UC1842/3/4/5
UC2842/3/4/5
UC3842/3/4/5
Current Mode PWM Controller
FEATURES
• Optimized For Off-line And DC
To DC Converters
• Low Start Up Current (<1mA)
• Automatic Feed Forward
Compensation
• Pulse-by-pulse Current Limiting
• Enhanced Load Response
Characteristics
• Under-voltage Lockout With
Hysteresis
• Double Pulse Suppression
• High Current Totem Pole
Output
• Internally Trimmed Bandgap
Reference
• 500khz Operation
• Low RO Error Amp
DESCRIPTION
The UC1842/3/4/5 family of control ICs provides the necessary features to
implement off-line or DC to DC fixed frequency current mode control schemes
with a minimal external parts count. Internally implemented circuits include
under-voltage lockout featuring start up current less than 1mA, a precision
reference trimmed for accuracy at the error amp input, logic to insure latched
operation, a PWM comparator which also provides current limit control, and a
totem pole output stage designed to source or sink high peak current. The
output stage, suitable for driving N Channel MOSFETs, is low in the off state.
Differences between members of this family are the under-voltage lockout
thresholds and maximum duty cycle ranges. The UC1842 and UC1844 have
UVLO thresholds of 16V (on) and 10V (off), ideally suited to off-line
applications. The corresponding thresholds for the UC1843 and UC1845 are
8.4V and 7.6V. The UC1842 and UC1843 can operate to duty cycles
approaching 100%. A range of zero to 50% is obtained by the UC1844 and
UC1845 by the addition of an internal toggle flip flop which blanks the output
off every other clock cycle.
BLOCK DIAGRAM
A/BNote 1: A = DIL-8 Pin Number. B = SO-14 and CFP-14 Pin Number.
Note 2: Toggle flip flop used only in 1844 and 1845.
SLUS223A - APRIL 1997 - REVISED MAY 2002