lm2735xsdx National Semiconductor Corporation, lm2735xsdx Datasheet - Page 17

• Manufacturer Part Number: lm2735xsdx
• Description: 520khz/1.6mhz - Space-efficient Boost And Sepic Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

maximum ambient air temperature keeps the silicon below
the 125°C temperature.
Procedure:
Place your application into a thermal chamber. You will need
to dissipate enough power in the device so you can obtain a
good thermal impedance value.
Raise the ambient air temperature until the device goes into
thermal shutdown. Record the temperatures of the ambient
air and/or the top case temperature of the LM2735. Calculate
the thermal impedances.
Example from previous calculations:
Pdiss = 475 mW
Ta @ Shutdown = 139°C
Tc @ Shutdown = 155°C
LLP & eMSOP typical applications will produce
in the range of 50°C/W to 65°C/W, and
18°C/W and 28°C/W. These values are for PCB’s with two
and four layer boards with 0.5 oz copper, and four to six ther-
mal vias to bottom side ground plane under the DAP.
For 5-pin SOT23 package typical applications, R
will range from 80°C/W to 110°C/W, and
50°C/W and 65°C/W. These values are for PCB’s with two &
four layer boards with 0.5 oz copper, with two to four thermal
vias from GND pin to bottom layer.
Here is a good rule of thumb for typical thermal impedances,
and an ambient temperature maximum of 75°C: If your design
requires that you dissipate more than 400mW internal to the
LM2735, or there is 750mW of total power loss in the appli-
cation, it is recommended that you use the 6 pin LLP or the 8
pin eMSOP package.
Note: To use these procedures it is important to dissipate an
amount of power within the device that will indicate a true
thermal impedance value. If one uses a very small internal
dissipated value, one can see that the thermal impedance
calculated is abnormally high, and subject to error. The graph
below shows the nonlinear relationship of internal power dis-
sipation vs .
LLP = 55°C/W
LLP = 21°C/W
.
will vary between
will vary between
θJA
numbers
numbers
17
SEPIC Converter
The LM2735 can easily be converted into a SEPIC converter.
A SEPIC converter has the ability to regulate an output volt-
age that is either larger or smaller in magnitude than the input
voltage. Other converters have this ability as well (CUK and
Buck-Boost), but usually create an output voltage that is op-
posite in polarity to the input voltage. This topology is a perfect
fit for Lithium Ion battery applications where the input voltage
for a single cell Li-Ion battery will vary between 3V & 4.5V and
the output voltage is somewhere in between. Most of the
analysis of the LM2735 Boost Converter is applicable to the
LM2735 SEPIC Converter.
SEPIC Design Guide:
SEPIC Conversion ratio without loss elements:
Therefore:
Small ripple approximation:
In a well-designed SEPIC converter, the output voltage, and
input voltage ripple, the inductor ripple and is small in com-
parison to the DC magnitude. Therefore it is a safe approxi-
mation to assume a DC value for these components. The
main objective of the Steady State Analysis is to determine
the steady state duty-cycle, voltage and current stresses on
all components, and proper values for all components.
In a steady-state converter, the net volt-seconds across an
inductor after one cycle will equal zero. Also, the charge into
a capacitor will equal the charge out of a capacitor in one cy-
cle.
Therefore:
FIGURE 14. R
θJA
and eMSOP-8 Package
vs Internal Dissipation for the LLP-6
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