參數(shù)資料
型號(hào)： SC4808AIMSTR
廠商： Semtech Corporation
英文描述： High Performance Dual Ended PWM Controller
中文描述： 高性能雙端PWM控制器
文件頁(yè)數(shù)： 21/31頁(yè)
文件大?。?/td> 483K
代理商： SC4808AIMSTR
21
2005 Semtech Corp.
www.semtech.com
SC4808A/B
POWER MANAGEMENT
Application Information (Cont.)
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.
SC4808 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
VCC
R28
10
2.2u,16V
C26
REF
0.1u,25V
C33
R33
10k
SYNC
U4
SC4808
4
5
3
2
10
1
6
7
8
9
FB
REF
CS
RC
LUVLO
SYNC
GND
OUTB
OUTA
VCC
R23
56.2k
Vin
56.2K
Csoft start
REF
MOCD207
6
5
NA
R37
1k
Approximate soft start duration can be calculated as be-
low:
37
R
C
T
SoftStart
SoftStart
×
START UP SEQUENCE
Initially during the power up, the SC4808 is in under volt-
age lock out condition. As the Vcc supply exceeds the UVLO
limit of the SC4808, the internal reference, oscillator, 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 SC4808.
The voltage level at the FB pin provides the duty cycle nec-
essary 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.
相關(guān)PDF資料
PDF描述
SC4808AIMSTRT High Performance Dual Ended PWM Controller
SC4808B High Performance Dual Ended PWM Controller
SC4808BIMSTR High Performance Dual Ended PWM Controller
SC4808BIMSTRT High Performance Dual Ended PWM Controller
SC4808B-1 High Performance Dual Ended PWM Controller
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
SC4808AIMSTRT 制造商:Semtech Corporation 功能描述:
SC4808B 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Dual Ended PWM Controller
SC4808B-1 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Dual Ended PWM Controller
SC4808B-1MSTRT 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Dual Ended PWM Controller
SC4808B-2 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Dual Ended PWM Controller
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