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SC4525DEVB 数据表(PDF) 13 Page - Semtech Corporation

部件名 SC4525DEVB
功能描述  18V, 3A, 350kHz Step-Down Switching Regulator
PDF  21 Pages
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制造商  SEMTECH [Semtech Corporation]
网页  http://www.semtech.com
标志 SEMTECH - Semtech Corporation

SC4525DEVB 数据表(HTML) 13 Page - Semtech Corporation

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© 2011 Semtech Corp.
www.semtech.com
SC4525D
13
Applications Information (Cont.)
Loop Compensation
The goal of compensation is to shape the frequency
response of the converter so as to achieve high DC
accuracy and fast transient response while maintaining
loop stability.
The block diagram in Figure 7 shows the control loops of a
buck converter with the SC4525D. The inner loop (current
loop) consists of a current sensing resistor (R
s=3.53mW)
and a current amplifier (CA) with gain (G
CA=18.5). The
outer loop (voltage loop) consists of an error amplifier
(EA), a PWM modulator, and a LC filter.
Since the current loop is internally closed, the remaining
task for the loop compensation is to design the voltage
compensator (C
5, R7, and C8).
Figure 7. Block diagram of control loops
For a converter with switching frequency F
SW, output
inductance L
1, output capacitance CO and loading R, the
control (V
C) to output (VO) transfer function in Figure 7 is
given by:
(8)
This transfer function has a finite DC gain
an ESR zero F
Z at
12
12
Fig.7: Block diagram of control loop
+
-
Vo
L1
Co
Resr
COMP
EA
REF
Vc
SW
CONTROLLER AND SCHOTTKY DIODE
FB
PWM
MODULATOR
Vramp
CA
R4
R6
C5
R7
C8
Io
Rs
12
12
Fig.7: Block diagram of control loop
+
-
Vo
L1
Co
Resr
COMP
EA
REF
Vc
SW
CONTROLLER AND SCHOTTKY DIODE
FB
PWM
MODULATOR
Vramp
CA
R4
R6
C5
R7
C8
Io
Rs
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
=
a dominant low-frequency pole F
P at
and double poles at half the switching frequency.
Including the voltage divider (R
4 and R6), the control to
feedback transfer function is found and plotted in Figure
8 as the converter gain.
Since the converter gain has only one dominant pole at
low frequency, a simple Type-2 compensation network
is sufficient for voltage loop compensation. As shown in
Figure 8, the voltage compensator has a low frequency
integrator pole, a zero at F
Z1, and a high frequency pole
at F
P1. The integrator is used to boost the gain at low
frequency. The zero is introduced to compensate the
excessive phase lag at the loop gain crossover due to the
integrator pole (-90deg) and the dominant pole (-90deg).
The high frequency pole nulls the ESR zero and attenuates
high frequency noise.
Figure 8. Bode plots for voltage loop design
Therefore, the procedure of the voltage loop design for
the SC4525D can be summarized as:
(1) Plot the converter gain, i.e. control to feedback transfer
function.
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
=
Fig.8: Bode plot of loop gains
Fp
Fsw/2
Fz1
Fp1
Fc
Fz
0.2K
2K
20K
200K
2M
-60
-30
0
30
60
FREQUENCY (Hz)
CONVE
RTER
GAIN
LOOP
GAIN
COMP
ENSAT
OR GA
IN
Fig.8: Bode plot of loop gains
Fp
Fsw/2
Fz1
Fp1
Fc
Fz
0.2K
2K
20K
200K
2M
-60
-30
0
30
60
FREQUENCY (Hz)
CONVE
RTER
GAIN
LOOP
GAIN
COMP
ENSAT
OR GA
IN



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