©2000 Fairchild Semiconductor International
www.fairchildsemi.com
Rev. 5.0
Features
• Low Start Up Current
• Maximum Duty Clamp
• UVLO With Hysteresis
• Operating Frequency Up To 500KHz
Description
The UC3842/UC3843/UC3844/UC3845 are fixed fre-
quency current-mode PWM controller. They are specially
designed for Off - Line and DC-to-DC converter applica-
tions with minimum external components. These inte-
grated 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-volt-
age 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
14-SOP
1
1
Internal Block Diagram
UC3842/UC3843/UC3844/UC3845
SMPS Controller
UC3842/UC3843/UC3844/UC3845
2
Absolute Maximum Ratings
Parameter Symbol Value Unit
Supply Voltage VCC 30 V
Output Current IO ±1 A
Analog Inputs (Pin 2.3) V(ANA) -0.3 to 6.3 V
Error Amp Output Sink Current ISINK (E.A) 10 mA
Power Dissipation (TA = 25°C) PD 1 W
UC3842/UC3843/UC3844/UC3845
3
Electrical Characteristics
(VCC=15V, RT=10KΩ, CT=3.3nF, TA= 0°C to +70°C, unless otherwise specified)
Parameter Symbol Conditions Min. Typ. Max. Unit
REFERENCE SECTION
Reference Output Voltage VREF TJ = 25°C, IREF = 1mA 4.90 5.00 5.10 V
Line Regulation ∆VREF 12V≤VCC≤25V - 6 20 mV
Load Regulation ∆VREF 1mA≤IREF≤20mA - 6 25 mV
Short Circuit Output Current ISC TA = 25°C - -100 -180 mA
OSCILLATOR SECTION
Oscillation Frequency f TJ = 25°C 47 52 57 KHz
Frequency Change with Voltage ∆f/∆VCC 12V≤VCC≤25V - 0.05 1 %
Oscillator Amplitude VOSC - - 1.6 - VP-P
ERROR AMPLIFIER SECTION
Input Bias Current IBIAS - - -0.1 -2 µA
Input Voltage VI(E>A) V1 = 2.5V 2.42 2.50 2.58 V
Open Loop Voltage Gain GVO 2V≤ VO ≤4V 65 90 - dB
Power Supply Rejection Ratio PSRR 12V≤ VCC ≤25V 60 70 - dB
Output Sink Current ISINK V2 = 2.7V, V1 = 1.1V 2 7 - mA
Output Source Current ISOURCE V2 = 2.3V, V1 = 5V -0.6 -1.0 - mA
High Output Voltage VOH V2 = 2.3V, RL = 15KΩ to GND 5 6 - V
Low Output Voltage VOL V2 = 2.7V, RL = 15KΩ to Pin 8 - 0.8 1.1 V
CURRENT SENSE SECTION
Gain GV (Note 1 & 2) 2.85 3 3.15 V/V
Maximum Input Signal VI(MAX) V1 = 5V(Note 1) 0.9 1 1.1 V
Power Supply Rejection Ratio PSRR 12V≤ VCC ≤25V (Note 1) - 70 - dB
Input Bias Current IBIAS - - -3 -10 µA
OUTPUT SECTION
Low Output Voltage
VOL
ISINK = 20mA - 0.08 0.4 V
ISINK = 200mA - 1.4 2.2 V
High Output Voltage
VOH
ISOURCE = 20mA 13 13.5 - V
ISOURCE = 200mA 12 13.0 - V
Rise Time tR TJ = 25°C, CL= 1nF (Note 3) - 45 150 ns
Fall Time tF TJ = 25°C, CL= 1nF (Note 3) - 35 150 ns
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold
VTH(ST)
UC3842/UC3844 14.5 16.0 17.5 V
UC3843/UC3845 7.8 8.4 9.0 V
Min. Operating Voltage
(After Turn On)
VOPR(MIN)
UC3842/UC3844 8.5 10.0 11.5 V
UC3843/UC3844 7.0 7.6 8.2 V
UC3842/UC3843/UC3844/UC3845
4
Electrical Characteristics (Continued)
(VCC=15V, RT=10KΩ, CT=3.3nF, TA= 0°C to +70°C unless otherwise specified)
Adjust VCC above the start threshould before setting at 15V
Note:
1. Parameter measured at trip point of latch
2. Gain defined as:
3.These parameters, although guaranteed, are not 100 tested in production.
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.
Parameter Symbol Conditions Min. Typ. Max. Unit
PWM SECTION
Max. Duty Cycle
D(max) UC3842/UC3843 95 97 100 %
D UC3844/UC3845 47 48 50 %
Min. Duty Cycle D(MIN) - - - 0 %
TOTAL STANDBY CURRENT
Start-Up Current IST - - 0.45 1 mA
Operating Supply Current ICC(OPR) V3=V2=ON - 14 17 mA
Zener Voltage VZ ICC = 25mA 30 38 - V
A
∆V1
∆V3
----------=
UC3842
,0 ≤ V3 ≤ 0.8V
UC3842/UC3843/UC3844/UC3845
5
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 (IS) is determined by the formula:
A small RC filter may be required to suppress switch transients.
UC3842/44 UC3843/45
IS MAX( ) 1.0VRS
------------=
UC3842/UC3843/UC3844/UC3845
6
Figure 5. Oscillator Waveforms and Maximum Duty Cycle
Oscillator timing capacitor, CT, is charged by VREF through RT, and discharged by an internal current source. During the dis-
charge time, the internal clock signal blanks the output to the low state. Selection of RT and CT therefore determines both
oscillator frequency and maximum duty cycle. Charge and discharge times are determined by the formulas:
tc = 0.55 RT CT
Frequency, then, is: f=(tc + td)-1
Figure 8. Shutdown Techniques
Figure 6. Oscillator Dead Time & Frequency Figure 7. Timing Resistance vs Frequency
tD RTCTIn
0.0063RT 2.7–
0.0063RT 4–
---------------------------------------- =
ForRT 5KΩ f 1.8
RTCT
---------------=,>
(Deadtime vs CT RT > 5kΩ)
UC3842/UC3843/UC3844/UC3845
7
Shutdown of the UC3842 can be accomplished by two methods; either raise pin 3 above 1V or pull pin 1 below a voltage two
diode drops above ground. Either method causes the output of the PWM comparator to be high (refer to block diagram). The
PWM latch is reset dominant so that the output will remain low until the next clock cycle after the shutdown condition at pins
1 and/or 3 is removed. In one example, an externally latched shutdown may be accomplished by adding an SOR which will be
reset by cycling Voc below the lower UVLO threshold. At this point the reference turns off, allowing the SCR to reset.
Figure 9. Slope Compensation
A fraction of the oscillator ramp can be resistively summed with the current sense signal to provide slope compensation for
converters requiring duty cycles over 50%. Note that capacitor, C, forms a filter with R2 to suppress the leading edge switch
spikes.
TEMPERATURE (°C)
Figure 10. TEMPERATURE DRIFT (Vref)
TEMPERATURE (°C)
Figure 11. TEMPERATURE DRIFT (Ist)
TEMPERATURE (°C)
Figure 12. TEMPERATURE DRIFT (Icc)
UC3842/UC3843
UC3842/UC3843/UC3844/UC3845
8
Mechanical Dimensions
Package
6.40 ±0.20
3.30 ±0.30
0.130 ±0.012
3.40 ±0.20
0.134 ±0.008
#1
#4 #5
#8
0.252 ±0.008
9.
20
±0
.2
0
0.
79
2.
54
0.
10
0
0.
03
1
(
)
0.
46
±0
.1
0
0.
01
8
±0
.0
04
0.
06
0
±0
.0
04
1.
52
4
±0
.1
0
0.
36
2
±0
.0
08
9.
60
0.
37
8
M
AX
5.08
0.200
0.33
0.013
7.62
0~15°
0.300
MAX
MIN
0.25
+0.10
–0.05
0.010
+0.004
–0.002
8-DIP
UC3842/UC3843/UC3844/UC3845
9
Mechanical Dimensions (Continued)
Package
8.
56
±0
.2
0
0.
33
7
±0
.0
08
1.
27
0.
05
0
5.72
0.225
1.55 ±0.10
0.061 ±0.004
0.05
0.002
6.00 ±0.30
0.236 ±0.012
3.95 ±0.20
0.156 ±0.008
0.60 ±0.20
0.024 ±0.008
8.
70
0.
34
3
M
AX
#1
#7 #8
0~
8°
#14
0.
47
0.
01
9
(
)
1.80
0.071
M
AX
0.
10
M
AX
0.
00
4
MAX
MIN
+
0.
10
-
0.
05
0.
20
+
0.
00
4
-
0.
00
2
0.
00
8
+
0.
10
-
0.
05
0.
40
6 +
0.
00
4
-
0.
00
2
0.
01
6
14-SOP
UC3842/UC3843/UC3844/UC3845
10
Ordering Information
Product Number Package Operating Temperature
UC3842N
8 DIP
0 ~ + 70°C
UC3843N
UC3844N
UC3845N
UC3842D
14 SOP
UC3843D
UC3844D
UC3845D
UC3842/UC3843/UC3844/UC3845
11
UC3842/UC3843/UC3844/UC3845
7/12/00 0.0m 001
Stock#DSxxxxxxxx
2000 Fairchild Semiconductor International
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
INTERNATIONAL. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body,
or (b) support or sustain life, and (c) whose failure to
perform when properly used in accordance with
instructions for use provided in the labeling, can be
reasonably expected to result in a significant injury of the
user.
2. A critical component in any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
www.fairchildsemi.com
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