UC284XA
UC384XA
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
NOT FOR NEW DESIGN
1
FEATURES
Figure 1. Package
■ TRIMMED OSCILLATOR DISCHARGE
CURRENT
■ CURRENT MODE OPERATION TO 500kHz
AUTOMATIC FEED FORWARD
COMPENSATION
DIP-8
SO-8
■ LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
■ INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGE LOCKOUT
■ HIGH CURRENT TOTEM POLE OUTPUT
UNDERVOLTAGE LOCKOUT WITH
HYSTERESIS
■ LOW START-UP CURRENT (< 0.5mA)
DOUBLE PULSE SUPPRESSION
Table 1. Order Codes
Part Number
Package
UC2842AD1; UC3842AD1;
UC2843AD1; UC3843AD1;
UC2844AD1; UC3844AD1;
UC2845AD1; UC3845AD1
SO-8
DIP-8
UC2842AN; UC3842AN;
UC2843AN; UC3843AN;
UC2844AN; UC3844AN;
UC2845AN; UC3845AN
2
DESCRIPTION
The UC384xA 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 im-
plemented circuits include a trimmed oscillator for
precise DUTY CYCLE CONTROL under voltage
lockout featuring start-up current less than 0.5mA,
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 de-
signed to source or sink high peak current. The
output stage, suitable for driving N-Channel MOS-
FETs, is low in the off-state.
Differences between members of this family are
the under-voltage lockout thresholds and maxi-
mum duty cycle ranges. The UC3842A and
UC3844A have UVLO thresholds of 16V (on) and
10V (off), ideally suited off-line applications The
corresponding thresholds for the UC3843A and
UC3845A are 8.5 V and 7.9V. The UC3842A and
UC3843A can operate to duty cycles approaching
100%. A range of the zero to < 50 % is obtained by
the UC3844A and UC3845A by the addition of an
internal toggle flip flop which blanks the output off
every other clock cycle.
Figure 2. Block Diagram (toggle flip flop used only in UC3844A and UC3845A)
7
Vi
UVLO
34V
8
6
5V
REF
VREF
5V 50mA
5
S/R
GROUND
INTERNAL
BIAS
2.50V
VREF GOOD
LOGIC
OUTPUT
4
RT/CT
OSC
ERROR AMP.
T
2R
+
-
S
2
1
3
VFB
R
PWM
LATCH
R
1V
COMP
CURRENT
SENSE
CURRENT
SENSE
COMPARATOR
D95IN331
REV. 5
1/16
May 2004