Datasheet

LM2727, LM2737
www.ti.com SNVS205D –AUGUST 2002–REVISED MARCH 2013
EFFICIENCY CALCULATIONS
A reasonable estimation of the efficiency of a switching controller can be obtained by adding together the loss is
each current carrying element and using the equation:
(13)
The following shows an efficiency calculation to complement the Circuit of Figure 26. Output power for this circuit
is 1.2V x 10A = 12W.
Chip Operating Loss
P
IQ
= I
Q-V
CC
*V
CC
(14)
2mA x 5V = 0.01W
FET Gate Charging Loss
P
GC
= n * V
CC
* Q
GS
* f
OSC
(15)
The value n is the total number of FETs used. The Si4442DY has a typical total gate charge, Q
GS
, of 36nC and
an r
ds-on
of 4.1mΩ. For a single FET on top and bottom: 2*5*36E
-9
*300,000 = 0.108W
FET Switching Loss
P
SW
= 0.5 * V
in
* I
O
* (t
r
+ t
f
)* f
OSC
(16)
The Si4442DY has a typical rise time t
r
and fall time t
f
of 11 and 47ns, respectively. 0.5*5*10*58E
-9
*300,000 =
0.435W
FET Conduction Loss
P
Cn
= 0.533W (17)
Input Capacitor Loss
(18)
(19)
4.28
2
*0.018/2 = 0.084W
Input Inductor Loss
P
Lin
= I
2
in
* DCR
input-L
(20)
(21)
2.82
2
*0.007 = 0.055W
Output Inductor Loss
P
Lout
= I
2
o
* DCR
output-L
(22)
10
2
*0.004 = 0.4W
System Efficiency
(23)
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