SFTB NSC [National Semiconductor], SFTB Datasheet - Page 10

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

www.national.com
Calculating Efficiency, and
Junction Temperature:
V
rms current is approximately 4.25mA. Total power losses
are:
η = 82%
Calculating the LM2734Z Junction
Temperature
Thermal Definitions:
T
T
R
R
Heat in the LM2734Z due to internal power dissipation 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:
silicon → package → lead frame → PCB.
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.
FIGURE 6. Cross-Sectional View of Integrated Circuit
J
A
BOOST
θJC
θJA
Design Example 1:
V
V
I
V
Freq
I
T
T
R
IND
D
= Chip junction temperature
OUT
Q
= Ambient temperature
RISE
FALL
IN
OUT
D
DSON
= Thermal resistance from chip junction to device case
= Thermal resistance from chip junction to ambient air
DCR
is normally between 3VDC and 5VDC. The I
Mounted on a Printed Circuit Board.
5.0V
2.5V
1.0A
0.35V
3MHz
1.5mA
8ns
8ns
330mΩ
75mΩ
0.568
Operating Conditions
P
P
P
P
P
P
P
P
P
OUT
DIODE
IND
SWF
SWR
COND
Q
BOOST
LOSS
(Continued)
2.5W
151mW
75mW
53mW
53mW
187mW
7.5mW
21mW
548mW
BOOST
20130373
10
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 and ground plane, and number of 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.
Four to six thermal vias should be placed under the exposed
pad to the ground plane if the LLP package is used. If the
Thin SOT23-6 package is used, place two to four thermal
vias close to the ground pin of the device.
The datasheet specifies two different R
Thin SOT23–6 package. The two numbers show the differ-
ence in thermal impedance for a four-layer board with 2oz.
copper traces, vs. a four-layer board with 1oz. copper. R
equals 120˚C/W for 2oz. copper traces and GND plane, and
235˚C/W for 1oz. copper traces and GND plane.
Method 1:
To accurately measure the silicon temperature for a given
application, two methods can be used. The first method
requires the user to know the thermal impedance of the
silicon junction to case. (R
the Thin SOT23-6 package. Knowing the internal dissipation
from the efficiency calculation given previously, and the case
temperature, which can be empirically measured on the
bench we have:
Therefore:
Design Example 2:
V
V
I
V
Freq
I
T
T
R
IND
D
OUT
Q
RISE
FALL
IN
OUT
D
DSON
DCR
5.0V
2.5V
1.0A
0.35V
3MHz
1.5mA
8ns
8ns
330mΩ
75mΩ
0.568
T
Operating Conditions
J
= (R
θJC
θJC
x P
) is approximately 80˚C/W for
P
P
P
P
P
P
P
P
P
LOSS
OUT
DIODE
IND
SWF
SWR
COND
Q
BOOST
LOSS
) + T
θJA
C
numbers for the
2.5W
151mW
75mW
53mW
53mW
187mW
7.5mW
21mW
548mW
θJA