------------> R6

    R3     --------------------------------> R3

    Rbias --------------------------------> R7

    C1    --------------------------------->  C17

    C2    ---------------------------------> C15

    C3    ---------------------------------> C16

    2- In the spreadsheet (picture attached), I assumed 500kHz switching frequency and used 1/5 of this value for Cross over frequency. However, I am not sure, where inside the IC I can set up the switching frequency?!

    3- Again in the spreadsheet, I assumed 10mohm ESR for output ceramic capacitor (100uF), is it a good estimation?

    Thanks,

  • 0 in reply to Babak Manou
    TI__Guru**** 191445 points
    Babak,

    Yes you are correct about the compensation and feedback. It is classic TYPE III compensation for voltage mode control. There is a lot of literature available on line.

    The switching frequency is set by a connection form COMP to GND. It is detailed in the datasheet. For 500 kHz, just use the regular type III with no resistor to GND (see section 7.3.4 in the datasheet).

    10 mohm is probably high. I typically use 3 mohm if I am estimating. If I want to be accurate, I look up the actual ESR at the capacitor manufacturer datasheet. Murata has an excellent tool called "Simsurfing". You can also find the important DC bias derating curve there also.
  • 0 in reply to JohnTucker
    Intellectual 420 points
    Thanks for your reply John. It really helped me.
    After mentioning section 7.3.4 in your reply, I noticed that I am looking at the evaluation module data sheet. I could find all the formula on the data sheet.

    Cheers,