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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 VFB ISENSE RT/CT 1 2 3 4 TOP VIEW 8 7 6 5 VREF VCC
OUTPUT GROUND
D Package
COMP 1 2 3 4 5 6 7 TOP VIEW 14 VREF 13 NC 12 V CC 11 V C 10 OUTPUT 9 8 GROUND POWER GROUND
FEATURES
NC VFB NC ISENSE NC RT/CT
* 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 VCC 34V (9)5 GND 16V 2.5V INTERNAL BIAS 6V UVLO S/R 5V REF V 8(14) REF 5.0V 50mA
(7)4 RT/CT OSC 6(10) OUTPUT ERROR AMP VFB 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 Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
ORDERING INFORMATION
DESCRIPTION 8-Pin Plastic Dual In-Line Package (DIP) 14-Pin Plastic Small Outline (SO) Package TEMPERATURE RANGE 0 to +70C 0 to +70C ORDER CODE UC3842N UC3842D DWG # 0404B 0405B
ABSOLUTE MAXIMUM RATINGS1
SYMBOL VCC VCC IOUT PARAMETER Supply voltage (ICC<30mA) Supply voltage (low impedance source) Output current 2, 3 Output energy (capacitive load) Analog inputs (Pin 2, Pin 3) Error amp output sink current PD TSTG TSOLD Power dissipation at TA70C (derate 12.5mW/C for TA Lead temperature (soldering, 10sec max) >70C)2 -65 to +150 300 C C Storage temperature range 30 1 5 -0.3 to 6.3 10 1 RATING UNIT Self-Limiting V A J V mA W
NOTES: 1. All voltages are with respect to Pin 5; all currents are positive into the specified terminal. 2. See section in application note on "Power Dissipation Calculation". 3. This parameter is guaranteed, but not 100% tested in production.
August 31, 1994
1101
Philips Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
DC AND AC ELECTRICAL CHARACTERISTICS
0TJ70C for UC3842; VCC=15V; RT=10kW; CT=3.3nF, unless otherwise specified. SYMBOL PARAMETER TEST CONDITIONS UC3842 Min Typ Max UNIT
Reference section
VOUT Output voltage Line regulation Load regulation Temp. stability1 Line, load, temp. 10Hzf10kHz, TJ=25C TJ=125C, 1000 Hrs. TJ=25 -55Oscillator section
Initial accuracy Voltage stability Temp. stability1 47 52 0.2 5 1.7 57 1 kHz % % V
Amplitude
Error amp section
Input voltage IBIAS AVOL Input bias current 65 0.7 0.5 60 2 -0.5 5 70 6 -0.8 6 0.7 1.1 2.50 -0.3 90 1 2.58 -2 V A dB MHz MHz dB mA mA V V
Current sense section
Gain 2, 3 Maximum input signal2 PSRR IBIAS Power supply rejection ratio2 Input bias current Delay to output1 3 1 70 -2 150 -10 300 3.15 1.1 V/V V dB A ns
August 31, 1994
1102
Philips Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
DC AND AC ELECTRICAL CHARACTERISTICS
0TJ70C for UC3842; VCC=154; RT=10k; CT=3.3nF, unless otherwise specified SYMBOL PARAMETER TEST CONDITIONS UC3842 Min Typ Max UNIT
Output section
VOL VOH tR tF Output Low-Level Output High-Level Rise time Fall time ISINK=20mA ISINK=200mA ISOURCE=20mA ISOURCE=200mA CL=1nF CL=1nF 14.5 8.5 13 12 0.1 1.5 13.5 13.5 50 50 150 150 0.4 2.2 V V ns ns
Undervoltage lockout section
Start threshold Min. operating voltage after turn on 16 10 17.5 11.5 V V
PWM section
Maximum duty cycle Minimum duty cycle 93 97 100 0 %
Total standby current
Start-up current ICC Operating supply current VCC zener voltage VPIN 2=VPIN 3=0V ICC=25mA 0.5 11 34 1 17 mA mA V
Maximum operating frequency section
Maximum operating frequency for all functions operating cycle-by-cycle NOTES: 1. These parameters, although guaranteed, are not 100% tested in production. 2. Parameter measured at trip point of latch with VPIN 2=0. DV PIN 1 3. Gain defined as: ; 0 v V PIN 3 v 0.8V A+ DV PIN 3 400 kHz
UNDERVOLTAGE LOCKOUT
7 VCC ON/OFF COMMAND TO REST OF IC
ERROR AMP CONFIGURATION
2.5V 0.5mA + ZI 2 VFB 1
-
VON VOFF
ICC
UC3842 16V 10V
COMP NOTE: Error AMP can source or sink up to 0.5mA.
<15mA <1mA VOFF NOTE: During Undervoltage 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. VON
VCC
August 31, 1994
1103
Philips Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
CURRENT SENSE CIRCUIT
ERROR AMP IS 2R CURRENT SENSE COMPARATOR R 1V
1 COMP R
3 CURRENT SENSE
RS 5 GND
NOTE: Peak current (IS) is determined by the formula: I S MAX + 1.0V Rs
A small RC filter may be required to suppress switch transients.
TYPICAL PERFORMANCE CHARACTERISTICS
Output Saturation Characteristics
4 100 SATURATION VOLTAGE (V) VOLTAGE GAIN (dB) 80 0 60 40 20 0 -20 0 0.01 0.03 0.05 0.1 0.3 0.5 1.0 OUTPUT CURRENT, SOURCE OR SINK (A) 10 100 1k 10k 100k 1M FREQUENCY (Hz) PHASE (DEG) Av -45 -90 -135 -180 -225 10M
Error Amplifier Open-Loop Frequency Response
45 0
3
2
1
August 31, 1994
1104
Philips Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
OPEN-LOOP LABORATORY TEST FIXTURE
VREF RT 2N2222 4.7k 100k 1 COMP ERROR AMP ADJUST 2 VFB 3 4.7k 5k ISENSE ADJUST ISENSE 4 RT/CT GND UC3842 8 VREF 7 VCC 6 OUTPUT 5 A VCC
0.1F
0.1F
1k 1W OUTPUT
GND CT NOTE: 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 5k potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to Pin 3.
SHUTDOWN TECHNIQUES
4.7k 8
1 COMP 4.7k 500 SHUTDOWN 3 ISENSE SHUTDOWN
TO CURRENT SENSE RESISTOR NOTE: 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 the examples shown, an externally-latched shutdown may be accomplished by adding an SCR which will be reset by cycling VCC below the lower UVLO threshold (10V). At this point all internal bias is removed, allowing the SCR to reset.
August 31, 1994
1105
Philips Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
OFF-LINE FLYBACK REGULATOR
T1 4.7 1W 4.7k 4W 56k 1W 0.01F 400V USD735 + 220F 250V 673-3
4700F 10V
DC (5V 2-5A) OUT
1N3613 - 1N3613
AC INPUT
16V 0.01F UC3842 20k 2 150k VFB 1 COMP 3.6k 100pF 8 VREF 10k 4 RT/CT 0.01F 0.0047F GND 5 3 CUR SEN 470pF 1k 0.85 6 OUT 20k NOTES: T1: Coilcraft E-4140-B Primary - 97 turns single AWG24 Secondary - 4 turns 4 parallel AWG22 control - 9 turns -3 parallel AWG28 27 UFN432 VCC 7 10F 20V 820pF 1N3613 2.5k
ISOLATION BOUNDARY
SPECIFICATIONS
Input line voltage: Input frequency: Switching frequency: Output power: Output voltage: Output current: Line regulation: Load regulation: Efficiency @ 25 W, VIN=90VAC: VIN=130VAC: Output short-circuit current: 70% 65% 2.5A average 90VAC to 130VAC 50 or 60Hz 40kHz10% 25W maximum 5V5% 2 to 5A 0.01%/V 8%/A* NOTE: This circuit uses a low-cost feedback scheme in which the DC voltage developed from the primary-side control winding is sensed by the UC3842 error amplifier. Load regulation is therefore dependent on the coupling between secondary and control windings, and on transformer leakage inductance. For applications requiring better load regulation, a UC1901 Isolated Feedback Generator can be used to directly sense the output voltage.
August 31, 1994
1106
Philips Semiconductors Linear Products
Product specification
Current-mode PWM controller
UC3842
SYNCHRONIZATION AND MAXIMUM DUTY CYCLE CLAMP
5V VCC 8 VCC 7
4 RA 7 RESET DISCH
RB 2 TRIG 6 THRESH
NE555
OUT
3
RT/CT 4 UC3842
5 GND GND 1 TO OTHER UC3842s
C NOTES: 1.44 f+ (R A ) 2R B) C RB RA ) 2R B
D
MAX
+
August 31, 1994
1107

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