lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 15

• Manufacturer Part Number: lm27342sdx
• Description: 2 Mhz 1.5a/2a Wide Input Range Step-down Dc-dc Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

Calculating Efficiency, and Junction
Temperature
The complete LM27341/LM27342 DC-DC converter efficien-
cy 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, where as 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
can be obtained from the Electrical Characteristics section of
the schottky diode datasheet. If the voltage drop across the
inductor (V
V
been omitted in the examples for simplicity.
SCHOTTKY DIODE CONDUCTION LOSSES
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.
INDUCTOR CONDUCTION LOSSES
Another significant external power loss is the conduction loss
in the output inductor. The equation can be simplified to:
MOSFET CONDUCTION LOSSES
The LM27341/LM27342 conduction loss is mainly associated
with the internal NFET:
DS
D
DCR
is the forward voltage drop across the Schottky diode. It
is the voltage drop across the internal NFET when it is
usually gives only a minor duty cycle change, and has
DCR
LOSS
) is accounted for, the equation becomes:
P
P
COND
) is the sum of two basic types of losses in
DIODE
V
P
IND
DS
= I
= V
= I
= I
OUT
OUT
OUT
D1
2
x I
x R
2
x R
x R
OUT
DSON
DSON
DCR
(1-D)
x D
15
MOSFET SWITCHING LOSSES
Switching losses are also associated with the internal NFET.
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
measuring the rise and fall times (10% to 90%) of the switch
at the switch node:
IC QUIESCENT LOSSES
Another loss is the power required for operation of the internal
circuitry:
I
2.4 mA.
MOSFET DRIVER LOSSES
The other operating power that needs to be calculated is that
required to drive the internal NFET:
V
current is dependant on switching frequency f
approximately 8.2 mA at 2 MHz and 4.4 mA at 1 MHz.
TOTAL POWER LOSSES
Total power losses are:
P
Losses internal to the LM27341/LM27342 are:
EFFICIENCY CALCULATION EXAMPLE
Operating conditions are:
Internal Power Losses are:
Total Power Losses are:
V
f
P
P
P
P
P
Q
SW
BOOST
LOSS
IN
COND
SW
Q
BOOST
INTERNAL
is the quiescent operating current, and is typically around
= 12V
= 2 MHz
P
Typical Rise and Fall Times vs Input Voltage
= P
INTERNAL
is normally between 3VDC and 5VDC. The I
COND
= I
= 2
= (V
= (12V x 2A x 2 MHz x 10ns)
= I
= 2.4 mA x 12V
= I
= 8.2 mA x 4.5V
= P
P
P
OUT
Q
BOOST
10V
15V
SWR
SWF
2
V
COND
5V
+ P
IN
= P
x V
x 0.15Ω x 0.314
IN
P
2
x I
SWR
= 1/2(V
= 1/2(V
BOOST
x R
IN
COND
OUT
x V
+ P
P
V
V
SW
OUT
D1
DSON
+ P
BOOST
SW
P
x f
+ P
= 0.5V
= P
= I
Q
IN
IN
SWF
= 3.3V
SW
+ P
= I
10ns
BOOST
t
x D
SWR
x I
x I
8ns
9ns
SWF
RISE
x t
Q
Q
+ P
OUT
OUT
x V
FALL
+ P
+ P
+ P
Q
x V
x f
x f
IN
+ P
BOOST
)
SWF
SWR
SW
SW
BOOST
BOOST
+ P
x t
x t
I
R
OUT
t
10ns
8ns
9ns
FALL
DCR
FALL
RISE
Q
+ P
= 2A
+ P
)
= 20 mΩ
)
SW
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DIODE
= 188 mW
= 480 mW
= 29 mW
= 37 mW
________
= 733 mW
BOOST
. I
BOOST
BOOST
+ P
rms
IND
is