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SC2441AEVB Datasheet(PDF) 33 Page - Semtech Corporation

Part # SC2441AEVB
Description  1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter
PDF  37 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2441AEVB Datasheet(HTML) 33 Page - Semtech Corporation

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 2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2441A
Applications Information
The inner current-loop is unstable (sub-harmonic
oscillation) unless the inductor current up-slope is steeper
than the inductor current down-slope. For stable
operation above 50% duty-cycle, a compensation ramp
is added to the sensed-current. In the SC2441A the
compensation ramp is made duty-ratio dependent. The
compensation ramp is approximately
D
0.734
SLOPE
e
D
230mV
(D)
V
×
×
×
=
D is the duty ratio.
The slope compensation voltage vs duty ratio is as shown
in Figure 22.
Figure 22. Slope Compensation Voltage Waveform
Illustrated as the picture above, as the duty ratio
increases, the slope compensation voltage added into
the control loop increases too. And the control loop
including the slope compensation is shown in Figure 23.
The voltage transconductance error amplifier (shown in
Figure 24) has a g
m of 315µA/V. C
2
, C
3
and R
2
construct
the compensation network for stable operation with
optimized load transient response.
The feedback gain h and the resistor values are
determined using the equations given in the “Setting the
Output Voltage” section with
.
V
5
.
0
h
o
=
Slope Compensation
Voltage
CS+
CS-
+
-
++
x 29
-
+
0.5V
FB
V
BE+1.13V
+ -
Compensation
Network
Comp
+
-
PWM
EA
Figure 23. Control Flow Chart with Slope
Compensation
k
Figure 24. A simple model of current-mode buck
converter
For the rated output current I
o, the first-order gain k is
determined as
.
V
I
k
c
o
D
D
=
k is the product of equivalent current sensing Rs and
current amplifier gain Gca=29. Furthermore the transfer
0.0
0.1
0.2
0.3
0.4
0.5
0.00.2
0.40.6
0.81.0
Duty Ratio (D)



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