SFTB NSC [National Semiconductor], SFTB Datasheet - Page 12

• Manufacturer Part Number: SFTB
• Description: Thin SOT23 1A Load Step-Down DC-DC Regulator
• Manufacturer: NSC [National Semiconductor]

www.national.com
LLP Package
For certain high power applications, the PCB land may be
modified to a "dog bone" shape (see Figure 7). By increasing
the size of ground plane, and adding thermal vias, the R
for the application can be reduced.
Design Example 4:
Package
V
V
I
V
Freq
I
I
V
T
T
R
IND
D
OUT
Q
BOOST
RISE
FALL
IN
OUT
D
BOOST
DSON
DCR
LLP-6
12.0V
3.3V
750mA
0.35V
3MHz
1.5mA
4mA
5V
8ns
8ns
400mΩ
75mΩ
30.3%
Operating Conditions
(Continued)
P
P
P
P
P
P
P
P
P
OUT
DIODE
IND
SWF
SWR
COND
Q
BOOST
LOSS
2.475W
523mW
56.25mW
108mW
108mW
68.2mW
18mW
20mW
902mW
θJA
12
This example follows example 2, but uses the LLP package.
Using a standard National Semiconductor LLP-6 demonstra-
tion board, use Method 2 to determine R
The four layer PCB is constructed using FR4 with 1/2oz
copper traces. The copper ground plane is on the bottom
layer. The ground plane is accessed by four vias. The board
measures 2.5cm x 3cm. It was placed in an oven with no
forced airflow.
The ambient temperature was raised to 113˚C, and at that
temperature, the device went into thermal shutdown.
If the junction temperature is to be kept below 125˚C, then
the ambient temperature cannot go above 73.2˚C.
Package Selection
To determine which package you should use for your specific
application, variables need to be known before you can
determine the appropriate package to use.
1. Maximum ambient system temperature
2. Internal LM2734Z power losses
3. Maximum junction temperature desired
4. R
The junction temperature must be less than 125˚C for the
worst-case scenario.
SOT23-6)
θJA
of the specific application, or R
T
J
- (R
θJA
x P
LOSS
) = T
A
θJA
θJC
of the board.
(LLP or Thin