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TPS2491: Condition of Ts(MAX)
Part Number: TPS2491
My customer has a question about equation(9) of datasheet P.21.
I would be grateful if you could advise.
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?)
I appreciate your great help in advance.
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.
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In reply to Rakesh:
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.
In reply to Shinichi Inoue:
RθJA should be considered for calculations as it includes the Junction to Case thermal resistance also.
I'll keep in mind this information.
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