Philips Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
DESCRIPTION
The UC3842 is available in an 8-Pin mini-DIP the necessary
features to implement off-line, fixed-frequency current-mode control
schemes with a minimal external parts count. This technique results
in improved line regulation, enhanced load response characteristics,
and a simpler, easier to design control loop. Topological advantages
include inherent pulse-by-pulse current limiting.
Protection circuitry includes built-in undervoltage lock-out and
current limiting. Other features include fully-latched operation, a 1%
trimmed bandgap reference, and start-up current less than 1mA.
These devices feature a totem-pole output designed to source and
sink high peak current from a capacitive load, such as the gate of a
power MOSFET. Consistent with N-channel power devices, the
output is low in the OFF-state.
PIN CONFIGURATIONS
N Package
COMP
V
FB
I
SENSE
R
T
/C
T
1
2
3
4
TOP VIEW
8
7
6
5
V
REF
V
CC
OUTPUT
GROUND
D Package
COMP
1
2
3
4
5
6
7
TOP VIEW
14
V
REF
13
NC
12
V
CC
11
V
C
10
OUTPUT
9
8
GROUND
POWER GROUND
FEATURES
NC
V
FB
NC
I
SENSE
NC
R
T
/C
T
•
Low start-up current (≤1mA)
•
Automatic feed-forward compensation
•
Pulse-by-pulse current limiting
•
Enhanced load response characteristics
•
Undervoltage lock-out with hysteresis
•
Double pulse suppression
•
High current totem-pole output
•
Internally-trimmed bandgap reference
•
400kHz operation, guaranteed min
BLOCK DIAGRAM
APPLICATIONS
•
Off-line switched mode power supplies
•
DC-to-DC converters UC3842
7(11)
(12)7
V
CC
34V
(9)5
GND
16V
2.5V
INTERNAL
BIAS
6V
UVLO
S/R
5V
REF
V
8(14)
REF
5.0V
50mA
(7)4
R
T
/C
T
OSC
6(10)
OUTPUT
ERROR
AMP
V
FB
COMP
CURRENT
SENSE
NOTE:
Pin numbers in parentheses refer to the D package.
(3)2
(1)1
(5)3
+
–
S
2R
R
R
1V
CURRENT
SENSE
COMPARATOR
PWM
LATCH
5(8)
August 31, 1994
1100
853-0614 13721
PHILIPS [ NXP SEMICONDUCTORS ]
