LM2735XMFSLASHNOPB datasheet(47/59 Pages) TI

47 / 59 page

0.1

0.2

0.3

0.4

0.5 0.6

0.7

0.8

500

PDISS

100

150

200

250

300

350

400

450

Dissipatio

- A

Dissipatio

- Case-Top

LM2735, LM2735-Q1

www.ti.com

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

10.3.5.2 Example From Previous Calculations

PDissipation = 475 mW

TA at Shutdown = 139°C

TCase-Top at Shutdown = 155°C

(61)

RθJA WSON = 55°C/W

RψJC WSON = 21°C/W

WSON & MSOP-PowerPAD typical applications will produce RθJA numbers in the range of 50°C/W to 65°C/W,

and RψJC will vary from 18°C/W to 28°C/W. These values are for PCB’s with two and four layer boards with 0.5-

oz copper, and 4 to 6 thermal vias to bottom side ground plane under the DAP.

For 5-pin SOT-23 package typical applications, RθJA numbers will range from 80°C/W to 110°C/W, and RψJC will

vary from 50°C/W to 65°C/W. These values are for PCBs with 2- and 4-layer boards with 0.5-oz copper, with 2 to

4 thermal vias from GND pin to bottom layer.

The following is a good rule of thumb for typical thermal impedances, and an ambient temperature maximum of

75°C: if the design requires more than 400 mW internal to the LM2735 be dissipated, or there is 750 mW of total

power loss in the application, TI recommends using the 6-pin WSON or the 8-pin MSOP-PowerPAD package.

NOTE

To use these procedures, it is important to dissipate an amount of power within the device

that will indicate a true thermal impedance value. If a very small internal dissipated value

is used, it can be determined that the thermal impedance calculated is abnormally high,

and subject to error. The graph below shows the nonlinear relationship of internal power

dissipation vs RθJA.

Figure 50. RθJA vs Internal Dissipation for the WSON

and MSOP-PowerPAD Package

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