sc4524a Semtech Corporation, sc4524a Datasheet - Page 14

• Manufacturer Part Number: sc4524a
• Description: Sc4524a 28v 2a Step-down Switching Regulator
• Manufacturer: Semtech Corporation

Applications Information (Cont.)
(3) Place the compensator zero, F
20% of the crossover frequency, F
(4) Use the compensator pole, F
F
(5) Then, the parameters of the compensation network
can be calculated by
where g
Example: Determine the voltage compensator for an
800kHz, 2V to 3.3V/2A converter with 22uF ceramic
output capacitor.
Choose a loop gain crossover frequency of 80kHz, and
place voltage compensator zero and pole at F
(20% of F
required compensator gain at F
Then the compensator parameters are
Select R
Compensator parameters for various typical applications
are listed in Table 4. A MathCAD program is also available
upon request for detailed calculation of the compensator
parameters.
PCB Layout Considerations
In a step-down switching regulator, the input bypass
Z
R
R
C
C
C
C
G
G
R
R
C
C
C
C
A
A
A
A
V
V
V
V
.
7
7
5
5
o
o
c
c
PWM
PWM
7
7
5
5
C
C
C
C
8
8
8
8
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
10
10
. 0
. 0
1 (
1 (
−
−
−
−
2
2
2
2
2
2
2
2
≈
≈
g
g
R
R
C
C
C
C
G
G
R
R
C
C
G
G
R
R
C
C
C
C
A
A
A
A
⋅ π
⋅ π
C
C
⋅ π
⋅ π
V
V
V
V
π
π
π
π
V
V
V
V
20
20
20
20
28
28
+
+
G
G
m
m
10
10
A
A
20
20
F
F
F
F
7
7
7
7
7
o
o
c
c
PWM
PWM
7
7
5
5
8
8
o
o
c
c
PWM
PWM
5
5
8
8
m
C
C
C
C
5
5
8
8
5
5
C
C
CA
CA
1
1
1
1
16
16
600
600
s
s
=22.k, C
Z
Z
P
P
1
1
=0.28mA/V is the EA gain of the SC4524A.
⋅
⋅
⋅
⋅
⋅
⋅
1
1
R
R
=
=
=
=
=
=
=
=
=
=
15
15
=
=
=
=
=
=
/
/
=
=
=
=
=
=
=
=
log
log
log
log
10
10
=
=
20
20
⋅
⋅
R
R
R
R
G
G
9 .
9 .
ω
ω
R
R
C
⋅
⋅
10
10
7
7
10
10
7
7
10
10
), and F
. 0
. 0
PWM
PWM
1 (
1 (
1 (
1 (
2
2
S
S
2
2
2
2
2
2
2
2
2
2
−
−
−
−
2
2
p
p
≈
≈
⋅
⋅
≈
≈


 
 








−
−
10
10
g
g
g
g
)
)
3
3
,
,
G
G
28
28
⋅ π
⋅ π
⋅ π
⋅ π
1 (
1 (
π
π
⋅ π
⋅ π
π
π
π
π
20
20
20
20
28
28
⋅ π
⋅ π
3
3
G
G
1
1
G
G
+
+
m
m
+
+
m
m
10
10
=
=
1 (
1 (
CA
CA
1
1
A
A
A
A
20
20
F
F
20
20
F
F
F
F
3
3
1
1
⋅
⋅
+
+
C
C
C
C
CA
CA
CA
CA
1
1
1
1
1
1
⋅
⋅
22
22
22
22
16
16
16
16
600
600
R
R
600
600
s
s
s
s
Z
Z
P
P
⋅
⋅
+
+
6
6
s
s
1
1
1
1
⋅
⋅
⋅
⋅
⋅
⋅
22
22
R
R
R
R
S
S
15
15
/
/
/
/
1 .
1 .
s
s
/
/
5
log
log
log
log
10
10
⋅
⋅
1
1
.
.
20
20
1 .
1 .
⋅
⋅
R
R
R
R
R
R
k 3
k 3
R
R
⋅
⋅
=0.47nF, and C
G
G
G
G
R
R
9 .
9 .
ω
ω
R
R
ω
ω
ω
ω
⋅
⋅
1 .
1 .
⋅
⋅
2
2
⋅
⋅
ESR
ESR
7
7
⋅
⋅
10
10
10
10
7
7
n
n
PWM
PWM
10
10
PWM
PWM
ω
ω
S
S
S
S
P
10
10
p
p
⋅
⋅
p
p
π
π
⋅
⋅
Q
Q


 
 








−
−
⋅
⋅
10
10
10
10
F
F
p
p
)
)
10
10
)
)
3
3
C
C
,
,
,
,
=600kHz. From Equation (9), the
1
1
G
G
28
28
+
+
1 (
1 (
1 (
1 (
C
C
−
−
≈
≈
3
3
3
3
O
O
3
3
3
3
C
C
1
1
s
s
1 (
1 (
=
=
1 (
1 (
CA
CA
3
3
1
1
)
)
1
1
R
R
=
=
O
O
3
3
3
3
⋅
⋅
2
2
⋅
⋅
1
1
⋅
⋅
+
+
+
+
1
1
C
C
=
=
2
2
⋅
⋅
22
22
22
22
. 0
. 0
22
22
R
R
/
/
⋅
⋅
⋅
⋅
⋅
⋅
+
+
+
+
O
O
⋅ π
⋅ π
6
6
s
s
s
s
12
12
ω
ω
V
V
22
22
22
22
V
V
45
45
S
S
,
,
FB
FB
1 .
1 .
2
2
n
n
s
s
s
s
O
O
/
/
/
/
.
.
1 .
1 .
1 .
1 .
1
1
80
80
pF
pF
)
)
k 3
k 3
R
R
R
R
⋅
⋅
nF
nF
ω
ω
ω
ω


 
 


1 .
1 .
1 .
1 .
2
2
⋅
⋅
ESR
ESR
ESR
ESR
⋅
⋅
⋅
⋅
⋅
⋅
n
n
n
n
10
10
C
ω
ω
ω
ω
10
10
10
10
10
10
π
π
P
Q
Q
Q
Q
⋅
⋅
⋅
⋅
is
p
p
p
p
F
F
10
10
10
10
8
C
C
, to cancel the ESR zero,
C
C
C
1
1
1
1
3
3
=0pF for the design.
+
+
+
+
C
C
−
−
.
≈
≈
≈
≈
3
3
3
3
O
O
O
O
Z
⋅
⋅
3
3
ω
ω
C
C
s
s
22
22
s
s
)
)
3
3
)
)
3
3
Z
Z
R
R
R
R
, between 0% and
=
=
O
O
⋅
⋅
2
2
2
2
=
=
1
1
1
1
C
C
C
C
2
2
=
=
=
=
⋅
⋅
. 0
. 0
/
/
/
/
⋅
⋅
10
10
R
R
O
O
O
O
⋅ π
⋅ π
12
12
12
12
ω
ω
ω
ω
V
V
ESR
ESR
V
V
45
45
,
,
,
,
−
−
1
1
FB
FB
n
n
2
2
n
n
O
O
2
2
6
6
C
C
80
80
pF
pF
pF
pF
)
)
)
)
⋅
⋅
nF
nF
O
O


 
 


1
1
3
3
,
,
0 .
0 .
3 .
3 .
⋅
⋅
10
10






Z
=
=
1
1
=6kHz
3
3
15
15
ω
ω
⋅
⋅
ω
ω
9 .
9 .
22
22
Z
Z
Z
Z
dB
dB
=
=
=
=
⋅
⋅
10
10
R
R
R
R
ESR
ESR
ESR
ESR
1
1
1
1
−
−
6
6
C
C
C
C
capacitor, the main power switch and the freewheeling
diode carry pulse current (Figure 9). For jitter-free
operation, the size of the loop formed by these components
should be minimized. Since the power switch is already
integrated within the SC4524A, connecting the anode of
the freewheeling diode close to the negative terminal of
the input bypass capacitor minimizes size of the switched
current loop. The input bypass capacitor should be placed
close to the IN pin. Shortening the traces of the SW and
BST nodes reduces the parasitic trace inductance at these
nodes. This not only reduces EMI but also decreases
switching voltage spikes at these nodes.
The exposed pad should be soldered to a large ground
plane as the ground copper acts as a heat sink for the
device. To ensure proper adhesion to the ground plane,
avoid using vias directly under the device.
⋅
⋅
O
O
O
O
1
1
3
3
,
,
,
,
0 .
0 .
3 .
3 .
Figure 9. Heavy lines indicate the critical pulse
V
V






IN
IN
=
=
15
15
9 .
9 .
current loop. The inductance of this
loop should be minimized.
dB
dB
Vin
Vin
+
+
Z
Z
L
L
V
V
OUT
OUT
4
C
C