FEATURES DESCRIPTION
BLOCK DIAGRAM
11
A/B
V
cc
7 12
5 9
GROUND
4 7
R
T
/C
T
2 3
V
FB
1 1COMP
3 5
CURRENT
SENSE
34 V
2.50 V
OSC
UVLO
S/R
5 V
REF
VREF
Good
Logic
Internal
BIAS
Error
Amp
2R
R
1 V
CURRENT
SENSE
COMPARATOR
PWM
LATCH
S
R
T
8 14
V
REF
5 V
50 mA
7
V
C
106
OUTPUT
85
POWER
GROUND
Note 1:
Note 2:
A = DIL−8 Pin Number. B = SO−14 and CFP−14 Pin Number.
Toggle flip flop used only in 1844 and 1845.
UC1842/3/4/5
UC2842/3/4/5
UC3842/3/4/5
SLUS223C – APRIL 1997 – REVISED JUNE 2007
CURRENT MODE PWM CONTROLLER
• Optimized For Off-line and DC-to-DC
The UC1842/3/4/5 family of control devices provides
Converters
the necessary features to implement off-line or
dc-to-dc fixed frequency current mode control
• Low Start-Up Current (<1 mA)
schemes with a minimal external parts count.
• Automatic Feed Forward Compensation
Internally implemented circuits include under-voltage
• Pulse-by-Pulse Current Limiting
lockout featuring start up current less than 1 mA, a
precision reference trimmed for accuracy at the error
• Enhanced Load Response Characteristics
amp input, logic to insure latched operation, a PWM
• Under-Voltage Lockout With Hysteresis
comparator which also provides current limit control,
• Double Pulse Suppression
and a totem pole output stage designed to source or
sink high peak current. The output stage, suitable for
• High Current Totem Pole Output
driving N-Channel MOSFETs, is low in the off state.
• Internally Trimmed Bandgap Reference
Differences between members of this family are the
• 500-kHz Operation
under-voltage lockout thresholds and maximum duty
• Low R
O
Error Amp
cycle ranges. The UC1842 and UC1844 have UVLO
thresholds of 16 V
ON
and 10 V
OFF
, ideally suited to
off-line applications. The corresponding thresholds
for the UC1843 and UC1845 are 8.4 V and 7.6 V.
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.
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas
Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date.
Copyright © 1997–2007, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
