LM2735XMFEVAL National Semiconductor, LM2735XMFEVAL Datasheet - Page 14

• Manufacturer Part Number: LM2735XMFEVAL
• Description: BOARD DEMO FOR LM2735 SOT23-5
• Manufacturer: National Semiconductor

Specifications of LM2735XMFEVAL

• Main Purpose: DC/DC, Step Up
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 12V
• Current - Output: 500mA
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Boost
• Frequency - Switching: 1.6MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2735
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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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
FIGURE 9. Boost PCB Layout Guidelines
Example of Proper PCB Layout
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guidelines). The electrical design considerations outweigh the
thermal considerations. Other factors that influence thermal
performance are thermal vias, copper weight, and number of
board layers.
Definitions
Heat energy is transferred from regions of high temperature
to regions of low temperature via three basic mechanisms:
radiation, conduction and convection.
Radiation: Electromagnetic transfer of heat between masses
at different temperatures.
Conduction: Transfer of heat through a solid medium.
Convection: Transfer of heat through the medium of a fluid;
typically air.
Conduction & Convection will be the dominant heat transfer
mechanism in most applications.
R
temperature.
R
temperature.
C
perature.
C
perature.
R
impedances, and most data sheets contain associated values
for these two symbols. The units of measurement are °C/
Watt.
R
thermal model below). The capacitors represent delays that
are present from the time that power and its associated heat
is increased or decreased from steady state in one medium
until the time that the heat increase or decrease reaches
steady state on the another medium.
θJA
θJC
θJC
θCA
θJA
θJA
FIGURE 10. Simplified Thermal Impedance Model
: Thermal impedance from silicon junction to ambient air
: Thermal impedance from silicon junction to device case
: Thermal Delay from silicon junction to device case tem-
: Thermal Delay from device case to ambient air tem-
is the sum of smaller thermal impedances (see simplified
& R
θJC
: These two symbols represent thermal
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