SC2446A Semtech Corporation, SC2446A Datasheet - Page 19

• Manufacturer Part Number: SC2446A
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

use R
With K = 1, it is further calculated that
use C
(magnitude and phase) is shown in Figure 13
Figure 13. The loop transfer function Bode plot of the
example.
POWER MANAGEMENT
Application Information (Cont.)
20 log G vc f ( ) C f ( )
arg G vc f ( ) C f ( )
2005 Semtech Corp.
⋅
(
(
3
2
= 10pF. The Bode plot of the loop transfer function
= 770k .
⋅
−
−
−
69.241
92.702
)
20.73
88.78
⋅
180
π
)
100
50
88
89
90
91
92
93
50
C
0
10
10
10
10
3
R
oesr
R
100
100
2
C
o
K
1 . 10
1 . 10
10
3
3
2 .
f
f
f
pF
1 . 10
1 . 10
,
4
4
1 . 10
1 . 10
5
5
3 10
3 10
×
1 . 10
×
1 . 10
5
5
6
6
19
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
In some initial prototypes, if the circuit noise makes the
control loop jittering, it is suggested to use a bigger C
value than the calculated one here. Effectively, the
converter bandwidth is reduced in order to reject some
high frequency noises. In the final working circuit, the
loop transfer function should be measured using network
analyzer and compared with the design to ensure circuit
stability under different line and load conditions. The load
transient response behavior is further tested and
measured to meet the specification.
c1
2
3
2
=2.139V, V
61.78pF. Select C
2.024nF. Select C
127.3k . Select R
c2
. Then, the first order AC gain is
c2
=2.457V. k is then calculated as 44.
k
3
2
V
2
I
= 47pF
= 2.2nF.
o
= 127k .
c
V
I
o
c
1
1
o
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.
I
V
o
2
c
2
o1
SC2446A
=1A, I
www.semtech.com
o2
=15A and
o1
and I
o2
c1
3