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TPS51396A: EVM circuit problem

Ning LU
Prodigy 70 points
Part Number: TPS51396A
Other Parts Discussed in Thread: TPS51396

Hi,

I purchased a TPS51396A EVM. In its user manual there is schematic of this EVM board,

However, the components R2, R3, and C13 are not in the reference design circuit in TPS51396A's datasheet. I also did not find any descriptions about these 3 components in datasheet.

So what is the function of these 3 components here? Under what circumstance will they be needed? Do I really need to include them in my application circuit?

Thank you.

/Roland

  • 0
    TI__Mastermind 18510 points

    Roland, R3/C13 is RFF and CFF(Table 8-2 in datasheet), which are used for transient performance improvement (for high transient response requirement, if transient meet the requirement, no need to use them), commonly RFF could be removed, only stuff CFF is ok. for <2.5Vout, CFF could be not mounted as well. if Vout >2.5V, 47pf to 100pf is common value for CFF.

    R2 is reserved for loop bode plot test, if you don't do bode plot test, it can be removed.

    Yuchang

  • 0 in reply to Yuchang Zhang
    Prodigy 70 points

    Yuchang,

    thanks for the detailed explaination.

    I did some tests on the EVM. The test condition is as below:

    Input: 12V

    Output is set to 5.2V

    A 1ohm load resistor is connected to the output, so the load current is ~5A.

    The power inductor becomes extremely hot. Is it normal? How can i reduce the temperature of the inductor?

    /Roland

  • 0 in reply to Ning LU
    TI__Expert 4360 points

    Hi LU

    could you help share the spec information for used inductor? thanks!

  • 0 in reply to Ning LU
    TI__Mastermind 18510 points

    Roland, after change Vout to 5.2V, you need to change inductor bigger to reduce the peak peak inductor current, you may refer to datasheet for recommended inductor value.

    Yuchang

  • 0 in reply to Yuchang Zhang
    Prodigy 70 points

    Yuchang,

    Thank you for the reminder. I did forget to change inductor.

    Then I used a ASPIAIG F5020 1.5uH instead to replace the default one. F5020's spec is as following:

     

    The other test condition remains the same. I used a thermal meter to measure the temperature and the reading is about 61C degree (room temperature about 28C, still air). 

    It is cooler than the default inductor, but this temperature still seems bit too high if compared with the above datasheet curve.

    Any suggestions?

    /Roland

  • 0 in reply to Ning LU
    TI__Expert 8100 points

    Hi Roland,

    My colleague will give you reply soon!

    BRs

    Zixu

  • +1 in reply to Ning LU
    TI__Mastermind 18510 points

    Roland, the temperature in inductor datasheet does not mean the temperature rise @different load. it only shows inductor value vs temperature rise.

    from your test result, it is not high value, if you would decrease the temperature of inductor, you may choose lower DCR inductor instead.

    Yuchang

  • 0 in reply to Yuchang Zhang
    Prodigy 70 points

    Yuchang,

    I not sure i understood your point.

    The curve figure shows 2 groups of data, one is inductor value vs. load current, which is plotted in blue; the other is the temperature rise vs. load current, which is plotted in red. i do not see a curve plot of inductor value vs. temperature in the picture as you mentioned. 

    And what do you mean by "not high value"? Do you mean 61C degree is normal for TPS51396 on EVM?

    /Roland

  • 0 in reply to Ning LU
    TI__Mastermind 18510 points

    Roland, sorry for typo, that is DC current vs load and Saturation current vs inductor value drop, actually inductor temperature rise is also related to AC loss, so we can't use the chart to evaluate the thermal.

    As experience, 61C is not a high value, as I mentioned, if you would decrease it, 2.2uH with lower DCR should be helpful.

    Yuchang

  • 0 in reply to Yuchang Zhang
    Prodigy 70 points

    Yuchang, 

    Thanks for the clarification.

    Larger inductor value usually lead to larger DCR, so probably difficult to find one can do both. But i can try some different ones first.

    In EVM's user guide there is a thermal picture showing the temperature of the IC under 8A load. I can not find the power indcutor's temperature in the document though. Is there a number of inductor's temperature you have tested so i can take as a reference on further testings on EMV and on my own hardware?

    /Roland

  • 0 in reply to Ning LU
    TI__Mastermind 18510 points

    Roland, sorry we didn't catch inductor thermal, but it should be lower than IC.

    Yuchang

  • 0 in reply to Yuchang Zhang
    Prodigy 70 points

    OK, I probably can try to reproduce it under same condition first.

  • 0 in reply to Ning LU
    TI__Mastermind 18510 points
    • Okay, thanks for understanding