2000 Fairchild Semiconductor International
www.fairchildsemi.com
Rev. 5.0
Features
Low Start Current 0.2mA (typ)
Operating Range Up To 500KHz
Cycle by Cycle Current Limiting
Under Voltage Lock Out With Hysteresis
Short Shutdown Delay Time: typ.100ns
High Current Totem-pole Output
Output Swing Limiting: 22V
Description
The UC3842A/UC3843A are fixed PWM controller for Off-
Line and DC to DC converter applications. The internal cir-
cuits include UVLO, low start up current circuit, tempera-
ture compensated reference, high gain error amplifier,
current sensing comparator, and high current totem-pole out-
put for driving a POWER MOSFET. Also UC3842A/
UC3843A provide low start up current below 0.3mA and
short shutdown delay time typ. 100ns. The UC3842A has
UVLO threshold of 16V(on) and 10V(off). The UC3843A is
8.4V(on) and 7.6V(off). The UC3842A and UC3843A can
operate within 100% duty cycle.
8-DIP
8-SOP
1
1
Internal Block Diagram
8
+
-
7
5
SET/
RESET
5V
V
REF
Internal
Bias
Good LOGIC
2
R
S
OSCILLATOR
1
3
4
T
7
6
5
UVLO
29V
22V
1/3
Error Amp
1/2V
REF
1V
C.S
Comp.
PWM
LATCH
V
REF
V
FB
COMP
C.S
R
T
/C
T
OUTPUT
V
CC
GND
PWR
V
C
PWR
GND
UC3842A/UC3843A
SMPS Controller
UC3842A/UC3843A
2
Absolute Maximum Ratings
Electrical Characteristics
(V
CC
= 15V, R
T
= 10K
, C
T
= 3.3nF, T
A
= 0
C to + 70
C ,Unless otherwise specified)
Parameter
Symbol
Value
Unit
Supply Voltage
V
CC
30
V
Output Current
I
O
1
A
Analog Inputs (pin 2, 3)
V
I(ANA)
- 0.3 to 6.3
V
Error Amp. Output Sink Current
I
SINK(EA)
10
mA
Power Dissipation
P
D
1
W
Parameter
Symbol
Conditions
Min.
Typ. Max. Unit
REFERENCE SECTION
Output Voltage
V
REF
T
J
= 25
C, I
O
= 1mA
4.9
5.0
5.1
V
Line Regulation
V
REF
V
CC
= 12V to 25V
-
6
20
mV
Load Regulation
V
REF
I
O
= 1mA to 20mA
-
6
25
mV
Output Short Circuit
I
SC
T
a
= 25
C
-
- 100 - 180
mA
OSILLATOR SECTION
Initial Accuracy
F
OSC
T
J
= 25
C
47
52
57
KHz
Voltage Stability
ST
V
V
CC
= 12V to 25V
-
0.2
1
%
Amplitude
V
OSC
V
PIN4
, Peak to Peak
-
1.7
-
V
Discharge Current
I
DISCHG
T
J
= 25
C
, Pin4 = 2V
7.8
8.3
8.8
mA
CURRENT SENSE SECTION
Gain
G
V
(NOTE 2, 3)
2.85
3
3.15
V/V
Maximum Input Signal
V
I(MAX)
V
PIN1
= 5V(NOTE 2)
0.9
1.0
1.1
V
PSRR
PSRR
V
CC
= 12V to 25V (NOTE 1, 2)
-
70
-
dB
Input Bias Current
I
BIAS
-
-
- 2
-10
uA
Delay to Output
T
D
V
PIN3
= 0 V to 2V (NOTE1)
-
100
200
ns
UC3842A/UC3843A
3
Electrical Characteristics (Continued)
(V
CC
= 15V, R
T
= 10K
, C
T
= 3.3nF, T
A
= 0
C to + 70
C, Unless otherwise specified)
* Adjust V
CC
above the start threshold before setting at 15V
Notes :
1. These parameters, although guaranteed, are not 100% tested in production.
2. Parameter measured at trip point of latch with V2 = 0V.
3. Gain defined as: G
V
=
V
PIN1
V
PIN3
(V
PIN3
= 0 to 0.8V)
Parameter
Symbol
Conditions
Min.
Typ.
Max.
Unit
ERROR AMPLIFIER SECTION
Input Voltage
V
I
T
PIN1
= 2.5V
2.42
2.50
2.58
V
Input Bias Current
I
BIAS
-
-
-0.3
- 2
uA
Open Loop Gain
G
VO
V
O
= 2V to 4V (NOTE 1)
65
90
-
dB
Unity Gain Bandwidth
GBW
T
J
= 25
C
(NOTE 1)
0.7
1
-
MHz
PSRR
PSRR
V
CC
= 12V to 25V (NOTE 1)
60
70
-
dB
Output Sink Current
I
SINK
V
PIN2
= 2.7V
V
PIN1
= 1.1V
2
6
-
mA
Output Source Current
I
SOURCE
V
PIN2
= 2.3V
V
PIN1
= 5.0V
-0.5
-0.8
-
mA
Output High Voltage
V
OH
V
PIN2
= 2.3V
R1 = 15K
to GND
5
6
-
V
Output Low Voltage
V
OL
V
PIN2
= 2.7V
R1 = 15K
to Pin8
-
0.8
1.1
V
OUTPUT SECTION
Output Low Level
V
OL
I
SINK
= 20mA
-
0.1
0.4
V
I
SINK
= 200mA
-
1.5
2.2
V
Output High Level
V
OH
I
SOURCE
= 20mA
13
13.5
-
V
I
SOURCE
= 200mA
12
13.5
-
V
Rise Time
t
R
T
J
= 25
C
, C1 = 1nF (NOTE 1)
-
40
100
ns
Fall Time
t
F
T
J
= 25
C
, C1 = 1nF (NOTE 1)
-
40
100
ns
Output Voltage Swing Limit
V
OLIM
V
CC
= 27V, C1 = 1nF
-
22
-
V
UNDER VOLTAGE LOCKOUT SECTION
Start Threshold
V
TH
UC3842A
15
16
17
V
UC3843A
7.8
8.4
9.0
V
Min. Operating Voltage
( After turn on )
V
TL
UC3842A
9
10
11
V
UC3843A
7.0
7.6
8.2
V
PWM SECTION
Maximum Duty Cycle
D
MAX
UC3842A/UC3843A
94
96
100
%
Minimum Duty Cycle
D
MIN
-
-
-
0
%
TOTAL STANDBY CURRENT
Start-Up Current
I
ST
-
-
0.2
0.4
mA
Operating Supply Current
I
CC
V
PIN2
= V
PIN3
= 0V
-
11
17
mA
V
CC
Zener Voltage
V
Z
I
CC
= 25mA
-
29
-
V
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