Datasheet

P
CAP
=
(I
RMS_RIP
)
2
x ESR
n
2
K =
P
OUT
P
OUT
+ P
TOTAL
x 100%
LM2745, LM2748
www.ti.com SNOSAL2E –APRIL 2005–REVISED APRIL 2013
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 Typical Application Circuit 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 the low side and high side 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
The FDS6898A has a typical turn-on rise time t
r
and turn-off fall time t
f
of 15 ns and 16 ns, respectively. The
switching losses for this type of dual N-Channel MOSFETs are 0.061W.
FET Conduction Loss (P
CND
)
P
CND
= P
CND1
+ P
CND2
P
CND1
= I
2
OUT
x R
DS(ON)
x k x D
P
CND2
= I
2
OUT
x R
DS(ON)
x k x (1-D)
R
DS(ON)
= 13 mΩ and the factor is a constant value (k = 1.3) to account for the increasing R
DS(ON)
of a FET due to
heating.
P
CND1
= (4A)
2
x 13 mΩ x 1.3 x 0.364
P
CND2
= (4A)
2
x 13 mΩ x 1.3 x (1 - 0.364)
P
CND
= 98.42 mW + 172 mW = 270.42 mW
There are few additional losses that are taken into account:
IC Operating Loss (P
IC)
P
IC
= I
Q_VCC
x V
CC
,
where I
Q-VCC
is the typical operating V
CC
current
P
IC
= 1.7 mA x 3.3V = 5.61 mW
FET Gate Charging Loss (P
GATE
)
P
GATE
= n x V
CC
x Q
GS
x f
SW
P
GATE
= 2 x 3.3V x 3 nC x 300 kHz
P
GATE
= 5.94 mW
The value n is the total number of FETs used and Q
GS
is the typical gate-source charge value, which is 3 nC. For
the FDS6898A the gate charging loss is 5.94 mW.
Input Capacitor Loss (P
CAP
)
where,
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