SC4525ASETRT Semtech, SC4525ASETRT Datasheet - Page 11

• Manufacturer Part Number: SC4525ASETRT
• Description: IC STEP-DWN SW REG 3A 28V 8-SOIC
• Manufacturer: Semtech
• Type: Step-Down (Buck)r

Specifications of SC4525ASETRT

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 3A
• Frequency - Switching: 300kHz ~ 1.3MHz
• Voltage - Input: 8 ~ 28 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Power - Output: -
• Other names: SC4525ASETR

Applications Information (Cont.)
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
where F
inductance.
An inductor ripple current between 20% to 50% of the
maximum load current, I
among efficiency, cost and size. Re-arranging Equation (3)
and assuming 35% inductor ripple current, the inductor is
given by
If the input voltage varies over a wide range, then choose
L
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

based on the nominal input voltage. Always verify
R
R
R
R
R
R
D
D
L
L
D
D
I
I
L
L
I
I
C
C
D
D
D
D
D
D
D
D
D
D
L
L
C
C
I
I
D
D
D
D
RMS
RMS
RMS
RMS
RMS
RMS
4
4
4
4
4
4
1
1
1
1
1
1
I
I
I
I
V
V
IN
IN
V
V
I
I
IN
IN
V
V
=
=
L
L
=
=
=
=
L
L
L
L
=
=
=
=
SW
O
O
=
=
O
O
=
=
=
=
=
=
=
=
_
_
=
=
_
_
>
>
>
>
=
=
_
_
V
V
V
V
V
V
R
R
R
R
R
R
=
=
=
=
=
=
CIN
CIN
CIN
CIN
V (
V (
CIN
CIN
V (
V (
V (
V (
IN
IN
IN
IN
IN
IN
6
6
V (
V (
4
4
6
6
is the switching frequency and L
V (
V (
6
6
V (
V (
4
4
D
D
D
D
D
D
20
20












O
O
20
20
O
O
35
35






O
O
+
+
+
+
+
+
D ⋅
D ⋅
O
O
D ⋅
D ⋅
=
=
O
O
I
I
O
O
=
=
I
I
=
=
I
I
1
1
V
V
1
1
V
V
1
1
V
V
+
+
L
L
L
L
+
+
+
+
V
V
V
V
V
V
O
O
O
O
O
O
V
V
0 .
0 .
+
+
V
V
I
I
V
V
I
I
0 .
0 .
%
%
+
+
0 .
0 .
+
+
%
%
I
I
⋅
⋅
%
%
V
V
⋅
⋅
⋅
⋅
D
D
D
D
O
O
O
O
D
D
V
V
F
F
V
V
F
F
O
O
F
F
V
V
O
O
O
O
O
O
V
V
I
I
I
I
+
+
+
+


 
 




 
 


+
+




IN
IN
O
O
IN
IN
V
V
SW
SW
D
D
V
V
SW
SW
D
D
O
O
V
V
V
V
⋅
⋅
V
V
SW
SW
−
−
V
V
⋅
⋅
−
−
⋅
⋅
⋅
⋅
−
−
D
D
ESR
ESR
⋅
⋅
ESR
ESR
⋅
⋅
ESR
ESR
I
I
I
I
D
D
)
)
D
D
V
V
V
V
D
D
)
)
O
O
I
I
V
V
O
O
)
)
⋅
⋅
⋅
⋅
O
O
−
−
V
V
−
−
⋅
⋅
V
V
−
−
V
V
D
D
⋅
⋅
)
)
D
D
)
)
D
D
)
)
F
F
F
F
⋅
⋅
D
D
⋅
⋅
⋅
⋅
D
D
1 (
1 (
⋅
⋅
⋅
⋅
D
D
1 (
1 (
⋅
⋅
CESAT
CESAT
CESAT
CESAT
CESAT
CESAT
⋅
⋅
1
1
L
L
⋅
⋅
⋅
⋅
⋅
⋅
1
1
F
F
L
L
1 (
1 (
1
1
L
L
SW
SW
F
F
SW
SW
1 (
1 (
1 (
1 (
F
F
1 (
1 (
⋅
⋅
SW
SW
⋅
⋅
⋅
⋅
1
1
+
+
SW
SW
1
1
+
+
1
1
+
+












−
−






−
−
1 (
1 (
SW
SW
1 (
1 (
1 (
1 (
−
−
−
−
) D
) D
−
−
8
8
8
8
−
−
) D
) D
) D
) D
−
−
−
−
−
−
) D
) D
O
) D
) D
) D
) D
⋅
⋅
⋅
⋅
, gives a good compromise
F
F
F
F
) D
) D
) D
) D
) D
) D
SW
SW
SW
SW
1
1
1
1
1
1
⋅
⋅
⋅
⋅
C
C
C
C
O
O
O
O


 
 




 
 






(3)
(4)
(5)

is the
R
R
C
C
C
C
R
R
G
G
R
R
R
R
A
A
A
A
C
C
C
C
C
C
R
R
G
G
C
C
C
C
C
C
C
C
C
C
G
G
A
A
A
A
A
A
A
A
V
V
V
V
V
V
V
V
V
V
V
V
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
where DV
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
expressed as
where C
Since the inductor ripple current DI
decreases (Equation (3)), the output ripple voltage is
therefore the highest when V
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
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
7
7
7
7
o
o
c
c
PWM
PWM
7
7
o
o
PWM
PWM
7
7
C
C
C
C
C
C
5
5
C
C
8
8
5
5
8
8
c
c
5
5
7
7
8
8
5
5
o
o
8
8
c
c
PWM
PWM
C
C
C
C
5
5
8
8
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
10
10
. 0
. 0
1 (
1 (
10
10
2
2
. 0
. 0
1 (
1 (
2
2
. 0
. 0
1 (
1 (
−
−
−
−
−
−
−
−
2
2
2
2
2
2
2
2
2
2
2
2
2
2
≈
≈
−
−
−
−
≈
≈
2
2
≈
≈
g
g
g
g
⋅ π
⋅ π
R
R
⋅ π
⋅ π
⋅ π
⋅ π
π
π
20
20
20
20
28
28
⋅ π
⋅ π
⋅ π
⋅ π
π
π
π
π
π
π
20
20
20
20
28
28
20
20
20
20
28
28
⋅ π
⋅ π
I
I
D
D
L
L
I
I
C
C
D
D
C
C
D
D
D
D
G
G
G
G
+
+
m
m
+
+
m
m
G
G
10
10
+
+
RMS
RMS
RMS
RMS
10
10
10
10
A
A
4
4
20
20
F
F
F
F
A
A
20
20
F
F
F
F
1
1
C
C
V
V
IN
IN
I
I
V
V
IN
IN
C
C
O
CA
CA
1
1
CA
CA
16
16
1
1
1
1
1
1
CA
CA
=
=
600
600
s
s
16
16
600
600
16
16
600
600
L
L
s
s
Z
Z
P
P
Z
Z
s
s
P
P
=
=
O
O
O
O
1
1
1
1
1
1
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
=
=
⋅
⋅
R
R
1
1
IN
⋅
⋅
is the output capacitance.
⋅
⋅
⋅
⋅
R
R
⋅
⋅
R
R
15
15
15
15
/
/
/
/
15
15
/
/
_
_
>
>
=
=
_
_
>
>
log
log
log
log
log
log
10
10
log
log
10
10
log
log
log
log
10
10
V
V
20
20
20
20
⋅
⋅
20
20
⋅
⋅
⋅
⋅
R
R
R
R
R
R
R
R
R
R
=
=
G
G
=
=
G
G
G
G
CIN
CIN
9 .
9 .
CIN
CIN
9 .
9 .
R
R
R
R
is the allowable input ripple voltage.
V (
V (
ω
ω
ω
ω
9 .
9 .
ω
ω
R
R
⋅
⋅
⋅
⋅
IN
IN
⋅
⋅
4
4
6
6
V (
V (
4
4
10
10
7
7
7
7
7
7
10
10
10
10
7
7
PWM
PWM
PWM
PWM
S
S
PWM
PWM
D
D
D
D
p
p
S
S
S
S
⋅
⋅
⋅
⋅
p
p
p
p
⋅
⋅
20
20








 
 










 
 
−
−


O
O






−
−


 
 








−
−
+
+
10
10
10
10
10
10
D ⋅
D ⋅
)
)
D ⋅
D ⋅
)
)
3
3
,
,
)
)
=
=
I
I
O
O
=
=
3
3
I
I
,
,
3
3
,
,
1
1
V
V
G
G
28
28
G
G
28
28
1 (
1 (
G
G
28
28
1 (
1 (
1 (
1 (
L
L
+
+
L
L
3
3
V
V
3
3
3
3
1
1
1
1
O
O
1
1
I
I
V
V
0 .
0 .
+
+
I
I
=
=
%
%
1 (
1 (
1 (
1 (
CA
CA
1
1
=
=
=
=
1 (
1 (
⋅
⋅
CA
CA
⋅
⋅
1
1
CA
CA
1
1
V
V
V
V
O
O
D
D
O
O
F
F
V
V
3
3
3
3
3
3
O
O
1
1
1
1
⋅
⋅
⋅
⋅
⋅
⋅
+
+
1
1
+
+
I
I
I
I
+
+


 
 


+
+


 
 


IN
IN
IN
IN
⋅
⋅
O
O
O
O
V
V
SW
SW
22
22
D
D
22
22
⋅
⋅
22
22
22
22
⋅
⋅
V
V
R
R
⋅
⋅
22
22
22
22
−
−
R
R
R
R
⋅
⋅
⋅
⋅
ESR
ESR
⋅
⋅
+
+
ESR
ESR
⋅
⋅
⋅
⋅
+
+
+
+
6
6
I
I
6
6
s
s
6
6
s
s
s
s
D
D
)
)
V
V
O
O
22
22
22
22
22
22
S
S
⋅
⋅
S
S
S
S
⋅
⋅
−
−
V
V
1 .
1 .
⋅
⋅
D
D
1 .
1 .
s
s
1 .
1 .
)
)
/
/
s
s
s
s
/
/
/
/
F
F
.
.
1 .
1 .
F
F
D
D
1
1
⋅
⋅
D
D
.
.
1 .
1 .
.
.
1 .
1 .
1 (
1 (
⋅
⋅
1
1
1
1
CESAT
CESAT
k 3
k 3
k 3
k 3
R
R
⋅
⋅
⋅
⋅
R
R
k 3
k 3
R
R
1
1
L
L
⋅
⋅
⋅
⋅
F
F
ω
ω
SW
SW
SW
SW
ω
ω
ω
ω
1 .
1 .
1 .
1 .
1 .
1 .
1 (
1 (
2
2
⋅
⋅
⋅
⋅
2
2
⋅
⋅
⋅
⋅
ESR
ESR
2
2
⋅
⋅
+
+
1
1
SW
SW
+
+
ESR
ESR
ESR
ESR






⋅
⋅
⋅
⋅
−
−
n
n
⋅
⋅
10
10
n
n
n
n
10
10
10
10
1 (
1 (
1 (
1 (
ω
ω
ω
ω
ω
ω
10
10
π
π
π
π
10
10
10
10
π
π
Q
Q
Q
Q
Q
Q
⋅
⋅
⋅
⋅
⋅
⋅
8
8
−
−
) D
) D
8
8
p
p
F
F
F
F
10
10
F
F
p
p
p
p
10
10
10
10
C
C
C
C
1
1
1
1
C
C
1
1
−
−
−
−
+
+
C
C
−
−
+
+
+
+
) D
) D
C
C
−
−
C
C
−
−
⋅
⋅
⋅
⋅
≈
≈
3
3
3
3
≈
≈
≈
≈
3
3
O
O
O
O
O
O
3
3
C
C
3
3
3
3
C
C
C
C
F
F
F
F
) D
) D
) D
) D
s
s
s
s
s
s
3
3
)
)
)
)
3
3
3
3
)
)
O
R
R
IN
R
R
SW
SW
=
=
SW
SW
R
R
=
=
=
=
O
O
O
O
⋅
⋅
⋅
⋅
2
2
O
O
⋅
⋅
2
2
2
2
1
1
1
1
1
1
of a buck converter can be
C
C
1
1
C
C
1
1
2
2
=
=
C
C
2
2
=
=
2
2
=
=
is at its maximum.
. 0
. 0
. 0
. 0
/
/
/
/
. 0
. 0
⋅
⋅
/
/
⋅
⋅
⋅
⋅
O
O
⋅
⋅
⋅ π
⋅ π
⋅
⋅
O
O
O
O
⋅ π
⋅ π
⋅ π
⋅ π
12
12
12
12
ω
ω
12
12
V
V
V
V
ω
ω
ω
ω
V
V
C
C
V
V
V
V
45
45
C
C
45
45
V
V
45
45
,
,
,
,
,
,
FB
FB
FB
FB
FB
FB
n
n
O
O
O
O
2
2
2
2
n
n
2
2
n
n
O
O
O
O
O
O
80
80
80
80
pF
pF
80
80
pF
pF
pF
pF
)
)
)
)
)
)
nF
nF
nF
nF
nF
nF


 
 




 
 




 
 




 
 




 
 


⋅
⋅
⋅
⋅
⋅
⋅
10
10
10
10
10
10
L
increases as D
1
1
1
1
1
1
3
3
3
3
3
3
⋅
⋅
⋅
⋅
ω
ω
ω
ω
⋅
⋅
ω
ω
22
22
22
22
22
22
Z
Z
Z
Z
Z
Z
=
=
=
=
=
=
(6)
(7)
⋅
⋅
⋅
⋅
⋅
⋅
10
10
10
10
R
R
10
10
R
R
R
R
ESR
ESR
ESR
ESR
ESR
ESR
1
1
−
−
−
−
1
1
1
1
−
−
6
6
6
6
6
6
C
C
C
C
C
C
⋅
⋅
⋅
⋅
O
O
⋅
⋅
O
O
O
O

1
1
3
3
1
1
3
3
1
1
O
3
3
,
,
,
,
,
,
,
0 .
0 .
3 .
3 .
0 .
0 .
3 .
3 .
0 .
0 .
3 .
3 .


















R =
R =
R
R
C
C
C
C
G
G
R =
R =
G
G
A
A
C =
C =
A
A
C =
C =
C =
C =
C =
C =
V
V
V
V
V
V
V
V
=
=
=
=
=
=
o
o
c
c
PWM
PWM
7
7
7
7
o
o
c
c
PWM
PWM
7
7
C
C
5
5
C
C
8
8
5
5
8
8
5
5
8
8
15
15
15
15
15
15
=
=
=
=
=
=
=
=
=
=
=
=
=
=