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參數(shù)資料
| 型號： | 1725I |
| 廠商： | Linear Technology Corporation |
| 英文描述： | General Purpose Isolated Flyback Controller |
| 中文描述： | 通用隔離反激式控制器 |
| 文件頁數(shù)： | 17/28頁 |
| 文件大?。?/td> | 241K |
| 代理商： | 1725I |
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17
LT1725
1725f
V
IN
(6a) “Standard” UVLO
Divider Topology
UVLO
R1
R2
V
IN
(6b) Filter Capacitor
Directly On UVLO Node
UVLO
R1
R2
C1
100pF
V
IN
(6c) Recommended Topology to
Filter High Frequency Ripple
UVLO
R1/2
R1/2
R2
1725 F06
C2
C1
100pF
APPLICATIOU
W
U
U
Figure 6
speed supply ripple, while leaving the UVLO pin node
impedance relatively unchanged at high frequency.
INTERNAL WIDE HYSTERESIS
UNDERVOLTAGE LOCKOUT
The LT1725 is designed to implement isolated DC/DC
converters operating from input voltages of typically 48V
or more. The standard operating topology utilizes a third
transformer winding on the primary side that provides
both feedback information and local power for the LT1725
via its V
CC
pin. However, this arrangement is not inherently
self-starting. Start-up is effected by the use of an external
“trickle-charge” resistor and the presence of an internal
wide hysteresis undervoltage lockout circuit that monitors
V
CC
pin voltage (see Figure 7). Operation is as follows:
“Trickle charge” resistor R1 is connected to V
IN
and
supplies a small current, typically on the order of a single
mA, to charge C1. At first, the LT1725 is off and draws only
its start-up current. After some time, the voltage on C1
(V
CC
) reaches the V
CC
turn-on threshold. The LT1725 then
turns on abruptly and draws its normal supply current.
Switching action commences at the GATE pin and the
MOSFET begins to deliver power. The voltage on C1
begins to decline as the LT1725 draws its normal supply
current, which greatly exceeds that delivered by R1. After
some time, typically tens of milliseconds, the output
voltage approaches its desired value. By this time, the
third transformer winding is providing virtually all the
supply current required by the LT1725.
One potential design pitfall is undersizing the value of
capacitor C1. In this case, the normal supply current
+
I
VCC
1725 F07
R1
C1
V
IN
V
IN
I
VCC
V
VCC
V
ON
THRESHOLD
0
V
GATE
V
CC
LT1725
GATE
PGND
SGND
Figure 7
drawn by the LT1725 will discharge C1 too rapidly; before
the third winding drive becomes effective, the V
CC
turn-off
threshold will be reached. The LT1725 turns off, and the
V
CC
node begins to charge via R1 back up to the turn-on
threshold. Depending upon the particular situation, this
may result in either several on-off cycles before proper
operation is reached, or, permanent relaxation oscillation
at the V
CC
node.
Component selection is as follows:
Resistor R1 should be selected to yield a worst-case
minimum charging current greater than the maximum
rated LT1725 start-up current, and a worst-case maxi-
mum charging current less than the minimum rated
LT1725 supply current.
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