sc2441a Semtech Corporation, sc2441a Datasheet - Page 21

• Manufacturer Part Number: sc2441a
• Description: Sc2441a 1.8v To 20v Input 2-phase Synchronous Step-down Controllers With Step-up Converter
• Manufacturer: Semtech Corporation

switching frequency and very short duty cycle is not
practical. If the input voltage is 3.3V and the operating
frequency is 1MHz, the lowest output voltage will be 0.6V.
There will not be enough modulation headroom if the on
time is simply made equal to the minimum on time of the
SC2441A. For ease of control, we recommend that the
required pulse width be at least 1.5 times the minimum
on time.
Maximum Duty-cycle Consideration
The top MOSFET turns off for at least 200ns every cycle
regardless of the switching frequency. This places an
upper bound on the voltage conversion ratio at a given
switching frequency.
If the desired output voltage requires high operating duty-
cycle, then operating frequency will have to be lowered
to allow modulating headroom.
RC Filtering network for V
A RC filtering network is recommended for the SC2441A
V
C
and V
from 3.3
1
C
and V
to the pins of the SC2441A to achieve the best
decoupling performance. Due to the different
functionalities of the V
placed between the V
SC2441A. And C
and the power ground of the SC2441A. The
recommended connections for the V
illustrated in Figure 8.
POWER MANAGEMENT
Applications Information
CC
µF
8
8
2006 Semtech Corp.
and R
and C
and V
.
IN
IN
pin respectively. The value of the R
pin. They should be placed as close as possible
IN
2
2
are the decoupling capacitors for the V
pin connections. As shown in Figure 1, R
plus C
to 5.11
8
2
should be placed between the V
are the filtering networks for V
IN
C
CC
pin and the signal ground of the
8
pin and V
and C
CC
and V
2
should be larger than
IN
CC
IN
pins
pin and V
pin, C
6
and R
2
should be
IN
2
pin are
ranges
6
CC
CC
CC
plus
pin
pin
pin
21
Step-Up Converter
The SC2441A features a step-up regulator and two step-
down controllers.
The boost section of the SC2441A comprises of pins 7,
8, 26 and 27. Pin 26 is the independent power ground
for the boost converter section, which should be
separated from the step-down section power ground pin
24 in layout to minimize the noise influence. The boost
section in SC2441A has an internal reference set at
1.25V. The output of the boost section can be
programmed with external resistors R
in Figure 1.
SC2441A utilizes a transconductance error amplifier for
the step-up controller and it can be compensated with
C
in the SC2441A employs cycle-by-cycle peak current limit
to protect the internal switching transistor. Current limit
threshold is typically 0.8A.
In the applications where only low input voltage is
available, the step-up converter in the SC2441A is very
useful for generating an auxiliary output to power the
gate drive of the step-down controllers.
Figure 8. RC Network Connections for V
3
, R
5
and C
5
24
as shown in Figure 1. The step-up controller
9
V
GND
PGND1
BOOST
SC2441A
C
=
C
8
2
1.25V
V
V
CC
IN
´
1
21
R
1
R
+
4
R
1
R
R
SC2441A
4
2
6
and R
IN
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and V
4
as shown
CC
pins