Datasheet

FLYBACK APPLICATION (A)
In this application (see Figure A, next page), complete
control is maintained on the primary side. Control power
is provided by R
IN and CIN during start-up, and by a pri-
mary-referenced low voltage winding, N2, for efficient op-
eration after start. The error amplifier loop is closed to
regulate the DC voltage from N2 with other outputs fol-
lowing through their magnetic coupling − a task made
even easier with the UC1841’s feed−forward line regula-
tion.
An extension to this application for more precise regula-
tion would be the use of the UC1901 Isolated Feedback
Generator for direct closed-loop control to an output.
Not shown, are protective snubbers or additional interface
circuitry which may be required by the choice of the high-
voltage switch, Qs, or the application; however, one ex-
ample of power transistor interfacing is provided on the
following page.
REGULATOR APPLICATION (B)
With the addition of a level shifting transistor, Q1, the
UC1841 is an ideal control circuit for DC to DC converters
such as the buck regulator shown in Figure B opposite. In
addition to providing constant current drive pulses to the
PIC661 power switch, this circuit has full fault protection
and high speed dynamic line regulation due to its feed-
forward capability. An additional feature is the ability to
UC1841
UC2841
UC3841
OPEN-LOOP TEST CIRCUIT
Nominal Frequency =
1
R
TCT
= 50 kHz
Start Voltage = 3



R1
+ R2 + R3
R2 + R3



+0.2R1 = 12V
UV Fault Voltage = 3



R1
+ R2 + R3
R2 + R3



= 8V
OV Fault Voltage = 3



R1
+ R2 + R3
R3



= 32V
Current Limit = 200mV
Current Fault Voltage = 600mV
Duty Cycle Clamp = 50%
6