Datasheet

K =
P
OUT
P
OUT
+ P
TOTAL
x 100%
-180
-156
-132
-108
-84
-60
PHASE (
o
)
100
1k 10k 100k 1M
FREQUENCY (Hz)
-40
-20
0
20
40
60
GAIN (dB)
100
1k 10k 100k 1M
FREQUENCY (Hz)
LM2743
SNVS276G –APRIL 2004–REVISED MARCH 2013 www.ti.com
The bandwidth and phase margin can be read graphically from Bode plots of H
EA
are shown in Figure 38.
Figure 38. Overall Loop Gain and Phase
The bandwidth of this example circuit is 59 kHz, with a phase margin of 60°.
EFFICIENCY CALCULATIONS
The following is a sample calculation.
A reasonable estimation of the efficiency of a switching buck controller can be obtained by adding together the
Output Power (P
OUT
) loss and the Total Power (P
TOTAL
) loss:
The Output Power (P
OUT
) for the Figure 1 design is (1.2V x 4A) = 4.8W. The Total Power (P
TOTAL
), with an
efficiency calculation to complement the design, is shown below.
The majority of the power losses are due to low and high side of MOSFET’s losses. The losses in any MOSFET
are group of switching (P
SW
) and conduction losses(P
CND
).
P
FET
= P
SW
+ P
CND
= 61.38 mW + 270.42 mW
P
FET
= 331.8 mW
FET Switching Loss (P
SW
)
P
SW
= P
SW(ON)
+ P
SW(OFF)
P
SW
= 0.5 x V
IN
x I
OUT
x (t
r
+ t
f
) x f
SW
P
SW
= 0.5 x 3.3V x 4A x 300 kHz x 31 ns
P
SW
= 61.38 mW
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