UC1842/3/4/5
UC2842/3/4/5
UC3842/3/4/5
www.ti.com
SLUS223C – APRIL 1997 – REVISED JUNE 2007
CURRENT MODE PWM CONTROLLER
FEATURES
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DESCRIPTION
Optimized For Off-line and DC-to-DC
Converters
Low Start-Up Current (<1 mA)
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
500-kHz Operation
Low RO Error Amp
The UC1842/3/4/5 family of control devices 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 1 mA, 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 16 VON and 10 VOFF, 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.
BLOCK DIAGRAM
Vcc
7 12
UVLO
34 V
S/R
GROUND
5
9
5V
REF
8 14
VREF
5V
50 mA
2.50 V
Internal
BIAS
VREF
Good
Logic
4
7
RT/CT
OSC
Error
Amp
VFB
2
COMP 1
CURRENT
SENSE
3
7 11
VC
3
1
6 10
OUTPUT
T
S
2R
R
R
1V
CURRENT
SENSE
COMPARATOR
PWM
LATCH
5
8
POWER
GROUND
5
Note 1: A/B A = DIL−8 Pin Number. B = SO−14 and CFP−14 Pin Number.
Note 2:
Toggle flip flop used only in 1844 and 1845.
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.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 1997–2007, Texas Instruments Incorporated
