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UC3705: Thermal Questions

Aaron Spaete
Expert 4330 points
Part Number: UC3705

Team,

My customer has the following questions:

I have a couple of thermal questions concerning the TI UC3705D  high speed power driver. 

 

  1. What is the RthetaJ-C of the part?  I could not find any mention of it in the datasheet.

  2. What is the max junction temperature of the part?  The operating temperature range is 0C to +70C, whereas the storage temperature range is -65C to +150C

Regards,

Aaron

 

  • 0
    TI__Genius 12990 points
    HI Aaron,

    Thanks for reaching out via e2e!

    Since this is an older part some of the common thermal characteristics are not there. What is there is the power dissipation at 25C. This can give us an estimate of Rja=(70-25)/1W = 45C/W where 1W is the dissipated power for a UC3705 SOIC package. However Rja is highly dependent on PCB layout in which the standard JEDEC board should be representative.

    To get a good indication of the thermal performance Rjc and Rca needs to be realized because Rja = Rjc + Rca. And a low value for Rja means a low value for Rjc. To estimate the junction temp from Rjc (if it was given) we have to have a heatsink on the top to assume all the heat is dissipated out the top. Thats why psi_JT is supposed to be used which is not the true thermal resistance because by assuming all the power is dissipated through the top, psi_JT compensates the junction temp which would otherwise be a lower value in reality.

    To find the junction temp you can first calculate the Rja_effective with the following app notes:
    -section 1 from the thermal metrics app note: www.ti.com/.../spra953c.pdf
    -and section 2 from www.ti.com/.../snva419c.pdf
    -also How to improve Rja app note: www.ti.com/.../snva183b.pdf

    Let me know if you have any more questions
    Thanks,
  • 0 in reply to Jeffrey Mueller
    Prodigy 30 points

    Hi Jeff,

    Thank you for responding to my question.  If I use Equation 4, the traditional, but invalid usage of RthetaJC, listed in Application Report, SPRA953C,  I calculate a Junction Temperature of 115C.  Is there anyway you can provide me with a definitive value of the maximum allowable junction temperature for the UC3705 part?  thank you, Charlie Famiglietti

  • 0 in reply to Charles Famiglietti
    TI__Genius 12990 points

    Hi Charles,

    update: great talk!

    UC3705 specs this part at 70C max operating amb temp. The datasheet test conditions spec Ta=Tj which means the part is soaked un-powered up to 70C then is tested and the junction is assumed to be equal. Therefore the max operating junction temp is larger than 70C. The abs max temp is not specd however the thermal shutdown of 155C can be considered since the abs max rating is not there. The abs max is therefore close to this thermal shutdown value. What the datasheet guarantees is the part can last the entire lifetime at the abs max temp and will survive double the lifetime for every 10C decrease in junction. We will update you asap with any confirmation I can obtain.

    Thanks,

  • 0 in reply to Jeffrey Mueller
    Prodigy 30 points
    Good morning Jeff,
    I estimated Rjc based on section 2.1 from spra953c. You provided a value of 45C/W for Rja. I assumed Rca was zero. Would it be possible for me to contact via phone because I have few more questions I think would be best discussed over the phone.
    thanks,
    Charlie
  • 0 in reply to Charles Famiglietti
    TI__Guru**** 189399 points
    Hi Charlie,

    Sure! Let me ask Aaron to set a conf call up.
  • 0 in reply to Don Dapkus
    TI__Genius 12990 points
    Hi Guys,

    I am going to close this thread. This is a popular space device that has thermal shutdown. The 155C spec for this thermal shutdown sheds light on the max junction the part can handle, 150C. There should be no issue with understanding this spec or allowing this conclusion to be assumed. Thanks for your help with everything Aaron and Charlie, I will be in touch with Aaron to wrap this up offline.

    Thanks,