sc4524d Semtech Corporation, sc4524d Datasheet - Page 11

• Manufacturer Part Number: sc4524d
• Description: 18v 2a Step-down Switching Regulator
• Manufacturer: Semtech Corporation

Applications Information (Cont.)
Closed-loop measurement shows that the SC4524D
minimum on time is about 120ns at room temperature
(Figure 4). If the required switch on time is shorter than
the minimum on time, the regulator will either skip cycles
or it will jitter.
To allow for transient headroom, the minimum operating
switch on time should be at least 20% to 30% higher than
the worst-case minimum on time.
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
100ns 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 gives a good compromise 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 SC4524D power transistor is at
1
based on the nominal input voltage. Always verify
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
RMS
RMS
RMS
RMS
4
4
4
4
1
1
1
1
I
I
I
I
V
V
IN
IN
V
V
=
=
L
L
=
=
L
L
=
=
=
=
SW
O
O
O
O
=
=
=
=
=
=
_
_
=
=
_
_
>
>
V
V
V
V
R
R
R
R
=
=
=
=
CIN
CIN
CIN
CIN
V (
V (
V (
V (
IN
IN
IN
IN
6
6
V (
V (
4
4
6
6
is the switching frequency and L
V (
V (
D
D
D
D
20
20












O
O
O
O
35
35
+
+
+
+
D ⋅
D ⋅
O
O
=
=
O
O
I
I
=
=
I
I
1
1
V
V
1
1
V
V
+
+
L
L
L
L
+
+
V
V
V
V
O
O
O
O
V
V
0 .
0 .
+
+
V
V
I
I
I
I
0 .
0 .
%
%
+
+
⋅
⋅
%
%
V
V
⋅
⋅
D
D
D
D
O
O
O
O
F
F
V
V
F
F
O
O
O
O
V
V
I
I
+
+
+
+


 
 




 
 


IN
IN
O
O
V
V
SW
SW
D
D
V
V
SW
SW
V
V
V
V
⋅
⋅
−
−
−
−
⋅
⋅
⋅
⋅
D
D
ESR
ESR
⋅
⋅
ESR
ESR
I
I
D
D
)
)
D
D
V
V
V
V
O
O
I
I
)
)
⋅
⋅
O
O
−
−
V
V
−
−
⋅
⋅
V
V
D
D
)
)
D
D
)
)
F
F
⋅
⋅
D
D
⋅
⋅
⋅
⋅
D
D
1 (
1 (
⋅
⋅
CESAT
CESAT
CESAT
CESAT
⋅
⋅
1
1
L
L
⋅
⋅
⋅
⋅
1
1
F
F
L
L
1 (
1 (
SW
SW
1 (
1 (
1 (
1 (
F
F
⋅
⋅
SW
SW
⋅
⋅
1
1
+
+
1
1
+
+












−
−
1 (
1 (
SW
SW
1 (
1 (
−
−
−
−
) D
) D
−
−
8
8
8
8
) D
) D
−
−
−
−
) D
) D
) D
) D
⋅
⋅
⋅
⋅
F
F
F
F
) D
) D
) D
) D
SW
SW
SW
SW
1
1
1
1
⋅
⋅
⋅
⋅
C
C
C
C
O
O
O
O


 
 




 
 


(3)
(4)
1
is the
R
R
C
C
C
C
R
R
G
G
R
R
A
A
A
A
C
C
C
C
C
C
G
G
C
C
A
A
A
A
V
V
V
V
V
V
V
V
least 2.6A. The maximum deliverable load current for the
SC4524D is 2.6A 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
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 10µF is adequate
for 200kHz to 500kHz switching frequency. For high
voltage applications, a small ceramic (1µF 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 10µF 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
7
7
7
7
o
o
c
c
PWM
PWM
7
7
o
o
PWM
PWM
C
C
C
C
C
C
5
5
C
C
8
8
5
5
8
8
c
c
5
5
8
8
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
10
10
. 0
. 0
1 (
1 (
2
2
. 0
. 0
1 (
1 (
2
2
−
−
−
−
−
−
−
−
2
2
2
2
2
2
2
2
≈
≈
≈
≈
g
g
R
R
⋅ π
⋅ π
R
R
⋅ π
⋅ π
π
π
D
D
L
L
I
I
20
20
20
20
28
28
⋅ π
⋅ π
⋅ π
⋅ π
π
π
D
D
D
D
C
C
20
20
20
20
28
28
D
D
L
L
I
I
D
D
L
L
I
I
C
C
D
D
D
D
C
C
D
D
D
D
G
G
G
G
+
+
m
m
+
+
RMS
RMS
10
10
RMS
RMS
RMS
RMS
10
10
4
4
A
A
4
4
20
20
F
F
F
F
A
A
1
1
1
1
1
1
I
I
V
V
IN
IN
C
C
I
I
V
V
IN
IN
I
I
V
V
IN
IN
C
C
O
CA
CA
1
1
CA
CA
=
=
16
16
1
1
=
=
L
L
=
=
600
600
/ s
/ s
16
16
600
600
L
L
L
L
/ s
/ s
1 Z
1 Z
P
P
=
=
O
O
=
=
=
=
O
O
O
O
1
1
=
=
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
=
=
⋅
⋅
R
R
IN
⋅
⋅
is the output capacitance.
R
R
15
15
15
15
=
=
_
_
>
>
=
=
_
_
>
>
=
=
_
_
>
>
V
V
log
log
log
log
log
log
10
10
log
log
10
10
V
V
V
V
20
20
20
20
⋅
⋅
⋅
⋅
R
R
R
R
R
R
R
R
=
=
CIN
CIN
=
=
G
G
=
=
G
G
V (
V (
CIN
CIN
9 .
9 .
CIN
CIN
9 .
9 .
R
R
R
R
IN
IN
V (
V (
is the allowable input ripple voltage.
V (
V (
ω
ω
ω
ω
⋅
⋅
⋅
⋅
IN
IN
IN
IN
6
6
V (
V (
4
4
V (
V (
4
4
6
6
V (
V (
4
4
10
10
7
7
7
7
10
10
PWM
PWM
PWM
PWM
D
D
S
S
D
D
D
D
p
p
S
S
⋅
⋅
⋅
⋅
p
p
O
O
20
20






20
20
20
20
O
O








 
 










 
 
−
−


O
O






−
−
+
+
+
+
+
+
10
10
D ⋅
D ⋅
10
10
O
O
D ⋅
D ⋅
1 ( )
1 ( )
D ⋅
D ⋅
1 ( )
1 ( )
=
=
I
I
3
3
,
,
O
O
=
=
I
I
O
O
=
=
3
3
I
I
,
,
1
1
V
V
V
V
1
1
V
V
L
L
G
G
28
28
G
G
28
28
+
+
L
L
+
+
L
L
+
+
V
V
3
3
V
V
V
V
3
3
O
O
1
1
1
1
V
V
O
O
O
O
0 .
0 .
+
+
%
%
I
I
+
+
I
I
V
V
0 .
0 .
+
+
I
I
%
%
=
=
%
%
1 (
1 (
1 (
1 (
⋅
⋅
CA
CA
1
1
=
=
D
D
⋅
⋅
V
V
CA
CA
⋅
⋅
1
1
F
F
O
O
V
V
V
V
D
D
V
V
O
O
D
D
O
O
F
F
V
V
F
F
V
V
3
3
3
3
O
O
O
O
I
I
1
1
1
1
⋅
⋅
⋅
⋅
+
+


 
 


+
+
+
+
I
I
I
I
+
+


 
 


+
+


 
 


IN
IN
V
V
SW
SW
D
D
IN
IN
O
O
IN
IN
V
V
⋅
⋅
O
O
V
V
SW
SW
D
D
⋅
⋅
O
O
V
V
SW
SW
22
22
D
D
22
22
⋅
⋅
22
22
22
22
−
−
⋅
⋅
⋅
⋅
V
V
R
R
⋅
⋅
ESR
ESR
−
−
R
R
−
−
⋅
⋅
⋅
⋅
ESR
ESR
⋅
⋅
+
+
ESR
ESR
⋅
⋅
+
+
I
I
D
D
I
I
6
6
I
I
6
6
/ s
/ s
V
V
)
)
/ s
/ s
D
D
)
)
D
D
)
)
V
V
O
O
V
V
O
O
O
O
22
22
22
22
⋅
⋅
S
S
⋅
⋅
S
S
⋅
⋅
−
−
V
V
⋅
⋅
V
V
−
−
V
V
D
D
)
)
⋅
⋅
1 .
1 .
⋅
⋅
D
D
D
D
1 .
1 .
R s
R s
)
)
F
F
)
)
R s
R s
⋅
⋅
D
D
F
F
.
.
1 .
1 .
F
F
⋅
⋅
1 (
1 (
⋅
⋅
D
D
1
1
⋅
⋅
D
D
.
.
1 .
1 .
CESAT
CESAT
1 (
1 (
⋅
⋅
1 (
1 (
⋅
⋅
1
1
CESAT
CESAT
CESAT
CESAT
⋅
⋅
k 3
k 3
k 3
k 3
1
1
⋅
⋅
L
L
F
F
⋅
⋅
⋅
⋅
L
L
1
1
SW
SW
L
L
⋅
⋅
F
F
F
F
ω
ω
SW
SW
SW
SW
ω
ω
1 .
1 .
1 (
1 (
1 .
1 .
1 (
1 (
1 (
1 (
⋅
⋅
SW
SW
2
2
⋅
⋅
1
1
⋅
⋅
+
+
2
2
⋅
⋅
⋅
⋅
ESR
ESR
1
1
SW
SW
+
+
1
1
SW
SW
+
+
ESR
ESR






−
−






⋅
⋅
⋅
⋅
−
−
n
n
−
−
1 (
1 (
10
10
n
n
10
10
1 (
1 (
1 (
1 (
ω
ω
ω
ω
10
10
π
π
π
π
10
10
Q
Q
Q
Q
⋅
⋅
⋅
⋅
−
−
) D
) D
8
8
−
−
) D
) D
8
8
−
−
) D
) D
8
8
p
p
F
F
F
F
10
10
p
p
10
10
C
C
C
C
1
1
1
1
−
−
−
−
−
−
+
+
) D
) D
C
C
−
−
+
+
) D
) D
) D
) D
C
C
−
−
⋅
⋅
⋅
⋅
⋅
⋅
≈
≈
3
3
3
3
≈
≈
O
O
O
O
3
3
F
F
C
C
3
3
) D
) D
C
C
F
F
F
F
) D
) D
) D
) D
s
s
s
s
3
3
)
)
)
)
3
3
SW
SW
O
R
R
IN
R
R
SW
SW
=
=
SW
SW
=
=
O
O
O
O
1
1
⋅
⋅
⋅
⋅
2
2
2
2
1
1
1
1
1
1
of a buck converter can be
C
C
1
1
C
C
2
2
=
=
2
2
=
=
is at its maximum.
. 0
. 0
. 0
. 0
/
/
/
/
⋅
⋅
⋅
⋅
⋅
⋅
O
O
⋅
⋅
⋅ π
⋅ π
⋅
⋅
O
O
⋅ π
⋅ π
12
12
12
12
ω
ω
V
V
V
V
ω
ω
C
C
C
C
V
V
V
V
45
45
C
C
45
45
,
,
,
,
FB
FB
FB
FB
n
n
O
O
O
O
2
2
2
2
n
n
O
O
O
O
O
O
80
80
80
80
pF
pF
pF
pF
)
)
)
)
nF
nF
nF
nF


 
 




 
 




 
 




 
 




 
 


⋅
⋅
⋅
⋅
10
10
10
10
L
increases as D
1
1
1
1
3
3
3
3
⋅
⋅
⋅
⋅
ω
ω
ω
ω
22
22
22
22
Z
Z
Z
Z
=
=
=
=
(5)
(6)
(7)
⋅
⋅
⋅
⋅
10
10
10
10
R
R
R
R
ESR
ESR
ESR
ESR
1
1
−
−
−
−
1
1
6
6
6
6
C
C
C
C
⋅
⋅
⋅
⋅
O
O
O
O
11
1
1
3
3
1
1
3
3
,
,
,
,
0 .
0 .
3 .
3 .
0 .
0 .
3 .
3 .












R
R
C
C
C
C
G
G
R =
R =
C =
C =
C =
C =
A
A
R
R
R =
R =
R
R
C
C
C
C
G
G
R =
R =
C
C
C
C
G
G
A
A
C =
C =
A
A
C =
C =
C =
C =
C =
C =
V
V
V
V
V
V
V
V
V
V
V
V
=
=
=
=
7
7
7
7
5
5
o
o
c
c
PWM
PWM
5
5
C
C
7
7
8
8
o
o
c
c
PWM
PWM
7
7
8
8
7
7
o
o
c
c
PWM
PWM
7
7
C
C
5
5
8
8
C
C
5
5
C
C
8
8
5
5
8
8
5
5
8
8
15
15
15
15
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=