Datasheet
P
COND
= (I
OUT
2
x D)
1
3
1 +
x
'i
L
I
OUT
2
R
DSON
D =
V
OUT
+ V
D
+ V
DCR
V
IN
+ V
D
+ V
DCR
- V
SW
LMR10510
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SNVS727B –OCTOBER 2011–REVISED APRIL 2013
The conduction losses in the free-wheeling Schottky diode are calculated as follows:
P
DIODE
= V
D
x I
OUT
x (1-D)
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.
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
The LMR10510 conduction loss is mainly associated with the internal PFET:
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
Switching losses are also associated with the internal PFET. 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.
Switching Power Loss is calculated as follows:
P
SWR
= 1/2(V
IN
x I
OUT
x F
SW
x T
RISE
)
P
SWF
= 1/2(V
IN
x I
OUT
x F
SW
x T
FALL
)
P
SW
= P
SWR
+ P
SWF
Another loss is the power required for operation of the internal circuitry:
P
Q
= I
Q
x V
IN
I
Q
is the quiescent operating current, and is typically around 3.3mA for the 1.6MHz frequency option.
Typical Application power losses are:
Table 1. Power Loss Tabulation
V
IN
5.0V
V
OUT
3.3V P
OUT
3.3W
I
OUT
1.0A
V
D
0.45V P
DIODE
150mW
F
SW
1.6MHz
I
Q
3.3mA P
Q
17mW
T
RISE
4nS P
SWR
16mW
T
FALL
4nS P
SWF
16mW
R
DS(ON)
150mΩ P
COND
100mW
IND
DCR
70mΩ P
IND
70mW
D 0.667 P
LOSS
369mW
η 88% P
INTERNAL
149mW
ΣP
COND
+ P
SW
+ P
DIODE
+ P
IND
+ P
Q
= P
LOSS
ΣP
COND
+ P
SWF
+ P
SWR
+ P
Q
= P
INTERNAL
P
INTERNAL
= 149mW
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