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參數(shù)資料
| 型號(hào)： | SI9124 |
| 廠商： | Vishay Intertechnology,Inc. |
| 元件分類： | DC/DC變換器 |
| 英文描述： | 500-kHz Push-Pull DC-DC Converter With Integrated Secondary Synchronous Rectification Control |
| 中文描述： | 500千赫推挽式直流與集成的次級(jí)同步整流控制DC轉(zhuǎn)換器 |
| 文件頁(yè)數(shù)： | 9/16頁(yè) |
| 文件大?。?/td> | 175K |
| 代理商： | SI9124 |
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Si9124
Vishay Siliconix
New Product
Document Number: 72099
S-03638—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 achievable duty
cycle during the soft start time. The duty cycle is increased
from zero to the final value at the rate set by the 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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