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參數資料
| 型號： | LM2744 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference |
| 中文描述： | 低壓N溝道MOSFET同步降壓穩壓控制器與外部參考 |
| 文件頁數： | 19/22頁 |
| 文件大小： | 663K |
| 代理商： | LM2744 |
Application Information
(Continued)
P
SW
= 0.5 * V
IN
* I
OUT
* (t
r
+ t
f
)* F
OSC
P
SW
= 0.5 x 3.3V x 4A x 300kHz x 31ns
P
SW
= 61.38mW
The FDS6898A has a typical turn-on rise time t
r
and turn-off
fall time t
of 15ns and 16ns, 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
2OUT
x R
DS(ON)
x k x D
P
CND2
= I
2OUT
x R
DS(ON)
x k x (1-D)
R
= 13m
and the factor is a constant value (k = 1.3)
to account for the increasing R
DS(ON)
of a FET due to heat-
ing.
P
CND1
= (4A)
2
x 13m
x 1.3 x 0.364
P
CND2
= (4A)
2
x 13m
x 1.3 x (1 - 0.364)
P
CND
= 98.42mW + 172mW = 270.42mW
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
= 1.5mA *3.3V = 4.95mW
FET Gate Charging Loss (P
GATE
)
P
GATE
= n * V
CC
* Q
GS
* F
OSC
P
GATE
= 2 x 3.3V x 3nC x 300kHz
P
GATE
= 5.94mW
The value n is the total number of FETs used and Q
GS
is the
typical gate-source charge value, which is 3nC. For the
FDS6898A the gate charging loss is 5.94mW.
Input Capacitor Loss (P
CAP
)
where,
Here n is the number of paralleled capacitors, ESR is the
equivalent series resistance of each, and P
is the dissi-
pation in each. So for example if we use only one input
capacitor of 24 m
.
P
CAP
= 88.8mW
Output Inductor Loss (P
IND
)
P
IND
= I
2OUT
* DCR
where DCR is the DC resistance. Therefore, for example
P
IND
= (4A)
2
x 11m
P
IND
= 176mW
Total System Efficiency
L
www.national.com
19
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