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SC1403 Arkusz danych(PDF) 12 Page - Semtech Corporation

Numer części SC1403
Szczegółowy opis  Mobile Multi-Output PWM Controller with Virtual Current SenseTM
PDF  30 Pages
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Producent  SEMTECH [Semtech Corporation]
Strona internetowa  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1403 Arkusz danych(HTML) 12 Page - Semtech Corporation

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 2002 Semtech Corp.
www.semtech.com
SC1403
PRELIMINARY
POWER MANAGEMENT
PRELIMINARY
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 subsequently calculated to be
3.9uH. For the interest of this design, Lo is chosen to be 4.7uH.
Choosing Current Sense Resistor
Since the SC1403 implements Virtual Current SenseTM, the exter-
nal current sense resistor is not required for the control loop. How-
ever, it is required for cycle-by-cycle current limit. Cycle-by-cycle
current limit is enabled when the voltage across the current sense
resistor exceeds 50mV nominal. Depending on the system re-
quirement, this current limit can vary, it is usually 10 to 30% higher
than the maximum load. Taking into consideration of the +/-20%
variation on the 50mV, the value of the current sense resistor can
be calculated using the following equation:
For a DC OC trip point between 6.3A to 9.8A, Rsense is chosen to
be 5.5m Ω . Considering the maximum power dissipation, two
Vishay WSL2010 11m Ω 1% resistors are used.
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,
Applying a derating of 70%, a 30V MOSFET is used in the design.
The peak current of the MOSFET is determined by
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 Ω @Tj=25 C
° and Vgs = 4.5V. In order to
find Rds(on)@ Tj=100 C
° , use 1.40*Rds(on)@25 C
° . Therefore,
Rds(on) @ Tj = 100 C
° is equal to 0.014 Ω .
where
= 7.1A,
= 4.9A and
The worst case conduction loss is calculated to be 112mW. 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 SC1403. 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 234mW 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
° ambient
temperature, 150 C
° junction temperature and 50 C
° /W ther-
mal resistance, its power handling is calculated as follows:
∆
T
J = 150°C;
T
A = 85°C;
= 50°C/W
N
/
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r
o
d
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e
V)
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(
S
D
V)
A
(
D
I)
n
0
(
s
d
R
V
5
.
4
@
)
m
h
o
(
e
g
a
k
c
a
P
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D
6
8
8
4
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33
15
3
1
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.
08
-
O
S
3
1
4
7
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R
I0
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08
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2
1
4
9
S
D
F0
39
.
76
3
0
.
08
-
O
S
L
0
3
F
N
2
1
S
T
S0
32
15
8
0
0
.
08
-
O
S
JA
θ
S
2
G
GATE
f
V
C
2
1
P
⋅
⋅
⋅
=
)
3
I
I
I
I
(
I
2
2
2
1
2
1
RMS
+
⋅
+
=
2
I
I
I
MAX
_
O
MAX
1
∆
+
=
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
nom
V
V
D
=
V
21
V
V
MAX
_
IN
MAX
_
DS
=
=
2
I
I
I
MAX
_
O
MAX
2
∆
−
=
1.30W
50
85
150
θ
T
T
P
JA
A
J
T
=
−
=
−
=
A
11
m
5
.
5
mV
60
IPEAK
=
Ω
=
OC
_
PK
(min)
SENSE
I
mV
40
R
=



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