SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Application Information (Cont.)
If the converter crossover frequency is set around 1/10
of the switching frequency, fc = 30kHz, the controller
parameters then can be calculated as
C2
=
gmhkRo
2πfc
≈ 0.328nF.
where, gm is the error amplifier transconductance gain
(260 µΩ−1).
If we use C2 = 0.33 nF,
R2
=
RoCo
C2
≈ 848.5kΩ,
− 88.78 88
89
90
( ) arg G vc(f)⋅C(f)
180
⋅
π
91
92
− 92.702 93
10
10
f
100
1 .103
1 .104
1 .105
1 .106
f
3×105
use R2 = 770kΩ.
With K = 1, it is further calculated that
C3
=
Roesr Co
R2
K
≈ 10.2pF,
use C3 = 10pF. The Bode plot of the loop transfer function
(magnitude and phase) is shown in Figure 18
69.241100
50
( ) 20⋅log G vc(f) C(f)
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 91o.
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. Io1 and Io2
and the corresponding error amplifier output voltage Vc1
and Vc2. Then, the first order AC gain is
0
k=
∆Io
∆Vc
=
Io1 − Io2
Vc1 − Vc2
− 20.73 50
10
10
100 1.103 1.104 1.105 1.106 With this k value, one can further calculate the improved
f
3×105 compensator parameter value using the previous
equations.
For example, if one measured that Io1=1A, Io2=15A and
Vc1=2.139V, Vc2=2.457V. k is then calculated as 44.
Substituting this parameter to the equations before, one
can derive that
C2 ≈ 2.024nF. Select C2 = 2.2nF.
R2 ≈ 127.3kΩ. Select R2 = 127kΩ.
C3 ≈ 61.78pF. Select C3 = 47pF
2004 Semtech Corp.
24
www.semtech.com
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