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Original question:

UCC28063: Tset resistor change

UCC28064: Tset resistor setting

James Gilbert
Prodigy 30 points
Part Number: UCC28064
Other Parts Discussed in Thread: UCC28060, , UCC28063

Can you please help clarify what value or range of values is appropriate for RTSET and how that is determined?  The UCC 28046 Design Calculator spreadsheet gives a value, but that value is related only to Eq (45) of the datasheet for RTH, but does not give a value for RTL, which in most cases (?) is the stated result when solving Eq (44) for RT = min(RTH, RTL).  More confusing, the example of section 9.2.2.9 in the datasheet says the value chosen for RT is 91K, but that is not even close to the result of Eq. 44 for the values given in the example, and instead should be more like 68K.  The example also seems to follow the UCC28064EVM board requirements, but the eval board has 115K loaded for RT == R15.  Following the WEBENCH example did not help either, as it yielded yet another value of 88.7K that didn't seem to change when I changed the value of the inductor, even after I plugged the same values that were relatively close to the datasheet requirements/results in.  So what exactly is the right equation or method for determining the correct value or range of values for RTSET?

Jim

  • 0
    TI__Guru* 78546 points

    Hello Jim,

    I looked through the UCC28064 product folder and could not find an excel design tool  I am guessing this is either the UCC28060/1 or 3 design tool.  These tools did calculate the RT resistor based on the maximum on time at the minimum line voltage and typical values and the minimum switching frequency.  The equation in the data sheet for the UCC28064 RTH and RTL is trying to take into account the KTH variation for selecting RT.  However, as you mentioned it does not seem to match what is in the EVM.  This does seem to match the calculations for the UCC28060/1/3 tools and data sheets.

    Equation 44 uses KTHmin which happens at high AC (265 V) and does not make sense so that is in error. 

    The UCC28063 excel tool calculated an RT of 122 kohms and the UCC28063 EVM used a 121 kOhm.  This is not that far off from the 115k ohms that was used in the UCC28064 EVM board as well. For now I would use the UCC28063 calculations and design tool to set the RT value.

    I will internally look into this further but I believe there is an error in the UCC28064 RT calculations.  The UCC28063 calculations seem to be used for the UCC28064 EVM design and work so I woulds use those calculations for now.

    Regards,

    Mike

     

  • 0 in reply to Mike O'
    Prodigy 30 points
    The spreadsheet for the 064 is SLUC645a.xls (www.ti.com/.../sluc645) in the zip folder "Design Files (1)-->UCC28064 Controller Setup Tool (Rev. A)" under technical documents for the 064: www.ti.com/.../technicaldocuments

    I've just downloaded the 063 spreadsheet and will go over that after the weekend.

    Thanks.

    Jim Gilbert
  • 0 in reply to James Gilbert
    TI__Guru* 78546 points

    Hello Jim,

    I discussed this with the systems engineer for the UCC28064 and the equation for Ton_max was an over estimate.  I have attached the corrected equations and the minimum Rth calculates to be 112.6 k ohms which is close to what is being used in the EVM for 340 uH boost inductors.  This is close to what is being used in the EVM. 

    The UCC28064 tool calculated that 106.9K be used for RTSET.  That is not far off from the correct equations.  I would use the hand calculations for now and use this value in your design.

    I have contacted the applications engineer that is responsible for the development and maintenance of the tool.  The engineer will correct the data sheet and tool when they have a chance.

    Sorry for the inconvenience and best regards,

    Mike

    Corrected Equation for Rtset UCC28064.pdf