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TPS53319 RTrip Values Datasheet vs. Webench

Other Parts Discussed in Thread: TPS53319, TPS53318

Hello team,

We have encountered some misalignments regarding the RTrip values when designing on Webench with the TPS53319.

For a 12V VIN, 1.03V VOUT, 500kHz FSW, 14A IOUT design, Webench recommends a 280kOhms RTrip and a 510nH inductor. If we use the formula on the datasheet for calculating IOCP (page 16), we get an IOCP of 24A, but the TPS53319 can only output 14A. If we use the formula to calculate RTrip (page 23) for 14A and using the same 510nH inductor, we get a RTrip of 149.49kOhms. Similarly, if we calculate using the same inductor and 1A IOCP we get negative resistance values, -10.40kOhms. 

Now, if we modify the Webench design for the same voltage specs but a 1A IOUT, Webench recommends a 301kOhms RTrip and a 5uH inductor. Doing the same exercise of calculating IOCP with these values we get IOCP=24A . Calculating the correct RTrip value gives 169kOhms. I have attached an excel file with these calculations.

The question is: what is the correct way to calculate RTrip? Do you need to keep the inductance value fixed based on the current limit? Which is correct: Webench or datasheet?

Also, what is the maximum RTrip value for the TPS53319, I read on another post it was 300kOhms, but the datasheet doesn't make any mention of this for the TPS53319; it does mention the limit for TPS53318 which is 150kOhms.

Let me know if you have any questions. Thanks.

TPS53319 RTRIP Calc.xlsx

  • The inductor value is calculated based on ripple current as a percentage of Iout. Those webench values appear reasonable to me. It looks like webench always sets the Rtrip to the maximum current limit of about 25 A. I think this is a mistake. I'll move this to the webench forum and get them to correct it.
  • Hi Fernando,
    I am currently looking into this issue. I will get back to you with a response soon.
    Regards,--Hadi
  • Hello Hadi,

    Thanks for your support here.

    I went ahead and simulated the design example on the datasheet for the TPS 53319 on Webench to see if the model works. The initial recommended values for the passive components are incorrect, but after modifying the model to match the design example for an 11A IOCP, the simulation shows the part tripping at the correct threshold.

    One thing I noticed though was the fact that during the time Vout trips, it actually drops into negative voltage. See below a screenshot of the simulation. Design specs are: Vin:5-18V, Vout=1.2V, fsw =850kHz, L=0.5uH, RTrip=120kOhms, Iout=8A, ICOP=11A.

  • HI Fernando,

    I believe you are using a current source at the load to do this current limit test. When Vout goes to 0V, you are probably still drawing (forcing) current out of Vout from this current source. This will (in simulation), discharge Vout further and hence it goes negative.

    Please try an equivalent simulation with a resistive load step and you should see Vout clamp to 0V and not go negative.

    Thanks,