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TPS548D22: design review

Sirui Zhu
Intellectual 2090 points
Part Number: TPS548D22

Hi team,

My customer is designing a system power supply based on TPS548D22, and have below requirement, please help to give your professional advice.

Vin=12V, Vout=5V, Iout_max=30A, Iout_ripple<200mV. Now there are two inductor selection:

1. L=220nH, Irms=33A, Isat=40A, 8*8mm

2. L=120nH, Irms=60A, Isat=80A, 10*7mm

Customer want to select an inductor as small as possible. Could you please comments on the inductor selection or you have other recommendation?

BR

Sirui

  • 0
    TI__Guru 51885 points

     

    At 12V to 5V and 650kHz, the inductor sees 4.5μV-s  (Vin - Vout) x Vout/Vin x 1/Fsw 

    A 220nH inductor will have a ripple current of 20.4A

    A 120nH inductor will have a ripple current of 37.4A

    Both of those are well above the typical 40% full load current, which would be 12A for a 30A max load current.  Further complicating the issue is the fact that the pin-programming is selecting Autoskip, which will reduce the switching frequency when the load current falls below 1/2 the inductor peak to peak current of about 10A and 19A respectively.  While this does not change the On-time or the volt-seconds on the inductor, it will increase the output ripple voltage to as much as 8x the ripple voltage under continuous conduction, which could make meeting your 200mV output ripple voltage difficult.

    Additionally, the 220nH inductor will exceed it's 40A saturation rating at 30A of load current with 20.4A of peak to peak ripple.

    Increasing the switching frequency to 850kHz would help reduce the ripple current about 25%, buying a little margin for the 220nH inductor at 30A load current to reduce the risk of saturation, but the low inductance will likely require selecting a different ramp option than the datasheet recommendations, likely the 1xR ramp, in order to keep the loop stable.

    Looking up some alternate inductors, the XAL7070-301 would provide 300nH of inductance in an 8mm x 8mm package with an Isat of 55A and Irms of 33A.  At 850kHz that would get the ripple current down to the 40% recommendation. - https://www.coilcraft.com/en-us/products/power/shielded-inductors/molded-inductor/xal/xal7070/xal7070-301/ 

    At 650kHz the ripple current would be 15A, high for a 30A application, but manageable with a 55A saturation inductor.

    Do you have the transient specification requirements?

    Amplitude of load change

    Rate of load change or rise/fall time of load change

    Maximum allowable voltage drop during load change

    Do you have the DC bias derating curves and ESR values for the 6.3V 100μF capacitors?  Those capacitors will have significantly less than 100μF of capacitance at 5V.