SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441AEVB
(Rev.:2006)
(Rev.:2006)
Semtech Corporation
SC2441AEVB Datasheet PDF : 37 Pages
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SC2441A
POWER MANAGEMENT
Applications Information
Loop Compensation for the Step-Up Converter
Soft-Starting the Step-Down Controllers
A simple small signal model for current-mode boost
converter in continuous-conduction mode is shown in
Figure 10.
The soft-start of the two step-down converters are
independently controlled through SS1 pin and SS2 pin.
As lillustrated in Figure 4, if VCC is below 4.5V, Q1 will be
on, keeping CSS discharged. When FAULT goes low, Q1 is
turned off, C gets charged via R from V . Values of
SS
SS
CC
RSS 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 10µA current
sink. R must be large enough to allow the soft-start
SS
a
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
b
coincidently. The method is shown in Figure 11.
VCC
Figure 10. Small signal model of Boost converter.
In Figure 10, Co3 and Resr3 are the capacitance and the
ESR of the output capacitor, gm3 is the error amplifier
transconductance and k3 is the current loop gain. If one
specifies the loop crossover frequency fc, the
compensation component values are readily calculated
as
C4
=
h 3 gm3k
3
(1-
D3
)
Ro3
2
| 1-
fc
fz1
|
1
2pfc
R esr 3
,
Resr3 + 0.5Ro3
RSS1
D1
SS1/EN1 3
CSS1
SC2441A
CSS
VCC
RSS2
D2
SS2/EN2 17
CSS2
and
R4
=
1
C4
Co3 (Resr3
+
Ro3
2
)
C5
=
C4
2R esr 3
Ro3
fz1
=
(1- D3 )2 Ro3
2pL3
.
Figure 11. Coincident Soft-Start for Step-Down
Converters
The capacitance of C , as shown above, should be more
SS
than 3 times of the capacitance of the C and C .
SS1
SS2
h3
=
Rb
Ra + Rb
2006 Semtech Corp.
23
www.semtech.com
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