SC2446 Semtech Corporation, SC2446 Datasheet - Page 24

• Manufacturer Part Number: SC2446
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

If the converter crossover frequency is set around 1/10
of the switching frequency, f
parameters then can be calculated as
where,
(260 µ
If we use C
use R
With K = 1, it is further calculated that
use C
(magnitude and phase) is shown in Figure 18
POWER MANAGEMENT
Application Information (Cont.)
20 log G vc f ( ) C f ( )
2004 Semtech Corp.
⋅
(
3
2
= 10pF. The Bode plot of the loop transfer function
= 770k .
g
m is the error amplifier transconductance gain
1
).
−
2
69.241
20.73
= 0.33 nF,
)
100
50
50
C
C
0
R
10
10
3
2
2
g
R
R
m
2
oesr
C
R
hkR
o
100
C
2
2
f
C
c
o
o
o
K
848
c
1 . 10
. 0
= 30kHz, the controller
10
328
3
.
k 5
2 .
f
nF
pF
1 . 10
,
.
,
4
1 . 10
5
3 10
×
1 . 10
5
6
24
Figure 18. The loop transfer function Bode plot of the
example.
It is clear that the resulted crossover frequency is about
27.1 kHz with phase margin 91
It is noted that the current sensing gain k was first
estimated using the DC value in order to quickly get the
compensation parameter value. When the circuit is
operational and stable, one can further improve the
compensation parameter value using AC current sensing
gain. One simple and practical method is to effectively
measure the output current at two points, e.g. I
and the corresponding error amplifier output voltage V
and V
With this k value, one can further calculate the improved
compensator parameter value using the previous
equations.
For example, if one measured that I
V
Substituting this parameter to the equations before, one
can derive that
C
R
C
c1
2
3
2
arg G vc f ( ) C f ( )
=2.139V, V
(
61.78pF. Select C
2.024nF. Select C
127.3k . Select R
c2
. Then, the first order AC gain is
⋅
−
−
92.702
)
88.78
⋅
180
π
c2
=2.457V. k is then calculated as 44.
88
89
90
91
92
93
10
10
k
3
2
V
2
I
= 47pF
= 2.2nF.
100
o
= 127k .
c
V
I
o
c
1 . 10
1
1
o
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.
3
I
V
o
2
f
c
f
2
o1
1 . 10
=1A, I
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4
SC2446
o2
1 . 10
=15A and
o1
5
and I
3 10
×
1 . 10
5
o2
c1
6