Datasheet

P
CAP
=
(1.924A)
2
x 24 m:
1
2
I
RMS_RIP
= I
OUT
x
D(1 - D)
P
CAP
=
(I
RMS_RIP
)
2
x ESR
n
2
LM2743
www.ti.com SNVS276G –APRIL 2004–REVISED MARCH 2013
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
OUT
)
2
x R
DS(ON)
x k x D
P
CND2
= (I
OUT
)
2
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
= 1.5 mA x 3.3V = 4.95 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,
Here n is the number of paralleled capacitors, ESR is the equivalent series resistance of each, and P
CAP
is the
dissipation in each. So for example if we use only one input capacitor of 24 mΩ.
P
CAP
= 88.8 mW
Output Inductor Loss (P
IND
)
P
IND
= I
2
OUT
x DCR
where DCR is the DC resistance. Therefore, for example
P
IND
= (4A)
2
x 11 mΩ
P
IND
= 176 mW
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