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SC4525ASETRT 数据表(PDF) 11 Page - Semtech Corporation

部件名 SC4525ASETRT
功能描述  28V 3A Step-Down Switching Regulator
PDF  19 Pages
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制造商  SEMTECH [Semtech Corporation]
网页  http://www.semtech.com
标志 SEMTECH - Semtech Corporation

SC4525ASETRT 数据表(HTML) 11 Page - Semtech Corporation

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Minimum Off Time Limitation
The PWM latch in Figure 2 is reset every cycle by the
clock. The clock also turns off the power transistor to
refresh the bootstrap capacitor. This minimum off time
limits the attainable duty cycle of the regulator at a given
switching frequency. The measured minimum off time is
00ns typically. If the required duty cycle is higher than
the attainable maximum, then the output voltage will not
be able to reach its set value in continuous-conduction
mode.
Inductor Selection
The inductor ripple current for a non-synchronous step-
down converter in continuous-conduction mode is
(3)
where F
SW is the switching frequency and L is the
inductance.
An inductor ripple current between 20% to 50% of the
maximum load current, I
O, gives a good compromise
among efficiency, cost and size. Re-arranging Equation (3)
and assuming 35% inductor ripple current, the inductor is
given by
(4)
If the input voltage varies over a wide range, then choose
L
 based on the nominal input voltage. Always verify
converter operation at the input voltage extremes.
The peak current limit of SC4525A power transistor is at
least 3.6A. The maximum deliverable load current for the
SC4525A is 3.6A minus one half of the inductor ripple
current.
Input Decoupling Capacitor
The input capacitor should be chosen to handle the RMS
ripple current of a buck converter. This value is given by
(5)
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
SW
O
D
O
1
F
I
%
35
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
SW
O
D
O
1
F
I
%
35
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
The input capacitance must also be high enough to keep
input ripple voltage within specification. This is important
in reducing the conductive EMI from the regulator. The
input capacitance can be estimated from
(6)
where DV
IN is the allowable input ripple voltage.
Multi-layer ceramic capacitors, which have very low ESR
(a few mW) and can easily handle high RMS ripple current,
are the ideal choice for input filtering. A single 4.7µF
X5R ceramic capacitor is adequate for 500kHz or higher
switching frequency applications, and 0µF is adequate
for 200kHz to 500kHz switching frequency. For high
voltage applications, a small ceramic (µF or 2.2µF) can be
placed in parallel with a low ESR electrolytic capacitor to
satisfy both the ESR and bulk capacitance requirements.
Output Capacitor
The output ripple voltage DV
O of a buck converter can be
expressed as
(7)
where C
O is the output capacitance.
Since the inductor ripple current DI
L increases as D
decreases (Equation (3)), the output ripple voltage is
therefore the highest when V
IN is at its maximum.
A 22µF to 47µF X5R ceramic capacitor is found adequate
for output filtering in most applications. Ripple current
in the output capacitor is not a concern because the
inductor current of a buck converter directly feeds C
O,
resulting in very low ripple current. Avoid using Z5U
and Y5V ceramic capacitors for output filtering because
these types of capacitors have high temperature and high
voltage coefficients.
Freewheeling Diode
Use of Schottky barrier diodes as freewheeling rectifiers
reduces diode reverse recovery input current spikes,
easing high-side current sensing in the SC4525A. These
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
CESAT
D
IN
D
O
V
V
V
V
V
D
−
+
+
=
−
=
1
V
0
.
1
V
R
R
O
6
4
1
SW
D
O
L
L
F
)
D
1
(
)
V
V
(
I
⋅
−
⋅
+
=
D
SW
O
D
O
1
F
I
%
20
)
D
1
(
)
V
V
(
L
⋅
⋅
−
⋅
+
=
)
D
1
(
D
I
I
O
CIN
_
RMS
−
⋅
⋅
=


⋅
⋅
+
⋅
D
=
D
O
SW
L
O
C
F
8
1
ESR
I
V
SW
IN
O
IN
F
V
4
I
C
⋅
D
⋅
>
,
R
G
R
G
S
CA
PWM
⋅
≈
)
/
s
Q
/
s
1
()
/
s
1
(
)
C
R
s
1
(
G
V
V
2
n
2
n
p
O
ESR
PWM
c
o
ω
+
ω
+
ω
+
+
=
7
1
Z
5
R
F
2
1
C
π
=
7
1
P
8
R
F
2
1
C
π
=
,
C
R
1
O
p ≈
ω
,
C
R
1
O
ESR
Z =
ω
k
3
.
22
10
28
.
0
10
R
3
7
20
9
.
15
=
⋅
=
−
nF
45
.
0
10
1
.
22
10
16
2
1
C
3
3
5
=
⋅
⋅
⋅
⋅
π
=
pF
12
10
1
.
22
10
600
2
1
C
3
3
8
=
⋅
⋅
⋅
⋅
π
=


⋅
π
⋅
⋅
−
=
O
FB
O
C
S
CA
C
V
V
C
F
2
1
R
G
1
log
20
A
dB
9
.
15
3
.
3
0
.
1
10
22
10
80
2
1
10
1
.
6
28
1
log
20
A
6
3
3
C
=
⋅
⋅
⋅
⋅
⋅
π
⋅
⋅
⋅
⋅
−
=
−
−
m
7
g
10
R
20
C
A
=
SC4525A

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