lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 17

• Manufacturer Part Number: lm27342sdx
• Description: 2 Mhz 1.5a/2a Wide Input Range Step-down Dc-dc Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

R
R
Method 2:
of Board
of Board
FIGURE 8. Estimate of Thermal Resistance vs. Ground
Number
Number
θJA
4 (Eval
θJA
4 (Eval
Layers
Board)
Layers
Board)
2
2
2
2
2
2
2
2
values for the eMSOP @ 1Watt dissipation:
values for the LLP @ 1Watt dissipation:
Eight Thermal Vias and Natural Convection
Bottom Layer
Connected to
Bottom Layer
Connected to
0.5625 in
1.3225 in
0.5625 in
1.3225 in
0.25 in
3.25 in
0.25 in
3.25 in
Copper
Copper
Size of
Size of
DAP
1 in
DAP
1 in
2
2
2
2
2
2
2
2
2
2
Copper Area
Layer Copper
Layer Copper
Connected to
Connected to
Size of Top
Size of Top
0.05 in
0.05 in
0.05 in
0.05 in
2.25 in
0.05 in
0.05 in
0.05 in
0.05 in
2.25 in
Dap
Dap
2
2
2
2
2
2
2
2
2
2
of 10 mil
Thermal
Thermal
Number
Number
of 8 mil
Vias
Vias
14
15
8
8
8
8
8
8
8
8
30005690
78 °C/
50 °C/
80.6 °
70.9 °
62.1 °
54.6 °
35.3 °
65.6 °
58.6 °
30.7 °
R
C/W
C/W
C/W
C/W
C/W
R
C/W
C/W
C/W
W
W
θJA
θJA
17
The second method requires the user to know the thermal
impedance of the silicon junction to case. (R
mately 9.5°C/W for the eMSOP package or 9.1°C/W for the
LLP. The case temperature should be measured on the bot-
tom 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 dis-
sipation from the efficiency calculation given previously, and
the case temperature (T
Therefore:
METHOD 2 EXAMPLE
The operating conditions are the same as the previous Effi-
ciency Calculation:
Internal Power Losses are:
The junction temperature can now be estimated as:
A National Semiconductor eMSOP evaluation board was
used to determine the T
layer PCB is constructed using FR4 with 2oz copper traces.
There is a ground plane on the internal layer directly beneath
the device, and a ground plane on the bottom layer. The
ground plane is accessed by fourteen 10 mil vias. The board
measures 2in x 2in (50.8mm x 50.8mm). It was placed in a
container with no airflow. The case temperature measured on
this LM27342MY Demo Board was 48.7°C. Therefore,
To keep the Junction temperature below 125 °C for this lay-
out, the ambient temperature must stay below 94.33 °C.
Method 3:
The third method can also give a very accurate estimate of
silicon junction temperature. The first step is to determine
V
f
P
P
P
P
P
SW
IN
COND
SW
Q
BOOST
INTERNAL
= 12V
= 2 MHz
= I
= 2
= (V
= (12V x 2A x 2 MHz x 10ns)
= I
= 1.5 mA x 12V
= I
= 7 mA x 4.5V
= P
T
T
J
OUT
Q
BOOST
A_MAX
2
COND
= (9.5 °C/W x 733 mW) + 48.7 °C
IN
x V
x 0.15Ω x 0.314
T
2
J
x I
T
T
x R
IN
= (R
A_MAX
J
= 125 °C - 55.66 °C + 25 °C
OUT
x V
+ P
V
V
= (R
T
OUT
D1
DSON
A_MAX
BOOST
θJC
T
SW
J
C
x f
J
= 0.5V
of the LM27341/LM27342. The four
θJC
) we have:
= T
= 3.3V
= 55.66 °C
SW
+ P
x P
x D
= 94.33 °C
J_MAX
x P
x t
Q
INTERNAL
LOSS
FALL
+ P
- T
BOOST
)
) + T
J
) + T
+T
I
R
C
OUT
A
DCR
C
θJC
= 2A
= 20 mΩ
) is approxi-
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= 188 mW
= 480 mW
= 29 mW
= 37 mW
________
= 733 mW