Datasheet

K =
=
P
OUT
P
OUT
+ P
LOSS
6.6 W
6.6 W + 1.499 W
= 81 %
LM27341, LM27342, LM27341-Q1, LM27342-Q1
SNVS497E –NOVEMBER 2008–REVISED APRIL 2013 www.ti.com
TOTAL POWER LOSSES
Total power losses are:
P
LOSS
= P
COND
+ P
SWR
+ P
SWF
+ P
Q
+ P
BOOST
+ P
DIODE
+ P
IND
(37)
Losses internal to the LM27341/LM27342 are:
P
INTERNAL
= P
COND
+ P
SWR
+ P
SWF
+ P
Q
+ P
BOOST
(38)
EFFICIENCY CALCULATION EXAMPLE
Operating conditions are:
V
IN
= 12V V
OUT
= 3.3V I
OUT
= 2A
f
SW
= 2 MHz V
D1
= 0.5V R
DCR
= 20 mΩ
Internal Power Losses are:
P
COND
= I
OUT
2
x R
DSON
x D
= 2
2
x 0.15Ω x 0.314
= 188 mW (39)
P
SW
= (V
IN
x I
OUT
x f
SW
x t
FALL
)
= (12V x 2A x 2 MHz x 10ns)
= 480 mW (40)
P
Q
= I
Q
x V
IN
= 2.4 mA x 12V
= 29 mW (41)
P
BOOST
= I
BOOST
x V
BOOST
= 8.2 mA x 4.5V
= 37 mW (42)
P
INTERNAL
= P
COND
+ P
SW
+ P
Q
+ P
BOOST
= 733 mW (43)
Total Power Losses are:
P
DIODE
= V
D1
x I
OUT
(1 - D)
= 0.5V x 2 x (1 - 0.314)
= 686 mW (44)
P
IND
= I
OUT
2
x R
DCR
= 2
2
x 20 mΩ
= 80 mW (45)
P
LOSS
= P
INTERNAL
+ P
DIODE
+ P
IND
= 1.499 W (46)
The efficiency can now be estimated as:
(47)
With this information we can estimate the junction temperature of the LM27341/LM27342.
CALCULATING THE LM27341/LM27342 JUNCTION TEMPERATURE
Thermal Definitions:
T
J
= IC junction temperature
T
A
= Ambient temperature
R
θJC
= Thermal resistance from IC junction to device case
R
θJA
= Thermal resistance from IC junction to ambient air
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