LM27341_14 datasheet(22/42 Pages) TI1

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RTJC =

TJ - TC

Power

LM27341, LM27342, LM27341Q, LM27342Q

SNVS497D – NOVEMBER 2008 – REVISED FEBRUARY 2010

www.ti.com

RθJA values for the MSOP-PowerPad @ 1Watt dissipation:

Number of Board

Size of Bottom Layer Copper

Size of Top Layer Copper

Number of 10 mil

RθJA

Layers

Connected to DAP

Connected to Dap

Thermal Vias

0.25 in2

0.05 in2

80.6 °C/W

0.5625 in2

0.05 in2

70.9 °C/W

1 in2

0.05 in2

62.1 °C/W

1.3225 in2

0.05 in2

54.6 °C/W

4 (Eval Board)

3.25 in2

2.25 in2

35.3 °C/W

RθJA values for the SON @ 1Watt dissipation:

Number of Board

Size of Bottom Layer Copper

Size of Top Layer Copper

Number of 8 mil

RθJA

Layers

Connected to DAP

Connected to Dap

Thermal Vias

0.25 in2

0.05 in2

78 °C/W

0.5625 in2

0.05 in2

65.6 °C/W

1 in2

0.05 in2

58.6 °C/W

1.3225 in2

0.05 in2

50 °C/W

4 (Eval Board)

3.25 in2

2.25 in2

30.7 °C/W

Figure 38. Estimate of Thermal Resistance vs. Ground Copper Area

Eight Thermal Vias and Natural Convection

Method 2:

The second method requires the user to know the thermal impedance of the silicon junction to case. (RθJC) is

approximately 9.5°C/W for the MSOP-PowerPad package or 9.1°C/W for the SON. The case temperature should

be measured on the bottom of the PCB at a thermal via directly under the DAP of the LM27341/LM27342. The

solder resist should be removed from this area for temperature testing. The reading will be more accurate if it is

taken midway between pins 2 and 9, where the NMOS switch is located. Knowing the internal dissipation from

the efficiency calculation given previously, and the case temperature (TC) we have:

(50)

Therefore:

TJ = (RθJC x PLOSS) + TC

(51)

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