L I N D O C # : 1844 SG1844/SG1845 Series CURRENT MODE PWM CONTROLLER T H E I N F I N I T E P O W E R I O F P N N O VA T I O N R O D U C T I O N DESCRIPTION The SG1844/45 family of control ICs provides all the necessary features to implement off-line fixed frequency, current-mode switching power supplies with a minimum number of external components. Current-mode architecture demonstrates improved line regulation, improved load regulation, pulse-bypulse current limiting and inherent protection of the power supply output switch. The bandgap reference is trimmed to ±1% over temperature. Oscillator discharge current is trimmed to less than ±10%. The SG1844/45 has undervoltage lockout, current-limiting circuitry OF and start-up current of less than 1mA. The totem-pole output is optimized to drive the gate of a power MOSFET. The output is low in the off state to provide direct interface to an N-channel device. Both operate up to a maximum duty cycle range of zero to <50% due to an internal toggle flip-flop which blanks the output off every other clock cycle. The SG1844/45 is specified for operation over the full military ambient temperature range of -55°C to 125°C. The SG2844/45 is specified for the industrial range of -25°C to 85°C, and the SG3844/45 is designed for the commercial range of 0°C to 70°C. S G 3 8 4 4 I N A F LY B A C K C O N V E R T E R S H E E T ■ OPTIMIZED FOR OFF-LINE CONTROL ■ LOW START-UP CURRENT (<1mA) ■ AUTOMATIC FEED FORWARD COMPENSATION ■ TRIMMED OSCILLATOR DISCHARGE CURRENT ■ PULSE-BY-PULSE CURRENT LIMITING ■ ENHANCED LOAD RESPONSE CHARACTERISTICS ■ UNDER-VOLTAGE LOCKOUT WITH 6V HYSTERESIS (SG1844 only) ■ DOUBLE PULSE SUPPRESSION ■ HIGH-CURRENT TOTEM-POLE OUTPUT ■ INTERNALLY TRIMMED BANDGAP REFERENCE ■ 500kHz OPERATION ■ UNDERVOLTAGE LOCKOUT SG1844 - 16 volts SG1845 - 8.4 volts ■ LOW SHOOT-THROUGH CURRENT <75mA OVER TEMPERATURE HIGH RELIABILITY FEATURES RST ■ AVAILABLE TO MIL-STD-883B ■ LINFINITY LEVEL "S" PROCESSING AVAILABLE IST AC INPUT A T A K E Y F E AT U R E S PRODUCT HIGHLIGHT T Y P I C A L A P P L I C AT I O N D VCC SG3844 TA (°C) 0 to 70 -25 to 85 -55 to 125 MIL-STD/883 DESC M Plastic DIP 8-pin N Plastic DIP 14-pin PA C K A G E O R D E R I N F O R M AT I O N Plastic SOIC Plastic SOIC Ceramic DIP Ceramic DIP DM 8-pin D 14-pin Y 8-pin J 14-pin Cer. Flatpack F 10-pin Ceramic LCC L 20-pin SG3844M SG3844N SG3844DM SG3844D SG3844Y SG3844J — — SG3845M SG3845N SG3845DM SG3845D SG3845Y SG3845J — — SG2844M SG2844N SG2844DM SG2844D SG2844Y SG2844J — — SG2845M SG2845N SG2845DM SG2845D SG2845Y SG2845J — — — — — — SG1844Y SG1844J — SG1844L — — — — SG1845Y SG1845J — SG1845L — — — — SG1844Y/883B SG1844J/883B — SG1844L/883B — — — — SG1845Y/883B SG1845J/883B — SG1845L/883B — — — — SG1844Y/DESC SG1844J/DESC SG1844F/DESC SG1844L/DESC — — SG1845Y/DESC SG1845J/DESC Note: All surface-mount packages are available in Tape & Reel. SG1845F/DESC SG1845L/DESC F O R F U R T H E R I N F O R M AT I O N C A L L ( 7 1 4 ) 8 9 8 - 8 1 2 1 Copyright © 2000 Rev. 1.4 4/00 11861 W ESTERN A VENUE , G ARDEN G ROVE , CA. 92841 1 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N A B S O L U T E M A X I M U M R AT I N G S D A T A S H E E T PACKAGE PIN OUTS (Notes 1 & 2) Supply Voltage (ICC < 30mA) ............................................................... Self Limiting Supply Voltage (Low Impedance Source) ........................................................ 30V Output Current (Peak) ....................................................................................... ±1A Output Current (Continuous) ....................................................................... 350mA Output Energy (Capacitive Load) ....................................................................... 5µJ Analog Inputs (Pins 2, 3) ................................................................. -0.3V to +6.3V Error Amp Output Sink Current ..................................................................... 10mA Operating Junction Temperature Hermetic (J, Y, F, L Packages) ................................................................... 150°C Plastic (N, M, D, DM Packages) ................................................................ 150°C Storage Temperature Range .......................................................... -65°C to +150°C Lead Temperature (Soldering, 10 Seconds) .................................................. 300°C COM VFB ISENSE RT/CT COM N.C. VFB N.C. ISENSE N.C. RT/CT 95°C/W COM N.C. VFB N.C. ISENSE N.C. RT/CT 165°C/W D PACKAGE: 120°C/W Y PACKAGE: 130°C/W 80°C/W F PACKAGE: THERMAL RESISTANCE-JUNCTION TO CASE, θ JC THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 1 8 2 7 3 6 4 5 VREF VCC OUT UT GND 1 14 2 13 3 12 4 11 5 10 6 9 7 8 VREF N.C. VCC VC OUT UT GND WR GND 80°C/W 145°C/W 1 14 2 13 3 12 4 11 5 10 6 9 7 8 VREF N.C. VCC VC OUT UT GROUND OWER GND J & N PACKAGE (Top View) J PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 5 65°C/W DM PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 6 4 D PACKAGE (Top View) N PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 3 VREF VCC OUT UT GND DM PACKAGE (Top View) M PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 7 COM VFB ISENSE RT/CT T H E R M A L D ATA THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 8 2 M & Y PACKAGE (Top View) Note 1. Exceeding these ratings could cause damage to the device. Note 2. All voltages are with respect to Pin 5. All currents are positive into the specified terminal. THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 1 1. COM 2. VFB 3. ISENSE 4. RT/CT 5. OWER GND 1 2 3 4 5 10.VREF 9. VCC 8. VC 7. OUT UT 6. GND 10 9 8 7 6 L PACKAGE: THERMAL RESISTANCE-JUNCTION TO CASE, θ JC THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA F PACKAGE (Top View) 35°C/W 120°C/W Junction Temperature Calculation: TJ = TA + (PD x θJA). The θJA numbers are guidelines for the thermal performance of the device/pc-board system. All of the above assume no ambient airflow. 3 2 1 20 19 4 18 5 17 6 16 7 15 8 14 9 10 11 12 13 1. N.C. 2. N.C. 3. COM . 4. N.C. 5. VFB 6. N.C. 7. ISENSE 8. RT/CT 9. N.C. 10. N.C. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. N.C. WR GND GND N.C. OUT UT N.C. VC VCC N.C. VREF L PACKAGE (Top View) 2 Copyright © 2000 Rev. 1.4 4/00 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P D R O D U C T I O N A T A S H E E T R E C O M M E N D E D O P E R AT I N G C O N D I T I O N S Parameter Recommended Operating Conditions Min. Typ. Max. Symbol Supply Voltage Range Output Current (Peak) Output Current (Continuous) Analog Inputs (Pin 2, Pin 3) Error Amp Output Sink Current Oscillator Frequency Range Oscillator Timing Resistor Oscillator Timing Capacitor Operating Ambient Temperature Range: SG1844/45 SG2844/45 SG3844/45 Note 3. Range over which the device is functional. (Note 3) Units 30 ±1 200 0.1 0.52 0.1 500 150 1.0 V A mA V mA kHz kΩ µF -55 -25 0 125 85 70 °C °C °C 0 2.6 5 RT CT ELECTRICAL CHARACTERISTICS (Unless otherwise specified, these specifications apply over the operating ambient temperatures for SG1844/SG1845 with -55°C ≤ TA ≤ 125°C, SG2844/ SG2845 with -25°C ≤ TA ≤ 85°C, SG3844/SG3845 with 0°C ≤ TA ≤ 70°C, VCC = 15V (Note 7), RT = 10kΩ, and CT = 3.3nF. Low duty cycle pulse testing techniques are used which maintains junction and case temperatures equal to the ambient temperature.) Parameter Symbol Test Conditions SG1844/45 SG2844/45 SG3844/45 Units Min. Typ. Max. Min. Typ. Max. Min. Typ. Max. Reference Section Output Voltage Line Regulation Load Regulation Temperature Stability (Note 4) Total Output Variation (Note 4) Output Noise Voltage (Note 4) Long Term Stability (Note 4) Output Short Circuit TJ = 25°C, IO = 1mA 12 ≤ VIN ≤ 25V 1 ≤ IO ≤ 20mA VN Line, Load, Temp. 10Hz ≤ f ≤ 10kHz, TJ = 25°C TA = 125°C, 1000hrs 4.95 5.00 5.05 4.95 5.00 5.05 4.90 5.00 5.10 6 20 6 20 6 20 6 25 6 25 6 25 0.2 0.4 0.2 0.4 0.2 0.4 4.90 5.10 4.90 5.10 4.82 5.18 50 50 50 5 25 5 25 5 25 -30 -100 -180 -30 -100 -180 -30 -100 -180 V mV mV mV/°C V µV mV mA Oscillator Section Initial Accuracy (Note 8) Voltage Stability Temperature Stability (Note 4) Amplitude Discharge Current TJ = 25°C 12V ≤ VCC ≤ 25V TMIN ≤ TA ≤ TMAX VRT/CT (Peak to Peak) TJ = 25°C TMIN ≤ TA ≤ TMAX 47 9.3 9.5 kHz % % V mA mA 2.45 2.50 2.55 2.45 2.50 2.55 2.42 2.50 2.58 -0.3 -1 -0.3 1 -0.3 -2 65 90 65 90 65 90 0.7 1 0.7 1 0.7 1 60 70 60 70 60 70 2 6 2 6 2 6 -0.5 -0.8 -0.5 -0.8 -0.5 -0.8 5 6 5 6 5 6 0.7 1.1 0.7 1.1 0.7 1.1 V µA dB MHz dB mA mA V V 7.8 6.8 52 .02 5 1.7 8.3 57 1 47 9.1 9.3 7.5 7.2 52 0.2 5 1.7 8.4 57 1 47 9.3 9.5 7.5 7.2 52 0.2 5 1.7 8.4 57 1 Error Amp Section Input Voltage Input Bias Current Open Loop Gain Unity Gain Bandwidth (Note 4) Power Supply Rejection Ratio Output Sink Current Output Source Current VOUT High VOUT Low VCOMP = 2.5V AVOL PSRR 2 ≤ VO ≤ 4V TJ = 25°C 12 ≤ VCC ≤ 25V VVFB = 2.7V, VCOMP = 1.1V VVFB = 2.3V, VCOMP = 5V VVFB = 2.3V, RL = 15K to gnd VVFB = 2.7V, RL = 15K to VREF (Electrical Characteristics continue next page.) Copyright © 2000 Rev. 1.4 4/00 3 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S H E E T ELECTRICAL CHARACTERISTICS Parameter Symbol Test Conditions (Cont'd.) SG1844/45 SG2844/45 SG3844/45 Units Min. Typ. Max. Min. Typ. Max. Min. Typ. Max. Current Sense Section Gain (Notes 5 & 6) Maximum Input Signal (Note 5) Power Supply Rejection Ratio (Note 5) PSRR Input Bias Current Delay to Output (Note 4) VCOMP = 5V 12 ≤ VCC ≤ 25V 2.85 0.9 3 1 70 -2 150 3.15 2.85 1.1 0.9 0.1 1.5 13.5 13.5 50 50 0.4 2.2 150 150 -10 300 3 1 70 -2 150 3.15 2.85 1.1 0.9 0.1 1.5 13.5 13.5 50 50 0.4 2.2 150 150 -10 300 3 1 70 -2 150 3.15 1.1 0.1 1.5 13.5 13.5 50 50 0.4 2.2 150 150 V V V V ns ns -10 300 V/V V dB µA ns Output Section Output Low Level Output High Level Rise Time (Note 4) Fall Time (Note 4) ISINK = 20mA ISINK = 200mA ISOURCE = 20mA ISOURCE = 200mA T J = 25°C, CL = 1nF TJ = 25°C, CL = 1nF 13 12 1844 1845 1844 1845 15 7.8 9 7.0 16 8.4 10 7.6 17 9.0 11 8.3 15 7.8 9 7.0 16 8.4 10 7.6 17 9.0 11 8.2 14.5 7.8 8.5 7.0 16 8.4 10 7.6 17.5 9.0 11.5 8.2 V V V V 46 48 50 0 46 48 50 0 46 48 50 0 % % 0.5 11 34 1 17 13 12 13 12 Under-Voltage Lockout Section Start Threshold Min. Operation Voltage After Turn-On PWM Section Maximum Duty Cycle Minimum Duty Cycle Power Consumption Section Start-Up Current Operating Supply Current VCC Zener Voltage VFB = VISENSE = 0V ICC = 25mA Notes: 4. These parameters, although guaranteed, are not 100% tested in production. 5. Parameter measured at trip point of latch with VVFB = 0. 4 0.5 1 0.5 1 mA 11 17 11 17 mA 34 34 V ∆ VCOMP 6. Gain defined as: A = ∆ V ; 0 ≤ VISENSE ≤ 0.8V. ISENSE 7. Adjust VCC above the start threshold before setting at 15V. 8. Output frequency equals one half of oscillator frequency. Copyright © 2000 Rev. 1.4 4/00 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S H E E T BLOCK DIAGRAM VCC* 34V UVLO GROUND** 16V (1844) 8.4 (1845) VREF 5.0V 50mA 5V REF S/R 6V (1844) 0.8V (1845) INTERNAL BIAS 2.5V VREF GOOD LOGIC RT/CT VC* OSCILLATOR T ERROR AMP VFB COMP CURRENT SENSE OUTPUT S 2R R R 1V PWM LATCH POWER GROUND** CURRENT SENSE COMPARATOR * - VCC and VC are internally connected for 8-pin packages. ** - POWER GROUND and GROUND are internally connected for 8-pin packages. Copyright © 2000 Rev. 1.4 4/00 5 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S GRAPH / CURVE INDEX H E E T FIGURE INDEX Characteristic Curves Application Information FIGURE # FIGURE # 1. DROPOUT VOLTAGE vs. TEMPERATURE 12. OSCILLATOR TIMING CIRCUIT 2. OSCILLATOR TEMPERATURE STABILITY 13. OSCILLATOR FREQUENCY vs. RT FOR VARIOUS CT 3. CURRENT SENSE TO OUTPUT DELAY vs. TEMPERATURE 4. START-UP CURRENT vs. TEMPERATURE 5. REFERENCE VOLTAGE vs. TEMPERATURE FIGURE # 6. START-UP VOLTAGE THRESHOLD vs. TEMPERATURE 14. CURRENT SENSE SPIKE SUPPRESSION 7. START-UP VOLTAGE THRESHOLD vs. TEMPERATURE 15. MOSFET PARASITIC OSCILLATIONS 8. OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE 16. BIPOLAR TRANSISTOR DRIVE 9. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND TEMPERATURE (SINK TRANSISTOR) 17. ISOLATED MOSFET DRIVE Typical Applications Section 10. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT 18. ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH SOFTSTART 11. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND TEMPERATURE (SOURCE TRANSISTOR) 19. EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYNCHRONIZATION 20. OSCILLATOR CONNECTION 21. ERROR AMPLIFIER CONNECTION 6 Copyright © 2000 Rev. 1.4 4/00 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S H E E T CHARACTERISTIC CURVES FIGURE 1. — DROPOUT VOLTAGE vs. TEMPERATURE FIGURE 2. — OSCILLATOR TEMPERATURE STABILITY VIN = 15V Duty Cycle = 50% 0 10.0 SG1844 Frequency Drift - (%) Minimum Operating Voltage - (V) 2 9.6 9.2 8.8 8.4 8.0 -75 -25 0 -4 -6 -8 -10 SG1845 -50 -2 25 50 75 100 -75 125 -50 FIGURE 3. — CURRENT SENSE TO OUTPUT DELAY vs. TEMPERATURE 25 50 75 100 125 FIGURE 4. — START-UP CURRENT vs. TEMPERATURE 220 0.7 SG1844 Start-Up Current - (mA) Current Sense Delay - (nS) 0 Junction Temperature - (°C) Junction Temperature - (°C) 200 180 VPIN3 = 1.1V 160 140 0.6 0.5 0.4 0.3 SG1845 120 -75 0.2 -50 -25 0 25 50 75 Junction Temperature - (°C) Copyright © 2000 Rev. 1.4 4/00 -25 100 125 -75 -50 -25 0 25 50 75 100 125 Junction Temperature - (°C) 7 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S H E E T CHARACTERISTIC CURVES FIGURE 5. — REFERENCE VOLTAGE vs. TEMPERATURE FIGURE 6. — START-UP VOLTAGE THRESHOLD vs. TEMPERATURE 5.02 8.32 SG1845 Reference Voltage - (V) Reference Voltage - (V) 5.01 VCC = 15V 5.00 4.99 8.30 8.28 8.26 8.24 8.22 4.98 8.20 -75 -50 -25 0 25 50 75 100 8.18 -75 125 -50 Junction Temperature - (°C) 50 75 100 125 Oscillator Discharge Current - (mA) 8.2 16.06 Start Up Voltage - (V) 25 FIGURE 8. — OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE 16.08 SG1844 16.04 16.02 16.00 15.98 8.0 7.8 7.6 7.4 7.2 -50 -25 0 25 50 75 Junction Temperature - (°C) 8 0 Junction Temperature - (°C) FIGURE 7. — START-UP VOLTAGE THRESHOLD vs. TEMPERATURE -75 -25 100 125 -75 -50 -25 0 25 50 75 100 125 Junction Temperature - (°C) Copyright © 2000 Rev. 1.4 4/00 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S H E E T CHARACTERISTIC CURVES FIGURE 9. — OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT & TEMPERATURE FIGURE 10. — CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT 1.0 2.5 C -55° °C +25 1.5 5°C +12 1.0 VIN = 15V Duty Cycle < 5% 0.5 0.8 12 5°C 25 °C -55 °C Saturation Voltage - (V) 2.0 Current Sense Threshold - (V) 0.9 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 100 200 300 400 500 Output Current - (mA) 1.0 2.0 3.0 4.0 5.0 Error Amp Output Voltage - (V) FIGURE 11. — OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT & TEMPERATURE VIN = 15V Duty Cycle < 5% Saturation Voltage - (V) 4.0 3.0 °C 25 +1 5°C +2 °C -55 °C +25 2.0 5°C +12 1.0 0 100 200 300 400 500 Output Current - (mA) Copyright © 2000 Rev. 1.4 4/00 9 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S H E E T A P P L I C AT I O N I N F O R M AT I O N OSCILLATOR The oscillator of the 1844/45 family of PWM's is programmed by the external timing components (RT, CT) as shown in Figure 13. VREF 100K RT - ( ) F 1n F 2n 2. F 7n 4. nF 10 nF 22 RT/CT F 0n 10 RT 47 nF 10K CT GND F≈ 1.86 RTC T where RT ≥ 5kΩ FIGURE 12 — OSCILLATOR TIMING CIRCUIT 10 1K 100 1K 10K 100K 1M Oscillator Frequency - (Hz) FIGURE 13 — OSCILLATOR FREQUENCY vs. RT FOR VARIOUS CT Copyright © 2000 Rev. 1.4 4/00 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D A T A S H E E T T Y P I C A L A P P L I C AT I O N C I R C U I T S Pin numbers referenced are for 8-pin package and pin numbers in parenthesis are for 14-pin package. VCC VIN VCC 7 (12) 7 (12) 7 (11) SG1844/45 VIN 7 (11) Q1 Q1 R1 6 (10) SG1844/45 6 (10) IPK 5 (8) R IPK(MAX) = 1.0V RS 3 (5) RS C 5 (8) RS 3 (5) FIGURE 14. — CURRENT SENSE SPIKE SUPPRESSION FIGURE 15. — MOSFET PARASITIC OSCILLATIONS The RC low-pass filter will eliminate the leading edge current spike caused by parasitics of Power MOSFET. A resistor (R1) in series with the MOSFET gate reduce overshoot and ringing caused by the MOSFET input capacitance and any inductance in series with the gate drive. (Note: It is very important to have a low inductance ground path to insure correct operation of the I.C. This can be done by making the ground paths as short and as wide as possible.) VCC IB VC + » R2 VIN VIN Isolation Boundary 7 (12) VC1 » R1|| R2 _ 7 (11) VC VC1 6 (10) VGS Waveforms 7 (11) R2 SG1844/45 Q1 6 (10) C1 R1 + 0 _ SG1844/45 50% DC 5 (8) Q1 + R 0 _ 3 (5) C 5 (8) 3 (5) RS FIGURE 16. — BIPOLAR TRANSISTOR DRIVE The 1844/45 output stage can provide negative base current to remove base charge of power transistor (Q1) for faster turn off. This is accomplished by adding a capacitor (C1) in parallel with a resistor (R1). The resistor (R1) is to limit the base current during turn on. Copyright © 2000 Rev. 1.4 4/00 IPK = V (PIN 1) - 1.4 3RS RS NS NP 25% DC N P NS FIGURE 17. — ISOLATED MOSFET DRIVE Current transformers can be used where isolation is required between PWM and Primary ground. A drive transformer is then necessary to interface the PWM output with the MOSFET. 11 PRODUCT DATABOOK 1996/1997 SG1844/SG1845 Series CURRENT-MODE PWM CONTROLLER P R O D U C T I O N D S A T A H E E T T Y P I C A L A P P L I C AT I O N C I R C U I T S VCC (continued) VIN 8 (14) 7 (12) 1N4148 IPK = SG1844/45 1 (1) R1 6 (10) 555 TIMER RB 3 4 (7) 5 (8) 2N2907 R R1 R2 VC R 1 R2 tSOFTSTART = -ln 1 C V2 R1+R 2 0.5 where: V2 = 1+ R1 2 1+ Softstart and adjustable peak current can be done with the external circuitry shown above. 5V 5 (9) f= 1.44 (RA + 2RB)C f= RB RA + 2RB To other SG1844/45 R1 R2 FIGURE 18. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH SOFTSTART 2.8V 1 RS 5 (9) 1.43 - 0.23 2 C 3 (5) V1 Where: 0 ≤ V1 ≤ 1.0V RS and V1 = SG1844/45 Q1 2 (3) C 4 6 7 (11) 4 (7) R2 8 RA 8 (14) FIGURE 19. — EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYNCHRONIZATION Precision duty cycle limiting for a duty cycle of <50%, as well as synchronizing several 1844/45's is possible with the above circuitry. 7 (11) 2.5V 1.1V RT SG1844/45 0.5mA 6 (10) CT SG1844/45 2 (3) Ri 1 (1) RF Discharge Current Id = 8.2mA FIGURE 20. — OSCILLATOR CONNECTION The oscillator is programmed by the values selected for the timing components RT and CT. Refer to application information for calculation of the component values. 12 RF ³ 10K FIGURE 21. — ERROR AMPLIFIER CONNECTION Error amplifier is capable of sourcing and sinking current up to 0.5mA. Copyright © 2000 Rev. 1.4 4/00