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UC1851
UC2851
UC3851
Programmable, Off-Line, PWM Controller
FEATURES
• All Control, Driving, Monitoring, and Protection
Functions Included
• Low-Current Off Line Start Circuit
• Voltage Feed Forward or Current Mode Control
• High Current Totem Pole Output
• 50% Absolute Max Duty Cycle
• 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
• 1% Reference Accuracy
• 500kHz Operation
• 18 Pin DIL or 20 Pin PLCC Package
DESCRIPTION
The UC1851 family of PWM controllers are optimized for off-
line primary side control. These devices include a high current
totem pole output stage and a toggle flip-flop for absolute 50%
duty cycle limiting. In all other respects this line of controllers is
pin for pin compatible with the UC1841 series. Inclusion of all
major housekeeping functions in these high performance con-
trollers makes them ideal for use in cost sensitive applications.
Important features of these controllers include low current
start-up, linear feed-forward for constant volt-second operation,
and compatibility with both voltage or current mode control. In
addition, these devices include a programmable start thresh-
old, as well as programmable over-voltage, under-voltage, and
over current fault thresholds. The fault latch on these devices
can be configured for automatic restart, or latched off response
to a fault.
These devices are packaged in 18-pin plastic or ceramic dual-
in-line packages, or for surface mount applications, a 20 Pin
PLCC. The UC1851 is characterized for -55°C to +125°C op-
eration while the UC2851 and UC3851 are designed for -40°C
to +85°C and 0°C to +70°C, respectively.
BLOCK DIAGRAM
10/94
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