LT1725CGN Linear Technology, LT1725CGN Datasheet - Page 17

• Manufacturer Part Number: LT1725CGN
• Description: IC CTRLR ISO FLYBACK HV 16SSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1725CGN

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Voltage - Output: -
• Voltage - Input: -
• Power - Output: -

APPLICATIO S I FOR ATIO
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
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
“Trickle charge” resistor R1 is connected to V
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
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
CC
CC
pin voltage (see Figure 7). Operation is as follows:
) reaches the V
CC
pin. However, this arrangement is not inherently
(6a) “Standard” UVLO
Divider Topology
V
IN
R1
R2
CC
U
turn-on threshold. The LT1725 then
UVLO
U
W
(6b) Filter Capacitor
Directly On UVLO Node
U
C1
100pF
IN
V
IN
and
R1
R2
Figure 6
UVLO
drawn by the LT1725 will discharge C1 too rapidly; before
the third winding drive becomes effective, the V
threshold will be reached. The LT1725 turns off, and the
V
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
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.
CC
node begins to charge via R1 back up to the turn-on
CC
+
V
node.
IN
R1
C1
V
ON
(6c) Recommended Topology to
Filter High Frequency Ripple
THRESHOLD
PGND
V
V
GATE
LT1725
I
VCC
VCC
V
C2
C1
100pF
0
CC
Figure 7
I
SGND
VCC
V
GATE
IN
R1/2
R1/2
R2
UVLO
1725 F06
V
IN
1725 F07
LT1725
CC
turn-off
17
1725fa