鈥?/div>
All Control, Driving, Monitoring, and
Protection Functions Included
Low-current, Off-line Start Circuit
Voltage Feed Forward or Current
Mode Control
Guaranteed Duty Cycle Clamp
PWM Latch for Single Pulse per Period
Pulse-by-Pulse Current Limiting Plus
Shutdown for Over-Current Fault
No Start-up or Shutdown Transients
Slow Turn-on Both Initially and After
Fault Shutdown
Shutdown Upon Over- or
Under-Voltage Sensing
Latch Off or Continuous Retry After
Fault
PWM Output Switch Usable to 1A
Peak Current
1% Reference Accuracy
500kHz Operation
18 Pin DIL Package
DESCRIPTION
The UC1841 family of PWM controllers has been designed to increase
the level of versatility while retaining all of the performance features of
the earlier UC1840 devices. While still optimized for highly-efficient boot-
strapped primary-side operation in forward or flyback power converters,
the UC1841 is equally adept in implementing both low and high voltage
input DC to DC converters. Important performance features include a
low-current starting circuit, linear feed-forward for constant volt-second
operation, and compatibility with either voltage or current mode topologies.
In addition to start-up and normal regulating PWM functions, these de-
vices include built in protection from over-voltage, under-voltage, and
over-current fault conditions with the option for either latch-off or automat-
ic restart.
While pin compatible with the UC1840 in all respects except that the po-
larity of the External Stop has been reversed, the UC1841 offers the fol-
lowing improvements:
1. Fault latch reset is accomplished with slow start discharge rather
than recycling the input voltage to the chip.
2. The External Stop input can be used for a fault delay to resist
shutdown from short duration transients.
3. The duty-cycle clamping function has been characterized and
specified.
The UC1841 is characterized for -55掳C to +125掳C operation while the
UC2841 and UC3841 are designed for -25掳C to +85掳C and 0掳to +70掳C,
respectively.
BLOCK DIAGRAM
Note: Positive true logic, latch outputs high with set, reset has priority.
6/93
Powered by ICminer.com Electronic-Library Service CopyRight 2003
1
2
3
4
5
6
7
8
9
