FLYBACK APPLICATION (A)

In this application (see Figure A, next page), complete

control is maintained on the primary side. Control power

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

= 50 kHz

Start Voltage = 3







R1

R2

+ R3







+0.2R1 = 12V

UV Fault Voltage = 3







R1

R2

+ R3







= 8V

OV Fault Voltage = 3







R1

R3







= 32V

Current Limit = 200mV

Current Fault Voltage = 600mV

Duty Cycle Clamp = 50%

6