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TPS549D22: Inductance value

Oba
Genius 17325 points
Part Number: TPS549D22

Hello

 

I want to understand the range of inductance value that can be used.

I couldn’t find this kind of information on the datasheet.

Could you tell me that?

I want to use inductance as small as possible.

 

Also is it possible open the technical background of inductance value range?

 

Regards,

Oba

  • 0
    TI__Guru**** 191445 points
    Oba,
    The inductor is selected using the well known standard equation of Lout = (Vout/Vin) * (Vin - Vout) / (Fsw * Iout * Kind)
    Where Kind is the percentage of p-p inductor ripple current relative to the output current expressd as a decimal (e.g. 15% = 0.15). The datasheet calls for Kind between 0 and 15 but that is an error. for minimum inductor value, you can use 0.15 for kind. Since Fsw is in the denominator, the smallest inductor value results with the highest switching frequency of 1125 MHz. You should design with that frequency.
  • 0 in reply to JohnTucker
    TI__Genius 17325 points

    Hello John,

    Thanks for your reply, I read some documents including datasheet of TPS549D22 and other part numbers etc.
    Then Kind value seems not to be the same in these documents. Does Kind value depend on the device?
    Is this 0.15 applicable for only TPS549D22? Or is this general value?

    Regards,
    Satoshi Obata

  • 0 in reply to Oba
    TI__Guru**** 191445 points

    Oba,

    It is really just a rule of thumb.  When someone asks "How small value inductor value (small Kind value) can I use?", the answer is "how much inductor ripple current can you tolerate?"  General rule is 0.2 to 0.4, 0.3 typical.  There are certainly exceptions to this.  One case that comes to mind is designing circuits with output current well below the rated current for the device.  For current mode control, the inductor current waveform is essentially the PWM ramp.  If the ramp signal amplitude is too low increased jitter can occur.  So when designing for low current application I either use the full rated current value in teh inductor equation instead of the intended load current or increase the Kind value above 0.5.  I am not sure exactly where the 0.15 value for TPS549d22, but is is probably due to the large rated current of 40 A.  0.15 * 40 = 6 A p-p inductor ripple current.  I will check to see if there is a specific reason other than that for 0.15 and get back to you.