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# TPS2491: Calculation method of RθJC

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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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

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

Hi Rajesh,

I'll share your suggestion with the customer.

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

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

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

Hi Rakesh,

I see.

I'll keep in mind this information.