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UCC28065: Ton time at R_TSET = 47Kohm

Takehiro Sato
Intellectual 2755 points
Part Number: UCC28065

Tool/software:

Hello Team,

The customer is considering the UCC28065.
The customer wants to use R_TSET=47K Ω, which is lower than the recommended value.
Is there any information on the on time for this resistor?

Best regards,

Sato

  • 0
    TI__Mastermind 40580 points

    Hello Sato-san, 

    TI strongly urges your customer not to use any value for TSET resistor that is lower than the minimum recommended value of 66.5kR. 

    The reason is that the TSET resistance sets an internal current that is used not only for the minimum on-time but for several other timing and control functions.
    These other functions uses currents that are mirrored from the pin current and the mirrors have saturation limits that cannot accommodate current levels higher than what the 66.5kR value will program.  Some of the other timing functions have saturation limits as well. 

    The limits are not all coordinated and may vary from part to part, so it will be unpredictable how the controller may behave when a TSET value < 66.5kR is used. 
    One device may appear to function properly at room temperature but may malfunction at temperature extremes.  Another device may not function properly at any temperature. 

    Please do not use a TSET value < 66.5kR.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Intellectual 2755 points

    Hello Ulrich-san,

    Thank you for answering.

    I can see that R_TSET is related to on-time, can you tell us a little more specifically about the expected defect? 

    Best regards,

    Sato

  • 0 in reply to Takehiro Sato
    TI__Genius 12795 points

    Sato,

    Are you asking what could go wrong inside the IC if R_TSET is below 66.5kR?

  • +1 in reply to Ning Tan
    TI__Mastermind 40580 points

    Hello Sato-san, 

    A similar issue with using R_TSET < 66.5kR came up in late 2020.

    At the time, we asked the actual designer of the IC about it and here is our question and his answer: 

    Q:   " [xxxxxx] team trying to decrease the Rtset to 40kohm in order to output full load.
    Since it is out of datasheet spec, can you comment on whether customer can lower the Rtset to such a low value and what is the drawback? "    

    A:   " In this case we cannot guarantee that the internal TSET Voltage source is able to sustain properly the current.
    Moreover even if the voltage source was able to sustain the current (2.65V / 40k) the same current is mirrored and used in downstream blocks that are not properly sized for such current.  

    We provided limits that are reported on datasheet please respect those limits. "

    So besides possibly working incorrectly at extreme temperatures, there can also be long-term degradation of tracks on the IC die due to excessive current density which is a long-term reliability concern.  The device might work okay in the product initially, but might start having high failure rates in the field after some unpredictable amount of time.    

    Unfortunately we cannot ask him for more information because the designer (a very good one, by the way) is no longer with TI, but as he stated: " We provided limits that are reported on datasheet please respect those limits. "

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Intellectual 2755 points

    Hello Ulrich-san,

    Thank you for answering our question.

    Best regards,

    Sato