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參數(shù)資料
| 型號： | SC1404 |
| 廠商： | Semtech Corporation |
| 英文描述： | Mobile Multi-Output PWM Controller with Virtual Current Sense |
| 中文描述： | 移動多輸出與虛擬PWM控制器電流檢測 |
| 文件頁數(shù)： | 12/27頁 |
| 文件大?。?/td> | 515K |
| 代理商： | SC1404 |
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12
2003 Semtech Corp.
www.semtech.com
SC1404
PRELIMINARY
POWER MANAGEMENT
Applications Information
where Vin_nom=15V, Vo=3.3V, D=Vo/Vin_nom, Fs=300KHz,
Ts=3.33uS and
Io=2.2 A. Lo is then calculated to be 3.9uH. For
the interest of this design, Lo is chosen to be 4.7uH for the 3.3V
output. For the 5V output, a coupled inductor with 6.4uH primary
(5V winding) inductance is used.
Choosing Current Sense Resistor
Since the SC1404 implements Virtual Current Sense
TM
, an exter-
nal current sense resistor is not needed by the control loop. But it
is required for cycle-by-cycle current limit. Cycle-by-cycle current
limit is reached when the voltage across the current sense resis-
tor exceeds 50mV nominal. Depending on the system require-
ment, this current limit can vary, it is usually 10 to 30% higher than
the maximum load. Taking into consideration lower limit of the
50mV, the value of the current sense resistor can be calculated
using the following equation:
mV
40
R
=
For a DC OC trip point between 8 to 12A, Rsense is chosen to be
5m
.
Choosing the Main Switching MOSFET
Before choosing the main switch MOSFET, we need to know two
critical parameters: voltage and current rating. In order to mini-
mize the conduction loss, we recommend using the lowest Rds(on)
for the same voltage and current rating. The maximum drain to
source voltage of the switch MOSFET is mainly decided by the
topology of the switcher. Since this is a buck topology,
V
21
V
V
MAX
_
IN
MAX
_
DS
=
=
Applying a derating of 70%, a 30V MOSFET is used in the design.
The peak current of the MOSFET is determined by
mV
60
I
PEAK
=
The following calculations are done to verify that the power dissi-
pation of the main switch MOSFET is well within 1.86W, which is
the maximum allowable power dissipation for the package.
where Rds(on) = 0.01
@Tj=25
C
find Rds(on)@ Tj=100
C
Rds(on) @ Tj = 100
C
°
and Vgs = 4.5V. In order to
°
, use 1.40*Rds(on)@25
C
°
is equal to 0.014
.
+
°
. Therefore,
where
= 7.1A, = 4.9A and
I
2
2
The worst case conduction loss is calculated to be 25mW. And the
switching loss of the MOSFET is given by,
=
where Crss is the reverse transfer capacitance of the MOSFET; it
is equal to 200pF for STS12NF30L, Ig is the gate driver current; it
is equal to 1A for SC1404. And Vin_nom = 15V, fs = 300KHz. The
switching loss is calculated to 81mW. And the gate loss is given by,
where Cg=11nF, V=5V and fs=300KHz. The gate loss is calcu-
lated to be 41mW.
So the total power dissipation is calculated to be 147mW and is
well within the maximum power dissipation allowance of the
MOSFET. No special heating sinking is required when laying out
the MOSFET.
According to the calculated voltage and current rating, Si4886DY,
IRF7413, FDS9412 or STS12NF30L meets the requirement. The
specs for these MOSFETs are listed in the table below. For the
purpose of this exercise, STS12NF30L is chosen. Next step is to
determine its power handling capability. Based on 85
C
temperature, 150
C
°
junction temperature and 50
C
mal resistance, its power handling is calculated as follows:
°
ambient
°
/W ther-
T
J
= 150°C; T
A
= 85°C; = 50°C/W
r
d
N
/
n
e
V
P
)
S
D
V
)
D
@
m
)
o
0
d
V
5
R
)
h
e
g
a
k
c
a
P
Y
D
6
8
8
4
0
3
3
1
5
3
1
0
8
s
3
1
4
7
F
R
0
3
3
1
1
1
0
8
s
2
1
4
9
S
D
F
0
3
9
6
3
0
8
s
-
F
N
2
1
S
T
L
S
0
0
3
2
1
5
8
0
0
8
s
S
2
G
GATE
P
f
V
C
2
1
=
nom
2
2
2
1
2
1
RMS
I
D
)
3
I
I
I
I
+
=
GATE
SWITCHING
CONDUCTION
DISS
_
TOTAL
P
P
P
P
+
+
=
=
nom
2
RMS
)
on
(
ds
CONDUCTION
D
I
R
P
G
OUT
S
2
IN
RSS
SWITCHING
I
I
f
V
C
P
NOM
_
IN
OUT
V
nom
V
D
=
2
1.30W
50
85
150
θ
T
T
P
JA
A
J
T
=
=
=
A
11
m
5
=
OC
_
PK
I
(min)
SENSE
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| SC1404ISSTR | Mobile Multi-Output PWM Controller with Virtual Current Sense |
| SC1404ITSTR | Mobile Multi-Output PWM Controller with Virtual Current Sense |
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相關(guān)代理商/技術(shù)參數(shù) |
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| SC1404_05 | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:Mobile Multi-Output PWM Controller with Virtual Current SenseTM |
| SC1404ISSTR | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:Mobile Multi-Output PWM Controller with Virtual Current SenseTM |
| SC1404ITSTR | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:Mobile Multi-Output PWM Controller with Virtual Current SenseTM |
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