lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 20

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
The FDS6898A has a typical turn-on rise time t
fall time t
losses for this type of dual N-Channel MOSFETs are
0.061W.
FET Conduction Loss (P
R
to account for the increasing R
ing.
There are few additional losses that are taken into account:
IC Operating Loss (P
where I
FET Gate Charging Loss (P
The value n is the total number of FETs used and Q
typical gate-source charge value, which is 3nC. For the
FDS6898A the gate charging loss is 5.94mW.
Input Capacitor Loss (P
DS(ON)
Q-VCC
= 13mΩ and the factor is a constant value (k = 1.3)
P
P
P
f
CND2
CND
P
SW
P
of 15ns and 16ns, respectively. The switching
P
SW
P
CND1
CND2
P
is the typical operating V
GATE
= 0.5 x 3.3V x 4A x 300kHz x 31ns
P
CND1
= 98.42mW + 172mW = 270.42mW
P
= 0.5 x V
= (4A)
IC
GATE
P
= 1.5mA x 3.3V = 4.95mW
= (4A)
= I
= 2 x 3.3V x 3nC x 300kHz
CND
P
= I
IC
P
P
2
= n x V
2
OUT
IC)
GATE
SW
2
= I
x 13mΩ x 1.3 x (1 - 0.364)
= P
OUT
2
IN
CAP
= 61.38mW
CND
x 13mΩ x 1.3 x 0.364
Q_VCC
x R
x I
CND1
= 5.94mW
x R
CC
)
GATE
OUT
)
DS(ON)
DS(ON)
DS(ON)
x Q
+ P
x V
x (t
)
GS
CC
CND2
x k x (1-D)
r
of a FET due to heat-
CC
+ t
x k x D
,
x f
f
SW
(Continued)
) x f
current
SW
r
and turn-off
GS
is the
20
where,
Here n is the number of paralleled capacitors, ESR is the
equivalent series resistance of each, and P
pation in each. So for example if we use only one input
capacitor of 24 mΩ.
Output Inductor Loss (P
where DCR is the DC resistance. Therefore, for example
Total System Efficiency
P
TOTAL
= P
FET
P
P
IND
IND
P
P
+ P
CAP
IND
= (4A)
= I
IND
IC
= 88.8mW
= 176mW
2
OUT
+ P
)
2
GATE
x 11mΩ
* DCR
+ P
CAP
CAP
+ P
is the dissi-
IND