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TPS40140 design errors

Rod Stiles
Prodigy 40 points

Other Parts Discussed in Thread: TPS40140

I have created a design for the TPS40140 using SwitcherPro Online 3.17.

VinMin: 11.4V VinMax: 12.6V Vout: 1.5V Iout: 10A Vout2: 900mV Iout2: 18A

I found a few problems:

The RT resistor (R14) is shown as 127ohm. It should be 127k

The UVLO resistors are shown as 10k & 10k.  This would result in a startup at 4V which is too low. They appear to be not calculated.

The ripple current (peak to peak) chosen is quite high, 4.7A for output 1 and 8A for output 2.

The resistors on CS1 and CS2 are always 10k (R1, R2 R30 and R28). They should be calculated from the inductor specs.

The current Limit threshold is displayed as 10A for both outputs. Designing with a different max output current does not change this value or change the ILIM resistor values at R15 and R16. Changing the inductor does affect the ILIM resistors.

 

  • 0
    TI__Expert 3085 points

    Dear Rod:

    Thanks for your posting.  I will be going through on the list of things you reported and I will get back to you very soon.

    Regards,

    Surinder

  • 0
    TI__Expert 3085 points

    Hi Rod:

     

    I have gone through some of the questions, and here is the partial list of anwers.  The other answers I will get you shortly.

    The RT resistor calculation indeed has a bug. I aggree it should be 127 KOhm, not 127 Ohm.  We are missing a factor of 1000 in our calculation.  We will try to correct it soon.

    The UVLO resistors are also fixed to 10K each in the code. Let us see if this should not be the case.

    Regarding the remaining points, we will get back to you soon. I did not want you to be waiting while we are investigating.

     

    Thanks,

    Surinder

     

  • 0 in reply to Surinder Singh
    TI__Intellectual 890 points

    Hi Rod,

    1)    The RT resistor (R14) is shown as 127ohm. It should be 127k.

    You are right; we need to fix RT in SwitcherPro. We plan to fix this in near term.

    2)    The UVLO resistors are shown as 10k & 10k.  This would result in a startup at 4V which is too low. They appear to be not calculated.

    The startup is set at 4V as recommended by the design example in datasheet. Currently the resistors are always set to 10Kohms (not calculated) and there is no way user can change the start and stop voltage from advanced inputs panel.

    3)    The ripple current (peak to peak) chosen is quite high, 4.7A for output 1 and 8A for output 2.

    The reason SwitcherPro is giving higher ripple is because the inductor selection is configured to select smaller than the calculated target value and hence higher ripple.  

    4)    The resistors on CS1 and CS2 are always 10k (R1, R2 R30 and R28). They should be calculated from the inductor specs.

    I just confirmed that CS1 and CS2 calculations are not getting updated when you change the inductor. According to the datasheet, it should have. We will plan to fix this in near term.

    Your feedback is important to us. Thanks for your feedback.

    Regards,

    --Abhi

  • 0 in reply to Abhishek Gupta - WEBENCH Design Center
    Prodigy 40 points

    I didn't get an answer to my last question:

    The current Limit threshold is displayed as 10A for both outputs. Designing with a different max output current does not change this value or change the ILIM resistor values at R15 and R16. Changing the inductor does affect the ILIM resistors.

     This is by far the most difficult calculation to do from the data sheet.

    Is it true that the current limit is calculated for 10A aways?


  • 0 in reply to Rod Stiles
    TI__Intellectual 890 points

    Hi Rod,

    Sorry for the delay in response.

    The current limit threshold value displayed in analysis output tab is incorrect. This is a bug in SwitcherPro so please ignore this value.

    The ILIM resistors are calculated based on equations 5-11, 5-12, 5-13, 5-14, 5-15 and 5-16 in the data sheet. Also, the resistor calculations are based on Ioc value of 30A.

    You are correct the output current will not change the resistor value but change in inductor will.

    Regards,

    --Abhi