SC4525A Semtech Corporation, SC4525A Datasheet - Page 14

• Manufacturer Part Number: SC4525A
• Description: 28V 3A Step-Down Switching Regulator
• Manufacturer: Semtech Corporation

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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/3A converter with 47uF 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 5. A MathCAD program is also available
upon request for detailed calculation of the compensator
parameters.
Z
R
R
C
C
C
C
G
G
C
C
C
C
A
A
A
A
R
R
V
V
V
V
.
o
o
c
c
PWM
PWM
7
7
C
C
C
C
7
7
5
5
8
8
5
5
8
8
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
10
10
1 (
1 (
−
−
−
−
. 0
. 0
2
2
2
2
2
2
2
2
≈
≈
g
g
R
R
R
R
C
C
C
C
G
G
C =
C =
C
C
C
C
G
G
C
C
C
C
A
A
A
A
R
R
⋅ π
⋅ π
⋅ π
⋅ π
V
V
π
π
π
π
V
V
V
V
20
20
20
20
V
V
28
28
+
+
G
G
m
m
A
A
20
20
F
F
F
F
10
10
7
o
o
PWM
PWM
7
7
o
o
c
c
PWM
PWM
7
7
c
c
5
5
8
8
5
5
8
8
m
C
C
C
C
7
7
5
5
8
8
C
C
5
5
CA
CA
1
1
16
16
1
1
600
600
s
s
=3.4k, C
Z
Z
P
P
=0.28mA/V is the EA gain of the SC4525A.
⋅
⋅
⋅
⋅
R
R
1
1
1
1
=
=
⋅
⋅
=
=
=
=
=
=
=
=
/
/
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
log
log
log
log
10
10
⋅
⋅
19
19
20
20
R
R
R
R
G
G
R
R
C
ω
ω
⋅
⋅
10
10
7
7
10
10
7
7
10
10
), and F
PWM
PWM
1 (
1 (
1 (
1 (
2
2
2
2
S
S
2
2
2
2
2
2
2
2
−
−
−
−
. 0
. 0
⋅
⋅
p
p
≈
≈
≈
≈


 
 








−
−
10
10
g
g
)
)
g
g
,
,
3
3
⋅ π
⋅ π
⋅ π
⋅ π
G
G
28
28
1 (
1 (
π
π
20
20
20
20
⋅ π
⋅ π
⋅ π
⋅ π
π
π
1
1
28
28
3
3
G
G
+
+
G
G
m
m
+
+
m
m
1 (
1 (
CA
CA
=
=
1
1
A
A
A
A
20
20
F
F
F
F
3
3
20
20
1
1
10
10
⋅
⋅
+
+
C
C
C
C
CA
CA
1
1
CA
CA
1
1
1
1
⋅
⋅
31
31
16
16
16
16
600
600
R
R
600
600
s
s
s
s
Z
Z
P
P
31
31
⋅
⋅
+
+
4
4
s
s
1
1
1
1
⋅
⋅
⋅
⋅
31
31
R
R
R
R
S
S
⋅
⋅
/
/
1 .
1 .
/
/
s
s
/
/
log
log
log
log
5
⋅
⋅
10
10
4 .
4 .
⋅
⋅
1
1
.
.
19
19
20
20
R
R
R
R
R
R
⋅
⋅
k 8
k 8
=0.33nF, and C
G
G
G
G
R
R
ω
ω
ω
ω
R
R
ω
ω
⋅
⋅
⋅
⋅
4 .
4 .
2
2
⋅
⋅
ESR
ESR
⋅
⋅
10
10
10
10
7
7
7
7
n
n
10
10
PWM
PWM
PWM
PWM
ω
ω
S
S
S
S
P
p
p
p
p
π
π
10
10
⋅
⋅
⋅
⋅


 
 


Q
Q






⋅
⋅
−
−
10
10
10
10
F
F
10
10
p
p
)
)
)
)
C
C
,
,
1
1
,
,
=600kHz. From Equation (9), the
3
3
G
G
28
28
+
+
1 (
1 (
1 (
1 (
C
C
−
−
≈
≈
3
3
3
3
O
O
3
3
3
3
1
1
1
1
C
C
s
s
1 (
1 (
1 (
1 (
3
3
)
)
1
1
CA
CA
=
=
1
1
=
=
R
R
O
O
3
3
3
3
⋅
⋅
⋅
⋅
1
1
⋅
⋅
2
2
+
+
+
+
1
1
C
C
2
2
=
=
⋅
⋅
22
22
. 0
. 0
31
31
R
R
31
31
/
/
⋅
⋅
⋅
⋅
⋅
⋅
+
+
+
+
⋅ π
⋅ π
4
4
O
O
s
s
s
s
8
8
ω
ω
V
V
V
V
31
31
22
22
31
31
S
S
,
,
FB
FB
1 .
1 .
5 .
5 .
s
s
O
O
2
2
n
n
s
s
/
/
/
/
4 .
4 .
1 .
1 .
1
1
80
80
.
.
)
)
R
R
⋅
⋅
R
R
k 8
k 8
nF
nF
pF
pF
ω
ω
ω
ω


 
 


4 .
4 .
1 .
1 .
2
2
⋅
⋅
ESR
ESR
ESR
ESR
⋅
⋅
⋅
⋅
⋅
⋅
n
n
n
n
10
10
C
ω
ω
ω
ω
10
10
10
10
π
π
10
10
P
Q
Q
Q
Q
⋅
⋅
⋅
⋅
is
p
p
F
F
p
p
10
10
10
10
8
C
C
C
C
C
, to cancel the ESR zero,
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
47
47
s
s
)
)
)
)
3
3
3
3
Z
Z
, between 0% and
R
R
R
R
=
=
=
=
O
O
⋅
⋅
2
2
2
2
P
P
P
P
P
P
P
P
P
P
P
P
=
=
1
1
1
1
C
C
C
C
2
2
=
=
=
=
⋅
⋅
. 0
. 0
C
C
. 0
. 0
BST
BST
D
D
IND
IND
/
/
TOTAL
TOTAL
SW
SW
/
/
⋅
⋅
10
10
R
R
O
O
O
O
⋅ π
⋅ π
12
12
8
8
ω
ω
ω
ω
V
V
ESR
ESR
V
V
=
=
=
=
31
31
45
45
1
1
,
,
,
,
FB
FB
−
−
5 .
5 .
n
n
n
n
O
O
=
=
2
2
2
2
=
=
6
6
=
=
D
D
1 (
1 (
C
C
80
80
pF
pF
)
)
)
)
nF
nF
⋅
⋅
nF
nF
pF
pF
O
O
=
=
1
1
1 (
1 (


 
 


2
2
D
D
⋅
⋅
1
1
3
3
,
,
−
−
V
V
P
P
0 .
0 .
3 .
3 .
⋅
⋅
1 .
1 .
⋅
⋅
⋅
⋅
10
10
CESAT
CESAT
) D
) D
C
C
t
t
V
V






S
S
Z
BST
BST
~
~
=
=
+
+
1
1
⋅
⋅
=6kHz
3
3
19
19
⋅
⋅
. 1
. 1
P
P
V
V
V
V
ω
ω
⋅
⋅
ω
ω
⋅
⋅
⋅
⋅
D
D
SW
SW
dB
dB
IN
IN
) 3
) 3
47
47
I
I
40
40
Z
Z
O
O
Z
Z
I
I
⋅
⋅
O
O
⋅
⋅
I
I
⋅
⋅
=
=
=
=
+
+
I
I
O
O
I
I
⋅
⋅
O
O
2
2
O
O
10
10
P
P
R
R
R
R
⋅
⋅
BST
BST
⋅
⋅
ESR
ESR
F
F
ESR
ESR
R
R
1
1
1
1
SW
SW
−
−
DC
DC
6
6
C
C
C
C
Thermal Considerations
For the power transistor inside the SC4525A, the
conduction loss P
circuit loss P
where V
switching time of the NPN transistor (see Table 4).
+
+
In addition, the quiescent current loss is
The total power loss of the SC4525A is therefore
The temperature rise of the SC4525A is the product of the
total power dissipation (Equation (2)) and q
which is the thermal impedance from junction to ambient
for the SOIC-8 EDP package.
It is not recommended to operate the SC4525A above
25
input voltage and high output current, the switching
frequency may need to be reduced to meet the thermal
requirement.
⋅
⋅
O
O
O
O
P
P
1
1
3
3
,
,
,
,
Q
Q
0 .
0 .
3 .
3 .
o
C junction temperature. In the applications with high






P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
Table 4. Typical switching time
=
=
TOTAL
TOTAL
C
C
SW
SW
BST
BST
D
D
IND
IND
Q
Q
TOTAL
TOTAL
C
C
SW
SW
BST
BST
D
D
IND
IND
BST
19
19
=
=
=
=
=
=
=
=
=
=
Input Voltage
Input Voltage
is the BST supply voltage and t
=
=
=
=
=
=
=
=
=
=
=
=
D
D
1 (
1 (
D
D
1 (
1 (
V
V
dB
dB
BST,
=
=
1
1
1 (
1 (
=
=
1 (
1 (
2
2
D
D
IN
IN
1
1
2
2
D
D
⋅
⋅
⋅
⋅
−
−
12V
12V
24V
24V
28V
28V
−
−
V
V
V
V
P
P
P
P
1 .
1 .
1 .
1 .
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
can be estimated as follows:
CESAT
CESAT
) D
) D
CESAT
CESAT
C
C
C
C
) D
) D
2
2
t
t
V
V
t
t
V
V
S
S
S
S
BST
BST
mA
mA
BST
BST
~
~
~
~
+
+
+
+
C
⋅
⋅
⋅
⋅
, the switching loss P
⋅
⋅
⋅
⋅
. 1
. 1
. 1
. 1
P
P
V
V
P
P
V
V
V
V
V
V
⋅
⋅
⋅
⋅
D
D
⋅
⋅
⋅
⋅
D
D
SW
SW
SW
SW
IN
IN
) 3
) 3
IN
IN
) 3
) 3
I
I
I
I
40
40
40
40
O
O
O
O
I
I
I
I
⋅
⋅
⋅
⋅
O
O
O
O
⋅
⋅
⋅
⋅
I
I
I
I
⋅
⋅
⋅
⋅
+
+
+
+
O
O
O
O
I
I
I
I
I
I
I
I
O
O
O
O
2
2
O
O
2
2
O
O
P
P
12.5ns
12.5ns
25.3ns
25.3ns
P
P
⋅
⋅
22ns
22ns
⋅
⋅
BST
BST
BST
BST
⋅
⋅
⋅
⋅
1A
1A
F
F
F
F
R
R
R
R
SW
SW
SW
SW
DC
DC
DC
DC
+
+
+
+
Load Current
Load Current
P
P
P
P
Q
Q
Q
Q
15.3ns
15.3ns
25ns
25ns
28ns
28ns
2A
2A
P
P
P
P
Q
Q
Q
Q
SW
=
=
=
=
S
is the equivalent
, and bootstrap
SC4525A
V
V
V
V
IN
IN
18ns
18ns
28ns
28ns
31ns
31ns
IN
IN
3A
3A
⋅
⋅
⋅
⋅
JA
2
2
2
2
mA
mA
(0)
()
mA
mA
(2)
(36
o
C/W),
4