sc4508a Semtech Corporation, sc4508a Datasheet - Page 16

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

where, the single dominant pole is:
Figure 9. A simple model of buck-boost converter with
current mode control.
and the zero due to the output capacitor ESR is:
and the RHP zero associated to the topoly is:
The dominant pole moves as input voltage and output
load varies.
The controller transfer function (from the converter
output (v
where
and
Application Information (Cont.)
POWER MANAGEMENT
2007 Semtech Corp.
o
) to the voltage error amplifier output (v
G
c
(
s
) s
zRHP
s
s
s
z
( s
1
p
z
1
2
C
1 (
2
R
g
R
1
R
m
oesr
o
D
2
D
C
1
1
C
C
D
C
)
3
o
2
2
L
)
o
1
1
R
o
s
s
s
s
p
z
2
2
c
) is:
16
The loop transfer function is then
To simplify design, we assume that C
With these assumptions, the controller zero is placed at
the converter dominant pole, the controller second pole
is placed at the converter ESR zero or RHP zero
depending on whichever is lower. The DC gain is finally
adjusted for desired phase margin. The controller
parameters are determined as following:
Assuming a DC gain
and
For example, if V
D=0.51, C
calculate that::
Set
calculated as:
where,
(100 µ
If we use C
ω
I
= 500, the controller parameters then can be
g
m is the error amplifier transconductance gain
1
).
o
2
=100uF, R
C
= 390 nF,
3
R
h
in
C
2
=12V, V
s
2
1
R
ω
p
R
s
2
o
V
T(s)=G
oesr
z
I
2
,
1
C
o
g
0
m
R
2
=35m , R
I
V
5 .
or
I
o
2
0
C
h
o
C
o
5 .
2
=-12V, I
g
C
vc
C
2
1
1
m
(s)G
400
2
I
12
s
3
C
p
C
. 0
1
3
3
C
nF
04
R
(s)h
2
s
o
=35m
SC4508A
=1A, f
1
s
3
<<C
zRHP
www.semtech.com
2
, R
s
=300kHz,
one can
oesr
<<R
o,