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SC2441AEVB Fiches technique(PDF) 23 Page - Semtech Corporation

No de pièce SC2441AEVB
Description  1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter
PDF  37 Pages
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Fabricant  SEMTECH [Semtech Corporation]
Site Internet  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2441AEVB Fiches technique(HTML) 23 Page - Semtech Corporation

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 2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2441A
Loop Compensation for the Step-Up Converter
A simple small signal model for current-mode boost
converter in continuous-conduction mode is shown in
Figure 10.
Figure 10. Small signal model of Boost converter.
In Figure 10, C
o3 and Resr3 are the capacitance and the
ESR of the output capacitor, g
m3 is the error amplifier
transconductance and k
3 is the current loop gain. If one
specifies the loop crossover frequency f
c,
the
compensation component values are readily calculated
as
,
R
5
.
0
R
R
f
2
1
|
f
f
1
|
2
R
)
D
1
(
k
g
h
C
3
o
3
esr
3
esr
c
1
z
c
3
o
3
3
3
m
3
4
+
p
-
-
=
)
2
R
R
(
C
C
1
R
3
o
3
esr
3
o
4
4
+
=
and
3
o
3
esr
4
5
R
R
2
C
C
=
.
L
2
R
)
D
1
(
f
3
3
o
2
3
1
z
p
-
=
b
a
b
3
R
R
R
h
+
=
Soft-Starting the Step-Down Controllers
The soft-start of the two step-down converters are
independently controlled through SS1 pin and SS2 pin.
As lillustrated in Figure 4, if V
CC is below 4.5V, Q1 will be
on, keeping C
SS discharged. When FAULT goes low, Q1 is
turned off, C
SS gets charged via RSS from VCC. Values of
R
SS and CSS set different start-up times.
As shown in Figure 4, if the output falls below 70% of its
setpoint, the Css will be discharged with a 10
µA current
sink. R
SS must be large enough to allow the soft-start
capacitor to be discharged below 0.5V. Soft-start process
should be long enough to allow the output to reach 70%
of its final value before hiccup is armed.
Coincident Soft-Start
The step-down controllers can be made to start
coincidently. The method is shown in Figure 11.
SS1/EN1
SC2441A
3
SS2/EN2
17
R
SS1
C
SS1
R
SS2
C
SS2
V
CC
V
CC
D
1
D
2
C
SS
Figure 11. Coincident Soft-Start for Step-Down
Converters
The capacitance of C
SS
, as shown above, should be more
than 3 times of the capacitance of the C
SS1
and C
SS2
.
Applications Information
b
a



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