lm2735xsdx National Semiconductor Corporation, lm2735xsdx Datasheet - Page 12

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

www.national.com
with this condition for large inductor values and high output
currents.
There are miscellaneous poles and zeros associated with
parasitics internal to the LM2735, external components, and
the PCB. They are located well over the crossover frequency,
and for simplicity are not discussed.
PCB Layout Considerations
When planning layout there are a few things to consider when
trying to achieve a clean, regulated output. The most impor-
tant consideration when completing a Boost Converter layout
is the close coupling of the GND connections of the C
pacitor and the LM2735 PGND pin. The GND ends should be
close to one another and be connected to the GND plane with
at least two through-holes. There should be a continuous
ground plane on the bottom layer of a two-layer board except
under the switching node island. The FB pin is a high
impedance node and care should be taken to make the FB
trace short to avoid noise pickup and inaccurate regulation.
The feedback resistors should be placed as close as possible
to the IC, with the AGND of R1 placed as close as possible to
the GND (pin 5 for the LLP) of the IC. The V
should be routed away from the inductor and any other traces
that are switching. High AC currents flow through the V
and V
sible. However, making the traces wide increases radiated
noise, so the designer must make this trade-off. Radiated
noise can be decreased by choosing a shielded inductor. The
remaining components should also be placed as close as
possible to the IC. Please see Application Note AN-1229 for
further considerations and the LM2735 demo board as an ex-
ample of a four-layer layout.
Below is an example of a good thermal & electrical PCB de-
sign. This is very similar to our LM2735 demonstration boards
that are obtainable via the National Semiconductor website.
The demonstration board consists of a two layer PCB with a
common input and output voltage application. Most of the
routing is on the top layer, with the bottom layer consisting of
a large ground plane. The placement of the external compo-
nents satisfies the electrical considerations, and the thermal
OUT
traces, so they should be as short and wide as pos-
OUT
trace to R2
OUT
IN
, SW
ca-
12
performance has been improved by adding thermal vias and
a top layer “Dog-Bone”.
Thermal Design
When designing for thermal performance, one must consider
many variables:
Ambient Temperature: The surrounding maximum air tem-
perature is fairly explanatory. As the temperature increases,
the junction temperature will increase. This may not be linear
though. As the surrounding air temperature increases, resis-
tances of semiconductors, wires and traces increase. This will
decrease the efficiency of the application, and more power
will be converted into heat, and will increase the silicon junc-
tion temperatures further.
Forced Airflow: Forced air can drastically reduce the device
junction temperature. Air flow reduces the hot spots within a
design. Warm airflow is often much better than a lower am-
bient temperature with no airflow.
External Components: Choose components that are effi-
cient, and you can reduce the mutual heating between de-
vices.
PCB design with thermal performance in mind:
The PCB design is a very important step in the thermal design
procedure. The LM2735 is available in three package options
(5 pin SOT23, 8 pin eMSOP & 6 pin LLP). The options are
electrically the same, but difference between the packages is
size and thermal performance. The LLP and eMSOP have
thermal Die Attach Pads (DAP) attached to the bottom of the
packages, and are therefore capable of dissipating more heat
than the SOT23 package. It is important that the customer
choose the correct package for the application. A detailed
thermal design procedure has been included in this data
sheet. This procedure will help determine which package is
correct, and common applications will be analyzed.
There is one significant thermal PCB layout design consider-
ation that contradicts a proper electrical PCB layout design
consideration. This contradiction is the placement of external
components that dissipate heat. The greatest external heat
contributor is the external Schottky diode. It would be nice if
you were able to separate by distance the LM2735 from the
Schottky diode, and thereby reducing the mutual heating ef-
fect. This will however create electrical performance issues.
It is important to keep the LM2735, the output capacitor, and
Schottky diode physically close to each other (see PCB layout
guidelines). The electrical design considerations outweigh the
FIGURE 9. Boost PCB Layout Guidelines
Example of Proper PCB Layout
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