www.fairchildsemi.com
UC3842/UC3843/UC3844/UC3845
SMPS Controller
Features
•
•
•
•
Low Start Up Current
Maximum Duty Clamp
UVLO With Hysteresis
Operating Frequency Up To 500KHz
Description
The UC3842/UC3843/UC3844/UC3845 are fixed
frequency current-mode PWM controller. They are
specially designed for Off - Line and DC-to-DC converter
applications with minimum external components. These
integrated circuits feature a trimmed oscillator for precise
duty cycle control, a temperature compensated reference,
high gain error amplifier. current sensing comparator, and a
high current totempole output Ideally suited for driving a
power MOSFET. Protection circuity Includes built in
under-voltage lockout and current limiting. TheUC3842 and
UC3844 have UVLO thresholds of 16V (on) and 10V (off)
The UC3843 and UC3845 are 8.5V (on) and 7.9V (off) The
UC3842 and UC3843 can operate within 100% duty cycle.
The UC3844and UC3845 can operate with 50% duty cycle.
8-DIP
1
14-SOP
1
Internal Block Diagram
Rev. 1.0.0
©2001 Fairchild Semiconductor Corporation
UC3842/UC3843/UC3844/UC3845
Absolute Maximum Ratings
Parameter
Supply Voltage
Output Current
Analog Inputs (Pin 2.3)
Error Amp Output Sink Current
Power Dissipation (T
A
= 25°C)
Symbol
V
CC
I
O
V
(ANA)
I
SINK (E.A)
P
D
Value
30
±1
-0.3 to 6.3
10
1
Unit
V
A
V
mA
W
2
UC3842/UC3843/UC3844/UC3845
Electrical Characteristics
(V
CC
=15V, R
T
=10KΩ, C
T
=3.3nF, T
A
= 0°C to +70°C, unless otherwise specified)
Parameter
REFERENCE SECTION
Reference Output Voltage
Line Regulation
Load Regulation
Short Circuit Output Current
OSCILLATOR SECTION
Oscillation Frequency
Frequency Change with
Voltage
Oscillator Amplitude
ERROR AMPLIFIER SECTION
Input Bias Current
Input Voltage
Open Loop Voltage Gain
Power Supply Rejection Ratio
Output Sink Current
Output Source Current
High Output Voltage
Low Output Voltage
CURRENT SENSE SECTION
Gain
Maximum Input Signal
Power Supply Rejection Ratio
Input Bias Current
OUTPUT SECTION
Low Output Voltage
High Output Voltage
Rise Time
Fall Time
Start Threshold
Min. Operating Voltage
(After Turn On)
V
OL
I
SINK
= 20mA
I
SINK
= 200mA
V
OH
t
R
t
F
I
SOURCE
= 20mA
I
SOURCE
= 200mA
T
J
= 25°C, C
L
= 1nF (Note 3)
T
J
= 25°C, C
L
= 1nF (Note 3)
UC3842/UC3844
UC3843/UC3845
UC3842/UC3844
UC3843/UC3844
-
-
13
12
-
-
14.5
7.8
8.5
7.0
0.08
1.4
13.5
13.0
45
35
16.0
8.4
10.0
7.6
0.4
2.2
-
-
150
150
17.5
9.0
11.5
8.2
V
V
V
V
ns
ns
V
V
V
V
G
V
V
I(MAX)
PSRR
I
BIAS
(Note 1 & 2)
V
pin1
= 5V(Note 1)
12V
≤
V
CC
≤
25V (Note 1)
-
2.85
0.9
-
-
3
1
70
-3
3.15
1.1
-
-10
V/V
V
dB
µA
I
BIAS
V
I(E>A)
G
VO
PSRR
I
SINK
I
SOURCE
V
OH
V
OL
V
pin1
= 2.5V
2V
≤
V
O
≤
4V
12V
≤
V
CC
≤
25V
V
pin2
= 2.7V, V
pin1
= 1.1V
V
pin2
= 2.3V, V
pin1
= 5V
V
pin2
= 2.3V, R
L
= 15KΩ to GND
V
pin2
= 2.7V, R
L
= 15KΩ to Pin 8
-
-
2.42
65
60
2
-0.6
5
-
-0.1
2.50
90
70
7
-1.0
6
0.8
-2
2.58
-
-
-
-
-
1.1
µA
V
dB
dB
mA
mA
V
V
f
∆f/∆V
CC
V
OSC
T
J
= 25°C
12V
≤
V
CC
≤
25V
-
47
-
-
52
0.05
1.6
57
1
-
KHz
%
V
P-P
V
REF
∆V
REF
∆V
REF
I
SC
T
J
= 25°C, I
REF
= 1mA
12V
≤
V
CC
≤
25V
1mA
≤
I
REF
≤
20mA
T
A
= 25°C
4.90
-
-
-
5.00
6
6
-100
5.10
20
25
-180
V
mV
mV
mA
Symbol
Conditions
Min.
Typ.
Max.
Unit
UNDER-VOLTAGE LOCKOUT SECTION
V
TH(ST)
V
OPR(MIN)
3
UC3842/UC3843/UC3844/UC3845
Electrical Characteristics (Continued)
(V
CC
=15V, R
T
=10KΩ, C
T
=3.3nF, T
A
= 0°C to +70°C, unless otherwise specified)
Parameter
PWM SECTION
Max. Duty Cycle
Min. Duty Cycle
TOTAL STANDBY CURRENT
Start-Up Current
Operating Supply Current
Zener Voltage
I
ST
I
CC(OPR)
V
Z
I
CC
= 25mA
-
V
pin3
=V
pin2
=ON
-
-
30
0.45
14
38
1
17
-
mA
mA
V
D
(max)
D
D
(MIN)
UC3842/UC3843
UC3844/UC3845
-
95
47
-
97
48
-
100
50
0
%
%
%
Symbol
Conditions
Min.
Typ.
Max.
Unit
Adjust V
CC
above the start threshould before setting at 15V
Note:
1. Parameter measured at trip point of latch
2. Gain defined as:
∆V
pin1
A
= -----------------
,0
≤
V
pin3
≤
0.8V
-
∆V
pin3
3.These parameters, although guaranteed, are not 100 tested in production.
UC3842
Figure 1. Open Loop Test Circuit
High peak currents associated with capacitive loads necessitate careful grounding techniques Timing and bypass capacitors
should be connected close to pin 5 in a single point ground. The transistor and 5KΩ potentiometer are used to sample the
oscillator waveform and apply an adjustable ramp to pin 3.
4
UC3842/UC3843/UC3844/UC3845
UC3842/44 UC3843/45
Figure 2. Under Voltage Lockout
During Under-Voltage Lock-Out, the output driver is biased to a high impedance state. Pin 6 should be shunted to ground with
a bleeder resistor to prevent activating the power switch with output leakage current.
Figure 3. Error Amp Configuration
Figure 4. Current Sense Circuit
Peak current (I
S
) is determined by the formula:
1.0V
I
S
(
MAX
)
= -----------
-
R
S
A small RC filter may be required to suppress switch transients.
5
stm32/stm8
