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TPS2491: Calculation method of RθJC

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Replies: 4

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Part Number: TPS2491

Dear Specislists,

My customer has a question about equation(9) of datasheet P.21.

I would be grateful if you could advise. 

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I'd like to know about calculation method of RθJC

RθCA is used in equation (9) of data sheet P.21.

In Equation (10), 30 ° C./W is used for RθCA.

I don't know if it's CSD19532KTT or CSD17552Q5B, (but I think either is a typo.)

RθCA is not described.

From each data sheet, subtract RJC from RθJA.

CSD19532KTT: 62 ℃ / W-0.6 ℃ / W = 61.4 ℃ / W

CSD17552Q5B: 50 ℃ / W-1.8 ℃ / W = 48.2 ℃ / W

In either case, it is not 30 ° C / W.

How was it calculated?

On the other hand, the Excel calculation tool uses θJA.

Here, TI EVM is listed as The EVM is 30 ° C / W number and a good starting point at 30 ° C / W.

In the data sheet, is the initial value of RθCA set to a typical value of 30 ° C / W for RθJA of EVM? 

(In this case, RθJA and RθJC are different, but are they ignored because RθJC is small?)

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I appreciate your great help in advance.

Best regards,

Shinichi

  • Hi Shinichi,

    Referring Figure-9 in http://www.ti.com/lit/an/slva673a/slva673a.pdf

    RθJA = RθJC + RθCA

    where RθJC is typically much lower than RθCA for MOSFETs with a thermal pad and RθCA is a strong function of the layout and the amount of copper that is connected to the drain of the MOSFET.

    For hot-swap applications, we recommend to use enough PCB area as heat sink for the MOSFET to achieve 30°C/W. The PCB area calculation is slightly complex. Please make use of http://www.ti.com/lit/an/spra953c/spra953c.pdf document.

    BR, Rajesh

    --------------------------------------------------------------------------------------------------------

    To find the latest information on Power Switches, Visit www.ti.com/powerswitch 

  • In reply to Rakesh:

    Hi Rajesh,

    Thank you for your reply.

    I'll share your suggestion with the customer.

    I'd like to confirm one more question, could you please advise.

    As you mention 30°C/W, which is correct, RθJA or RθCA?

    I appreciate your great help.

    Best regards,

    Shinichi

  • In reply to Shinichi Inoue:

    Hi Shinichi,

    RθJA  should be considered for calculations as it includes the Junction to Case thermal resistance also.

    BR, Rakesh

    --------------------------------------------------------------------------------------------------------

    To find the latest information on Power Switches, Visit www.ti.com/powerswitch 

  • In reply to Rakesh:

    Hi Rakesh,

    Thank you for your reply.

    I see.

    I'll keep in mind this information.

    I appreciate your great help.

    Best regards,

    Shinichi

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