LM2734ZEVAL National Semiconductor, LM2734ZEVAL Datasheet - Page 11

• 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: -

Method 1:
To accurately measure the silicon temperature for a given
application, two methods can be used. The first method re-
quires the user to know the thermal impedance of the silicon
junction to case. (R
SOT23-6 package. Knowing the internal dissipation from the
efficiency calculation given previously, and the case temper-
ature, which can be empirically measured on the bench we
have:
Therefore:
The second method can give a very accurate silicon junction
temperature. The first step is to determine R
cation. The LM2734Z has over-temperature protection cir-
cuitry. When the silicon temperature reaches 165°C, the
device stops switching. The protection circuitry has a hys-
teresis of 15°C. Once the silicon temperature has decreased
to approximately 150°C, the device will start to switch again.
Knowing this, the R
ing the early stages of the design by raising the ambient
temperature in the given application until the circuit enters
thermal shutdown. If the SW-pin is monitored, it will be obvi-
ous when the internal NFET stops switching indicating a
junction temperature of 165°C. Knowing the internal power
dissipation from the above methods, the junction temperature
and the ambient temperature, R
Once this is determined, the maximum ambient temperature
allowed for a desired junction temperature can be found.
V
V
I
V
Freq
I
T
T
R
IND
D
OUT
Q
RISE
FALL
IN
OUT
D
DSON
DCR
Design Example 2:
5.0V
2.5V
1.0A
0.35V
3MHz
1.5mA
8ns
8ns
330mΩ
75mΩ
0.568
T
θJA
θJC
J
= (R
) is approximately 80°C/W for the Thin
for any PCB can be characterized dur-
θJC
x P
P
P
P
P
P
P
P
P
P
θJA
LOSS
OUT
DIODE
IND
SWF
SWR
COND
Q
BOOST
LOSS
Operating Conditions
can be determined.
) + T
C
2.5W
151mW
75mW
53mW
53mW
187mW
7.5mW
21mW
548mW
θJA
of the appli-
11
Using a standard National Semiconductor Thin SOT23-6
demonstration board to determine the R
four layer PCB is constructed using FR4 with 1/2oz copper
traces. The copper ground plane is on the bottom layer. The
ground plane is accessed by two vias. The board measures
2.5cm x 3cm. It was placed in an oven with no forced airflow.
The ambient temperature was raised to 94°C, and at that
temperature, the device went into thermal shutdown.
If the junction temperature was to be kept below 125°C, then
the ambient temperature cannot go above 54.2°C.
The method described above to find the junction temperature
in the Thin SOT23-6 package can also be used to calculate
the junction temperature in the LLP package. The 6 pin LLP
package has a R
on the application. R
as described in method #2 (see example 3).
Design Example 3:
Package
V
V
I
V
Freq
I
I
V
T
T
R
IND
D
OUT
Q
BOOST
RISE
FALL
IN
OUT
D
BOOST
DSON
DCR
SOT23-6
12.0V
3.30V
750mA
0.35V
3MHz
1.5mA
4mA
5V
8ns
8ns
400mΩ
75mΩ
30.3%
θJC
T
Operating Conditions
J
θJA
= 20°C/W, and R
- (R
can be calculated in the same manner
θJA
x P
P
P
P
P
P
P
P
P
P
LOSS
OUT
DIODE
IND
SWF
SWR
COND
Q
BOOST
LOSS
) = T
θJA
θJA
can vary depending
A
of the board. The
2.475W
523mW
56.25mW
108mW
108mW
68.2mW
18mW
20mW
902mW
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