CURRENT-MODE PWM CONTROLLERS FEATURES ORDERING INFORMATION BLOCK DIAGRAM

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Melissa Anabel Thompson
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1 CURRENT-MODE PWM CONTROLLERS The KA3842B/3B/4B/5B are fixed frequency current-mode PWM controller. They are specially designed for Off - Line and DC-to-DC converter applications with minimal 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. The KA3842B and KA3844B have UVLO thresholds of 16V (on) and 10V (off) The KA3843B and KA3845B are 8.5V (on) and 7.9V (off) The KA3842B and KA3843B can operate within 100% duty cycle. The KA3844B and KA3845B can operate with 50% duty cycle. 8 DIP 14 SOP FEATURES Low Start Up Current Maximum Duty Clamp U/V Lockout With Hysteresis Operating Frequency Up To 500KHz ORDERING INFORMATION Device Package Operating Temperature KA384XB 8 DIP 0 ~ +70Î KA384XBD 14 SOP 0 ~ + 70Î BLOCK DIAGRAM Rev. B 1999 Fairchild Semiconductor Corporation

2 ABSOLUTE MAXIMUM RATINGS Characteristic Symbol Value Unit Supply Voltage V CC 30 V Output Current I O ä1 A Analog Inputs (Pin 2.3) V (ANA) -0.3 to 6.3 V Error Amp Output Sink Current I SINK (E.A) 10 ma Power Dissipation (T A = 25Î) P D 1 W ELECTRICAL CHARACTERISTICS ( V CC=15V, R T=10K`, C T=3.3nF, T A= 0Î to +70Î, unless otherwise specified) Characteristic Symbol Test Conditions Min Typ Max Unit REFERENCE SECTION Reference Output Voltage V REF T J = 25Î, I REF = 1mA V Line Regulation LV REF 12V'V CC'25V 6 20 mv Load Regulation LV REF 1mA'I REF'20mA 6 25 mv Short Circuit Output Current I SC T A = 25Î ma OSCILLATOR SECTION Oscillation Frequency f T J = 25Î KHz Frequency Change with Voltage Lf/LV CC 12V'V CC'25V % Oscillator Amplitude V (OSC) 1.6 V P-P ERROR AMPLIFIER SECTION Input Bias Current I BIAS sa Input Voltage V I(E>A) V 1 = 2.5V V Open Loop Voltage Gain G VO 2V'V O'4V db Power Supply Rejection Ratio PSRR 12V'V CC'25V db Output Sink Current I SINK V 2 = 2.7V, V 1 = 1.1V 2 7 ma Output Source Current I SOURCE V 2 = 2.3V, V 1 = 5V ma High Output Voltage V OH V 2 = 2.3V, R L = 15K` to GND 5 6 V Low Output Voltage V OL V 2 = 2.7V, R L = 15K` to Pin V CURRENT SENSE SECTION Gain G V (Note 1 2) V/V Maximum Input Signal V I(MAX) V 1 = 5V(Note 1) V Power Supply Rejection Ratio PSRR 12V'V CC'25V (Note 1) 70 db Input Bias Current I BIAS sa

3 ELECTRICAL CHARACTERISTICS (Continued) (V CC=15V, R T=10K`, C T=3.3nF, T A= 0Î to +70Î unless otherwise specified) Characteristic Symbol Test Conditions Min Typ Max Unit OUTPUT SECTION Low Output Voltage High Output Voltage I SINK = 20mA V I SINK = 200mA V I SOURCE = 20mA V I SOURCE = 200mA V Rise Time t R T J = 25Î, C L= 1nF (Note 3) ns Fall Time t F T J = 25Î, C L= 1nF (Note 3) ns UNDER-VOLTAGE LOCKOUT SECTION Start Threshold KA3842B/44B V KA3843B/45B V Min. Operating Voltage V OPR(MIN) KA3842B/44B V (After Turn On) KA3843B/45BG V PWM SECTION Max. Duty Cycle KA3842B/43B % KA3844B/45B % Min. Duty Cycle D (MIN) 0 % TOTAL STANDBY CURRENT V OL V OH V TH(ST) D (max) Start-Up Current I ST ma Operating Supply Current I CC(OPR) V 3=V 2=ON ma Zener Voltage V Z I CC = 25mA V Adjust VCC above the start threshould before setting at 15V Note 1. Parameter measured at trip point of latch 2. Gain defined as: 7 LV 1 A = ; 0'V 3'0.8V LV 3 3.These parameters, although guaranteed, are not 100 tested in production.

4 Fig. 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 5K` potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to pin 3. Fig. 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. Fig. 3 Error Amp Configuration

5 Fig. 4 Current Sense Circuit Peak current (I S) is determined by the formula: I S(MAX) = 1.0V R S A small RC filter may be required to suppress switch transients. Fig. 5 Oscillator Waveforms and Maximum Duty Cycle Oscillator timing capacitor, C T, is charged by V REF through R T, and discharged by an internal current source. During the discharge time, the internal clock signal blanks the output to the low state. Selection of R T and C T therefore determines both oscillator frequency and maximum duty cycle. Charge and discharge times are determined by the formulas: t C = 0.55 R T C T R t D = R T C T ln T R T-4 Frequency, then, is: f=(t c + t d) -1 For RT>5K`, f = 1.8 R T C T

6 Fig. 6 Oscillator Dead Time & Frequency DEADTIME vs CT (RT>5K) Fig. 7 Timing Resistance vs Frequency Fig. 8 Shutdown Techniques FREQUENCY (Hz) Shutdown of the KA3842B 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. Fig. 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.

7 Pig. 10 TEMPERATURE DRIFT (Vref) Fig. 11 TEMPERATURE DRIFT (1st) KA3842B KA3B42B TEMP ERAT URE (Î) TEMP ERAT URE (Î) TEMPERATURE (Î) TEMPERATURE (Î) Fig. 12 TEMPERATURE DRIFT (Icc) KA3842B TEMPERATURE (Î)

8 TRADEMARKS The following are registered and unregistered trademarks Fairchild Semiconductor owns or is authorized to use and is not intended to be an exhaustive list of all such trademarks. DISCLAIMER ACEx CoolFET CROSSVOLT E 2 CMOS TM FACT FACT Quiet Series FAST FASTr GTO HiSeC ISOPLANAR MICROWIRE POP PowerTrench QS Quiet Series SuperSOT -3 SuperSOT -6 SuperSOT -8 TinyLogic UHC VCX FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. 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 FAIRCHILD SEMICONDUCTOR CORPORATION. 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, or (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 significant injury to the user. PRODUCT STATUS DEFINITIONS Definition of Terms 2. A critical component is 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. Datasheet Identification Product Status Definition Advance Information Preliminary No Identification Needed Formative or In Design First Production Full Production This datasheet contains the design specifications for product development. Specifications may change in any manner without notice. This datasheet contains preliminary data, and supplementary data will be published at a later date. Fairchild Semiconductor reserves the right to make changes at any time without notice in order to improve design. This datasheet contains final specifications. Fairchild Semiconductor reserves the right to make changes at any time without notice in order to improve design. Obsolete Not In Production This datasheet contains specifications on a product that has been discontinued by Fairchild semiconductor. The datasheet is printed for reference information only.

UC3842/UC3843/UC3844/UC3845

SMPS Controller www.fairchildsemi.com 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