sc4508a Semtech Corporation, sc4508a Datasheet - Page 15

• Manufacturer Part Number: sc4508a
• Description: Sc4508a Buck Or Buck-boost Inverting Current-mode Controller
• Manufacturer: Semtech Corporation

assumptions, the controller parameters are determined
as follows:
and
For example, if V
R
and
If the converter crossover frequency is set around 1/10
of the switching frequency, f
parameters then can be calculated as:
where,
(100 µ
If we use C
use R
It is further calculated that:
use C
function (magnitude and phase) is shown in Figure 8.
Application Information (Cont.)
POWER MANAGEMENT
oesr
2007 Semtech Corp.
=10m , R
2
3
= 7.5k .
= 120pF. The Bode plot of the loop transfer
g
m is the error amplifier transconductance gain
1
).
2
= 22 nF,
s
=35m
C
C
o
2
=3.3V, I
3
R
R
k
h
2
o
C
g
C
R
2
R
m
3
2
8
0
R
kR
oesr
V
2
one can calculate that::
R
V
C
5 .
I
f
o
1
o
o
R
c
C
o
2
g
o
2
R
C
o
=2A, f
2
S
o
m
R
oesr
h
o
kR
C
R
o
. 0
f
. 1
C
c
c
2
2
. 7
= 30kHz, the controller
C
o
. 3
o
152
65
23
134
k 5
h
o
57
s
=300kHz, C
6 .
pF
nF
o
=100uF,
15
Figure 8. The loop transfer function Bode plot of the
buck example.
It is clear that the resulted crossover frequency is about
30kHz with phase margin 91
For buck-boost converter, a simplified equivalent circuit
model of the buck converter with current mode control
is shown in Figure 9 and the transfer function from the
voltage error amplifier output (v
(v
o
20 log T f ( )
0
180
) is:
π
⋅
V
V
arg T f ( )
o
c
(
(
(
) s
) s
(
−
−
:
−
100
85
95
40
G
)
)
16.67
vc
76.67
53.33
85
87
89
91
93
95
6.67
100
(
40
30
) s
1
1
1
1
k
1
1
10
10
D
D
100 1 . 10
R
o
.
100 1 . 10
o
c
) to the converter output
1 (
f
f
s
3
zRHP
3
1
SC4508A
s
1 . 10
1 . 10
www.semtech.com
s
)(
s
4
p
1
4
1
1 . 10
1 . 10
s
10 10
s
1 z
10 10
5
5
×
1 . 10
)
1 . 10
⋅
5
5
6
6