SFTB NSC [National Semiconductor], SFTB Datasheet - Page 11

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

Calculating the LM2734Z Junction
Temperature
The second method can give a very accurate silicon junction
temperature. The first step is to determine R
cation. The LM2734Z has over-temperature protection cir-
cuitry. When the silicon temperature reaches 165˚C, the
device stops switching. The protection circuitry has a hyster-
esis of 15˚C. Once the silicon temperature has decreased to
approximately 150˚C, the device will start to switch again.
Knowing this, the R
during the early stages of the design by raising the ambient
temperature in the given application until the circuit enters
thermal shutdown. If the SW-pin is monitored, it will be
obvious when the internal NFET stops switching indicating a
junction temperature of 165˚C. Knowing the internal power
dissipation from the above methods, the junction tempera-
ture and the ambient temperature, R
Once this is determined, the maximum ambient temperature
allowed for a desired junction temperature can be found.
Using a standard National Semiconductor Thin SOT23-6
demonstration board to determine the R
four layer PCB is constructed using FR4 with 1/2oz copper
traces. The copper ground plane is on the bottom layer. The
Design Example 3:
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
SOT23-6
12.0V
3.30V
750mA
0.35V
3MHz
1.5mA
4mA
5V
8ns
8ns
400mΩ
75mΩ
30.3%
Operating Conditions
θJA
(Continued)
for any PCB can be characterized
P
P
P
P
P
P
P
P
P
OUT
DIODE
IND
SWF
SWR
COND
Q
BOOST
LOSS
θJA
θJA
can be determined.
of the board. The
θJA
2.475W
523mW
56.25mW
108mW
108mW
68.2mW
18mW
20mW
902mW
of the appli-
11
ground plane is accessed by two vias. The board measures
2.5cm x 3cm. It was placed in an oven with no forced airflow.
The ambient temperature was raised to 94˚C, and at that
temperature, the device went into thermal shutdown.
If the junction temperature was to be kept below 125˚C, then
the ambient temperature cannot go above 54.2˚C.
The method described above to find the junction tempera-
ture in the Thin SOT23-6 package can also be used to
calculate the junction temperature in the LLP package. The 6
pin LLP package has a R
depending on the application. R
same manner as described in method #2 (see example 3).
LLP Package
The LM2734Z is packaged in a Thin SOT23-6 package and
the 6–pin LLP. The LLP package has the same footprint as
the Thin SOT23-6, but is thermally superior due to the ex-
posed ground paddle on the bottom of the package.
R
than that of the Thin SOT23-6 package for a similar PCB
configuration (area, copper weight, thermal vias).
θJA
of the LLP package is normally two to three times better
No Pullback LLP Configuration
T
FIGURE 7. Dog Bone
J
- (R
θJA
θJC
x P
= 20˚C/W, and R
LOSS
θJA
can be calculated in the
) = T
A
20130370
θJA
www.national.com
can vary
20130374