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參數(shù)資料
| 型號： | SI9123 |
| 廠商： | Vishay Intertechnology,Inc. |
| 元件分類： | DC/DC變換器 |
| 英文描述： | 500-kHz Half-Bridge DC-DC Converter With Integrated Secondary Synchronous Rectification Control |
| 中文描述： | 500千赫半橋型DC - DC與集成的次級同步整流控制轉(zhuǎn)換器 |
| 文件頁數(shù)： | 9/16頁 |
| 文件大?。?/td> | 181K |
| 代理商： | SI9123 |
Si9123
Vishay Siliconix
New Product
Document Number: 72098
S-03639—Rev. B, 20-Mar-03
www.vishay.com
9
The pre-regulator will remain on until V
CC
equals V
REG
but
between V
UVLO
and V
REG
, excessive current may result in
V
CC
falling below V
UVLO
and stopping soft-start operation. This
situation is avoided by the hysteresis between V
REG
and
V
UVLO
and correct sizing of the V
CC
capacitor, bootstrap
capacitor, the soft-start capacitor, the primary MOSFET gate
driving charge, and load on the SEC_SYNC output. The value
of the V
CC
capacitor should be chosen to be capable of
maintaining soft-start operation with V
CC
above V
UVLO
until the
V
CC
current can be supplied from the external circuit (e.g., via
an auxiliary winding on the secondary inductor).
C
4.7 F
GND
V
CC
V
REF
V
IN
V
INEXT
Figure 6.
High-Voltage Pre-Regulator Circuit
HV
DMOS
14.5 V
R
EXT
= 1.4 k
Auxillary
V
CC
The feedback voltage from the output of the auxiliary winding
must sustain V
CC
above V
REG
to fully disconnect the
pre-regulator, isolating V
CC
from V
INEXT
.
V
CC
is then
maintained above V
REG
for the duration of operation. In the
event of an over voltage condition on V
CC
, an internal voltage
clamp turns on at 14.5 V to shunt excessive current to GND.
In systems where operation is directly from a 12 V supply,
V
INEXT
and V
CC
can be connected to the 12 V bus.
The soft-start circuit is designed for the dc-dc converter to start
up in an orderly manner and reduce component stress.
Soft-start is achieved by ramping the maximum attainable duty
cycle during the soft-start time. The duty cycle is increased
from zero to the final value at the rate set by an external
capacitor, C
SS
as shown in
Figure 7
. The hiccup time is set by
an internal 20
μ
A current source charging C
SS
from 0 V to
2 V
be
, at which point switching begins. Then a 100
μ
A charging
current is applied to C
SS
to charge from 2 V
be
to the final value
controlling the duty cycle as it rises. In the event of UVLO,
shutdown or over current, the SS pin will be held low (<1 V)
disabling driver switching. A longer soft-start time may be
needed for highly capacitive loads and high peak-output
current applications. In the event of an over current condition
being detected, the soft-start pin will be pulled low and the
cycle will start again performing a hiccup as shown in
Figure 4
. The hiccup off-time, t
1
, is given by:
t
1
C
SS
1.2 V
20 A
The soft-start time t
2
is can be estimated as:
t
2
C
SS
(K
V
OUT
100 A)
n
where V
OUT
is the output of the converter, and n is the turns
ratio of the primary to each secondary winding, and K is the
ratio of the resistive divider from V
INEXT
to V
INDET
(typically
10/1).
-
+GM
C
SS
+
-
AV
A
V
150 mV
Peak Detect
Blank
CS1
CS2
SS Control
Figure 7.
Current-Sense and Soft-Start Circuit Block Diagram
A
V
100 mV
I
4I
V
CC
SS
SS Enable
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