sc2441a Semtech Corporation, sc2441a Datasheet - Page 25

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

With an arbitrary selection of
get C
value, we use 100nF capacitor for C
Pre-biased Start Up
Sometimes the step-down converter is to start into a
pre-biased output load. The pre-biased voltage is normally
lower than the output setpoint of the step-down
converter.
As described earlier, pre-bias startup process with the
SC2441A is seamless. The testing setup of the pre-biased
start-up is shown as in Figure 14.
R
POWER MANAGEMENT
Applications Information
R
1
2
2
2006 Semtech Corp.
µ
+
×
A
R
1
R
Figure 14. Test Setup for Pre-biased Start Up
1
Module
=83nF. Since 83nF is not a standard capacitance
3
×
µ
under
3
Test
(
A
R
=
R
×
2
2.5K
3
2.5
+
×
R
R
K
2
W
3
W
+
)
-
. And we can also derive:
R
+
V
R
R
+
O
LOAD
2
2
R
R
(
+
1
2
×
R
R
R
R
R
R
×
2
2
3
3
3
3
2.5K
+
= 6.80K
= 3.92K
×
Blocking Diode
×
R
R
C
1
1
3
W
D1
=
)
R
R
æ
ç
è
0.25ms
×
2
2
28A
æ
ç
è
W
W
+
28A
×
R
R
3
3
+
External
Voltage
Source
=
+
5A
1
2
. Consequently,
3.01K
5A
2
ö
÷
ø
=
ö
÷
ø
25mV
W
=
, we can
25mV
V
s
25
In Figure 14, V
V
from V
module under test. Before power-up the module, monitor
V
voltage. Then power-up the module. V
smoothly.
Free-running Operation
The internal oscillator of the SC2441A can either free-
run or it can be phase-locked to an external clock.
In free-running mode, the internal phase-locked loop is
disabled by tying an external resistor from the PLLF pin
to V
programs the channel frequency. The PLLF pull-up resistor
should be carefully selected so that the voltage at the
PLLF pin is above 1V. A value between 20K to 50K
recommended.
Pull-up resistor can also be tied to V
before the SC2441A starts to switch. The advantage tying
the pull-up resistor to V
regulated output from either a boost converter or a sepic
converter. The resistor from the PLLF pin can be tied to
V
that the V
output of the boost converter reaches the setpoint.
However, in some applications, a SEPIC converter is
employed to get stable V
range. In this case, the resistor from the PLLF pin should
not be connected to the V
blocking capacitor in the converter. The SC2441A will not
switch if the PLLF pin is at zero volt.
Applying more than 2.1V at the PLLF pin activates the
diode clamp circuit (see Figure 5(a)). The filtering
components (R
needed while free-running. The clamp activation will have
no effect on the PLL if V
The internal clock is brought out to the CKOUT pin. The
signal at CKOUT pin can be used as the synchronizing
clock for other SC2441As in a master-slave configuration.
O
O
CC
. D
to ensure that it is the desired pre-biased output
if V
IN
1
. The external resistor ROSC (see figure 5(a))
blocks the output of the power module under test
S
CC
during soft-start. R
is from a boost converter output. The reason is
CC
will be powered up from the input V
S
1
is the external power supply pre-biasing
,R
2
, C
1
and C
PLLF
CC
CC
CC
LOAD
due to the wide input voltage
is because that the V
>1V.
due to the presence of a DC
is the resistive load of the
2
in Figure 5(a)) are not
CC
SC2441A
if V
www.semtech.com
O
CC
should rise
is present
IN
before
CC
is a
is