LM2734ZEVAL National Semiconductor, LM2734ZEVAL Datasheet - Page 10

• Manufacturer Part Number: LM2734ZEVAL
• Description: BOARD EVALUATION LM2734Z
• Manufacturer: National Semiconductor

Specifications of LM2734ZEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 1A
• Voltage - Input: 3 ~ 20V
• Regulator Topology: Buck
• Frequency - Switching: 3MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2734
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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η = 82%
Calculating the LM2734Z Junction Temperature
Thermal Definitions:
T
T
Heat in the LM2734Z due to internal power dissipation is re-
moved through conduction and/or convection.
Conduction: Heat transfer occurs through cross sectional ar-
eas 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
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.
Thermal impedance is defined as:
Thermal impedance from the silicon junction to the ambient
air is defined as:
J
A
= Chip junction temperature
= Ambient temperature
→
package
FIGURE 6. Cross-Sectional View of Integrated Circuit Mounted on a Printed Circuit Board.
→
lead frame
→
PCB.
Design Example 1:
V
V
I
V
Freq
I
T
T
R
IND
D
OUT
Q
IN
OUT
D
RISE
FALL
DSON
DCR
5.0V
2.5V
1.0A
0.35V
3MHz
1.5mA
8ns
8ns
330mΩ
75mΩ
0.568
Operating Conditions
10
P
P
P
P
P
P
P
P
P
OUT
DIODE
IND
SWF
SWR
COND
Q
BOOST
LOSS
R
R
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 conduct 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.
θJC
θJA
= Thermal resistance from chip junction to ambient air
= Thermal resistance from chip junction to device case
2.5W
151mW
75mW
53mW
53mW
187mW
7.5mW
21mW
548mW
θJA
numbers for the
20130373
θJA