Datasheet

LM2738
SNVS556B –APRIL 2008–REVISED APRIL 2013 www.ti.com
Another significant external power loss is the conduction loss in the output inductor. The equation can be
simplified to:
P
IND
= I
OUT
2
x R
DCR
(32)
The LM2738 conduction loss is mainly associated with the internal NFET switch:
(33)
If the inductor ripple current is fairly small, the conduction losses can be simplified to:
P
COND
= I
OUT
2
x R
DSON
x D (34)
Switching losses are also associated with the internal NFET switch. They occur during the switch on and off
transition periods, where voltages and currents overlap resulting in power loss. The simplest means to determine
this loss is to empirically measure the rise and fall times (10% to 90%) of the switch at the switch node.
Switching Power Loss is calculated as follows:
P
SWR
= 1/2(V
IN
x I
OUT
x F
SW
x T
RISE
) (35)
P
SWF
= 1/2(V
IN
x I
OUT
x F
SW
x T
FALL
) (36)
P
SW
= P
SWR
+ P
SWF
(37)
Another loss is the power required for operation of the internal circuitry:
P
Q
= I
Q
x V
IN
(38)
I
Q
is the quiescent operating current, and is typically around 1.9mA for the 0.55MHz frequency option.
Typical Application power losses are:
Table 1. Power Loss Tabulation
V
IN
12.0V
V
OUT
3.3V P
OUT
4.125W
I
OUT
1.25A
V
D
0.34V P
DIODE
317mW
F
SW
550kHz
I
Q
1.9mA P
Q
22.8mW
T
RISE
8nS P
SWR
33mW
T
FALL
8nS P
SWF
33mW
R
DS(ON)
275mΩ P
COND
118mW
IND
DCR
70mΩ P
IND
110mW
D 0.275 P
LOSS
634mW
η 86.7% P
INTERNAL
207mW
ΣP
COND
+ P
SW
+ P
DIODE
+ P
IND
+ P
Q
= P
LOSS
(39)
ΣP
COND
+ P
SWF
+ P
SWR
+ P
Q
= P
INTERNAL
(40)
P
INTERNAL
= 207mW (41)
Thermal Definitions
T
J
= Chip junction temperature
T
A
= Ambient temperature
R
θJC
= Thermal resistance from chip junction to device case
R
θJA
= Thermal resistance from chip junction to ambient air
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