lm2738xsdx National Semiconductor Corporation, lm2738xsdx Datasheet - Page 13

• Manufacturer Part Number: lm2738xsdx
• Description: Lm2738 550khz/1.6mhz 1.5a Step-down Dc-dc Switching Regulator
• Manufacturer: National Semiconductor Corporation

Calculating Efficiency, and Junction
Temperature
The complete LM2738 DC/DC converter efficiency can be
calculated in the following manner.
Or
Calculations for determining the most significant power loss-
es are shown below. Other losses totaling less than 2% are
not discussed.
Power loss (P
the converter: switching and conduction. Conduction losses
usually dominate at higher output loads, whereas switching
losses remain relatively fixed and dominate at lower output
loads. The first step in determining the losses is to calculate
the duty cycle (D):
V
on, and is equal to:
V
diode. It can be obtained from the diode manufactures Elec-
trical Characteristics section. If the voltage drop across the
inductor (V
The conduction losses in the free-wheeling Schottky diode
are calculated as follows:
Often this is the single most significant power loss in the cir-
cuit. Care should be taken to choose a Schottky diode that
has a low forward voltage drop.
Another significant external power loss is the conduction loss
in the output inductor. The equation can be simplified to:
The LM2738 conduction loss is mainly associated with the
internal NFET switch:
SW
D
is the forward voltage drop across the Schottky catch
is the voltage drop across the internal NFET when it is
DCR
LOSS
) is accounted for, the equation becomes:
P
) is the sum of two basic types of losses in
DIODE
V
P
SW
IND
= V
= I
= I
OUT
D
OUT
x I
2
x R
OUT
x R
DSON
x (1-D)
DCR
13
If the inductor ripple current is fairly small, the conduction
losses can be simplified to:
Switching losses are also associated with the internal NFET
switch. They occur during the switch on and off transition pe-
riods, where voltages and currents overlap resulting in power
loss. The simplest means to determine this loss is to empiri-
cally measure the rise and fall times (10% to 90%) of the
switch at the switch node.
Switching Power Loss is calculated as follows:
Another loss is the power required for operation of the internal
circuitry:
I
1.9mA for the 0.55MHz frequency option.
Typical Application power losses are:
Thermal Definitions
T
T
R
R
Heat in the LM2738 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.
Q
J
A
θJC
θJA
is the quiescent operating current, and is typically around
= Chip junction temperature
= Ambient temperature
= Thermal resistance from chip junction to ambient air
= Thermal resistance from chip junction to device case
→
R
IND
ΣP
T
T
V
I
DS(ON)
F
ΣP
V
package
OUT
V
FALL
RISE
OUT
I
D
SW
η
COND
Q
IN
D
DCR
P
P
COND
SWR
SWF
+ P
P
Power Loss Tabulation
= 1/2(V
= 1/2(V
+ P
COND
→
SW
P
P
550kHz
275mΩ
1.9mA
86.7%
SWF
70mΩ
INTERNAL
lead frame
SW
12.0V
1.25A
0.34V
0.275
3.3V
8nS
8nS
+ P
= I
P
= P
IN
Q
IN
+ P
OUT
DIODE
= I
x I
x I
SWR
SWR
2
Q
OUT
OUT
= 207mW
x R
x V
+ P
P
→
+ P
+ P
x F
x F
P
P
INTERNAL
P
DSON
IN
P
P
P
P
PCB
DIODE
IND
COND
P
LOSS
SWR
SWF
OUT
SWF
IND
SW
Q
SW
Q
= P
+ P
x D
x T
x T
Q
INTERNAL
FALL
RISE
= P
22.8mW
4.125W
317mW
118mW
110mW
634mW
207mW
33mW
33mW
)
)
LOSS
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