Datasheet

P
COND

= (I
OUT
2
x D) x
1
3
1 +
x
'i
L
I
OUT
2
x R
DS (ON)
LMR10530
www.ti.com SNVS814A –JUNE 2012–REVISED APRIL 2013
Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky
diode with 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
(23)
The LMR10530 conduction loss is mainly associated with the internal power switch:
(24)
If the inductor ripple current is fairly small, the conduction losses can be simplified to:
P
COND
= I
OUT
2
x R
DS(ON)
x D (25)
Switching losses are also associated with the internal power 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 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
= 0.5 x (V
IN
x I
OUT
x f
SW
x T
RISE
) (26)
P
SWF
= 0.5 x (V
IN
x I
OUT
x f
SW
x T
FALL
) (27)
P
SW
= P
SWR
+ P
SWF
(28)
The power loss required for operation of the internal circuitry is given by:
P
Q
= I
Q
x V
IN
(29)
I
Q
is the quiescent operating current, and is typically around 3.2mA for the LMR10530X, and 4.3mA for the
LMR10530Y.
An example of efficiency calculation for a typical application is shown in Table 1:
Table 1. Power Loss Tabulation
Conditions Power loss
V
IN
5V
V
OUT
3.3V
I
OUT
3.0A P
OUT
9.9W
V
D
0.33V P
DIODE
277mW
R
DS(ON)
56mΩ P
COND
363mW
f
SW
1.5MHz
T
RISE
10ns
P
SW
225mW
T
FALL
10ns
IND
DCR
28mΩ P
IND
252mW
I
Q
3.2mA P
Q
16mW
η 89.7%
D is calculated to be 0.72
P
LOSS
= Σ ( P
COND
+ P
SW
+ P
Q
+ P
IND
+ P
DIODE
) (30)
P
LOSS
= 1.133W (31)
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