18 2006 Semtech Corp. www.semtech.com
SC4808B-1
POWER MANAGEMENT
SOFT START
During start up of the converter, the discharged output ca-
pacitor and the load current have large supply current re-
quirements. To avoid this a soft start scheme is usually
implemented where the duty cycle of the regulator is gradu-
ally increased from 0% until the soft start duration is
elapsed.
SC4808B-1 has an internal soft start circuit that limits the
duty cycle for a duration approximated by the formula be-
low. Also the soft start circuitry is activated if an over cur-
rent condition occurs. After an over current condition, OUTA
and OUTB are disabled and kept low for a duration of about
140µs. After the delay, the OUTA and OUTB are enabled
while the soft start limits the duty cycle. If the over current
condition persists, the soft start cycle repeats indefinitely.
Approximate internal soft start duration can be calculated
as below:
()
+
×
≅
1
R
R
2
VREF
Ramp
T
GND_to_CS
SlopeComp_Internal
SoftStart
SoftStart
If longer soft start durations are required, the simple exter-
nal circuit shown below can be implemented.
R27
15k
200p
C31
VCC
R28
10
2.2u,16V
C26
REF
0.1u,25V
C33
R33
10k
SYNC
U4
SC4808
4
5
3
2
101
6
7
8
9
FB
REF
CS
RC
LUVLOSYNC
GND
OUTB
OUTA
VCC
R23
56.2k
Vin
56.2K
Csoft start
REF
MOCD207
6
5
C40
NA
R37
1k
Approximate soft start duration can be calculated as be-
low:
37RCT
SoftStartSoftStart
×≅
Application Information (Cont.)
START UP SEQUENCE
Initially during the power up, the SC4808B-1 is in under
voltage lock out condition. As the Vcc supply exceeds the
UVLO limit of the SC4808B-1, the internal reference, oscil-
lator, and logic circuitry are powered up.
The OUTA and OUTB drivers are not enabled until the line
under voltage lock out limit is reached. At that point, once
the FB pin is above 1.5V, soft start circuitry starts the out-
put drivers, and gradually increases the duty cycle from
0%. The soft start duration is internally set (see formula in
Soft Start section).
As the output voltage starts to increase, the error signal
from the error amplifier starts to decrease. If isolation is
required, the error amplifier output can drive the LED of
the opto isolator. The output of the opto is connected in a
common emitter configuration with a pull-up resistor to a
reference voltage connected to the FB pin of the SC4808B-
1. The voltage level at the FB pin provides the duty cycle
necessary to achieve regulation.
If an over current condition occurs, the outputs are dis-
abled and after a soft start delay time of about 100µs, the
softstart sequence mentioned above is repeated.
