lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 16

• 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

www.national.com
The efficiency can now be estimated as:
Heat in the LM27341/LM27342 due to internal power dissi-
pation is removed through conduction and/or convection.
Conduction: Heat transfer occurs through cross sectional
areas of material. Depending on the material, the transfer of
heat can be considered to have poor to good thermal con-
ductivity properties (insulator vs conductor).
Heat Transfer goes as:
Convection: Heat transfer is by means of airflow. This could
be from a fan or natural convection. Natural convection occurs
when air currents rise from the hot device to cooler air.
Thermal impedance is defined as:
Thermal impedance from the silicon junction to the ambient
air is defined as:
This impedance can vary depending on the thermal proper-
ties of the PCB. This includes PCB size, weight of copper
used to route traces , the ground plane, and the number of
P
P
P
DIODE
IND
LOSS
= V
= 0.5V x 2 x (1 - 0.314)
= I
= 2
= P
OUT
2
D1
INTERNAL
x 20 mΩ
FIGURE 7. Cross-Sectional View of Integrated Circuit Mounted on a Printed Circuit Board.
Silicon
2
x I
x R
OUT
DCR
→
+ P
(1 - D)
Lead Frame
DIODE
+ P
IND
→
PCB
= 686 mW
= 80 mW
________
= 1.499 W
16
With this information we can estimate the junction tempera-
ture of the LM27341/LM27342.
CALCULATING THE LM27341/LM27342 JUNCTION
TEMPERATURE
Thermal Definitions:
T
T
R
R
layers within the PCB. The type and number of thermal vias
can also make a large difference in the thermal impedance.
Thermal vias are necessary in most applications. They con-
duct heat from the surface of the PCB to the ground plane.
Six to nine thermal vias should be placed under the exposed
pad to the ground plane. Placing more than nine thermal vias
results in only a small reduction to R
area. These vias should have 8 mil holes to avoid wicking
solder away from the DAP. See AN-1187 and AN-1520 for
more information on package thermal performance. If a com-
promise for cost needs to be made, the thermal vias for the
eMSOP package can range from 8-14 mils, this will increase
the possibility of solder wicking.
To predict the silicon junction temperature for a given appli-
cation, three methods can be used. The first is useful before
prototyping and the other two can more accurately predict the
junction temperature within the application.
Method 1:
The first method predicts the junction temperature by extrap-
olating a best guess R
and graph are for natural convection. The internal dissipation
can be calculated using the efficiency calculations. This al-
lows the user to make a rough prediction of the junction
temperature in their application. Methods two and three can
later be used to determine the junction temperature more ac-
curately.
The two tables below have values of R
eMSOP package.
J
A
θJC
θJA
= IC junction temperature
= Ambient temperature
= Thermal resistance from IC junction to ambient air
= Thermal resistance from IC junction to device case
θJA
from the table or graph. The tables
θJA
θJA
30005666
for the same copper
for the LLP and the