Datasheet

D =
V
OUT
+ V
D1
+ V
DCR
V
IN
+ V
D1
- V
DS
LM27341, LM27342, LM27341-Q1, LM27342-Q1
www.ti.com
SNVS497E NOVEMBER 2008REVISED APRIL 2013
V
DS
is the voltage drop across the internal NFET when it is on, and is equal to:
V
DS
= I
OUT
x R
DSON
(27)
V
D
is the forward voltage drop across the Schottky diode. It can be obtained from the Electrical Characteristics
section of the schottky diode datasheet. If the voltage drop across the inductor (V
DCR
) is accounted for, the
equation becomes:
(28)
V
DCR
usually gives only a minor duty cycle change, and has been omitted in the examples for simplicity.
SCHOTTKY DIODE CONDUCTION LOSSES
The conduction losses in the free-wheeling Schottky diode are calculated as follows:
P
DIODE
= V
D1
x I
OUT
(1-D) (29)
Often this is the single most significant power loss in the circuit. 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:
P
IND
= I
OUT
2
x R
DCR
(30)
MOSFET CONDUCTION LOSSES
The LM27341/LM27342 conduction loss is mainly associated with the internal NFET:
P
COND
= I
OUT
2
x R
DSON
x D (31)
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:
P
SWF
= 1/2(V
IN
x I
OUT
x f
SW
x t
FALL
) (32)
P
SWR
= 1/2(V
IN
x I
OUT
x f
SW
x t
RISE
) (33)
P
SW
= P
SWF
+ P
SWR
(34)
Table 1. Typical Rise and Fall Times vs Input Voltage
V
IN
t
RISE
t
FALL
5V 8ns 8ns
10V 9ns 9ns
15V 10ns 10ns
IC QUIESCENT LOSSES
Another loss is the power required for operation of the internal circuitry:
P
Q
= I
Q
x V
IN
(35)
I
Q
is the quiescent operating current, and is typically around 2.4 mA.
MOSFET DRIVER LOSSES
The other operating power that needs to be calculated is that required to drive the internal NFET:
P
BOOST
= I
BOOST
x V
BOOST
(36)
V
BOOST
is normally between 3VDC and 5VDC. The I
BOOST
rms current is dependant on switching frequency f
SW
.
I
BOOST
is approximately 8.2 mA at 2 MHz and 4.4 mA at 1 MHz.
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