L I N D O C # : 1552 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM T I H E N F I N I T E P O W E R O F I P N N O VA T I O N R O D U C T I O N DESCRIPTION The LX155X family of ultra-low start-up current (250µA max.), current mode control IC's offer new levels of energy efficiency for offline converter applications. They are ideally optimized for personal computer and CRT power supplies although they can be used in any number of off-line applications where energy efficiency is critical. Coupled with the fact that the LX155X series requires a minimal set of external components, the series offers an excellent value for cost conscious consumer applications. Optimizing energy efficiency, the LX155X series demonstrates a significant power reduction as compared with other similar off-line controllers. Table 1 compares the SG384X, UC384XA and the LX155X start-up resistor power dissipation. The LX155X offers an overall 4X reduction in power dissipa- D S A T A H E E T K E Y F E AT U R E S ■ ULTRA-LOW START-UP CURRENT (150µA typ.) ■ TRIMMED OSCILLATOR DISCHARGE CURRENT (±2% typ.) ■ INITIAL OSCILLATOR FREQUENCY BETTER THAN ±4% ■ OUTPUT PULLDOWN DURING UVLO ■ PRECISION 2.5V REFERENCE (±2% max.) p CURRENT SENSE DELAY TO OUTPUT (150ns typ.) p AUTOMATIC FEED FORWARD COMPENSATION p PULSE-BY-PULSE CURRENT LIMITING p ENHANCED LOAD RESPONSE CHARACTERISTICS p UNDER-VOLTAGE LOCKOUT WITH HYSTERESIS p DOUBLE PULSE SUPPRESSION p HIGH CURRENT TOTEM POLE OUTPUT (±1Amp peak) p 500kHz OPERATION tion. Additionally, the precise oscillator discharge current gives the power supply designer considerable flexibility in optimizing system duty cycle consistency. The current mode architecture demonstrates improved load regulation, pulse by pulse current limiting and inherent protection of the power supply output switch. The LX155X includes a bandgap reference trimmed to 1%, an error amplifier, a current sense comparator internally clamped to 1V, a high current totem pole output stage for fast switching of power mosfet's, and an externally programmable oscillator to set operating frequency and maximum duty cycle. The undervoltage lock-out circuitry is designed to operate with as little as 250µA of supply current permitting very efficient bootstrap designs. A P P L I C AT I O N S PRODUCT HIGHLIGHT ■ ECONOMY OFF-LINE FLYBACK OR FORWARD CONVERTERS ■ DC-DC BUCK OR BOOST CONVERTERS ■ LOW COST DC MOTOR CONTROL T YPICAL A PPLICATION OF LX155X U SING I TS M ICRO P OWER S TART -U P F EATURE TABLE 1 R ST I ST AC INPUT VCC LX1552 or LX1554 Design Using A VA I L A B L E O P T I O N S SG384x UC384xA LX155x Max. Start-up Current 1000µA 500µA Specification (IST) Typical Start-Up Resistor Value (RST) 62KΩ Ω Part # 250µA Ω 124KΩ Ω 248KΩ Max. Start-Up Resistor 2.26W 1.13W 0.56W Power Dissipation (PR) Note: Calculation is done for universal AC input specification of V ACMIN= 90VRMS to VACMAX= 265V RMS using the following equation: (Resistor curr ent is selected to be 2 * I ST at V ACMIN.) RST = PER P A RT # Start-Up Hysteresis Max. Duty Voltage Cycle LX1552 16V 6V <100% LX1553 8.4V 0.8V <100% LX1554 16V 6V <50% LX1555 8.4V 0.8V <50% VAC MIN 2V AC2 MAX , PR = √2 * IST RST PA C K A G E O R D E R I N F O R M AT I O N DIP M Plastic 8-pin TA (°C) SOIC DM Plastic 8-pin SOIC D Plastic 14-pin DIP Y Ceramic 8-pin PW TSSOP 20-pin 0 to 70 LX155xCM LX155xCDM LX155xCD — LX155xCPW -40 to 85 -55 to 125 LX155xIM — LX155xIDM — LX155xID — — LX155xMY — — Note: All surface-mount packages are available in Tape & Reel. Append the letter "T" to part number. (i.e. LX1552CDMT) 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 © 1994 Rev. 1.0a 1/01 11861 WESTERN A VENUE , G ARDEN G ROVE , CA. 92841 1 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM 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 (Note 1) Supply Voltage (Low Impedance Source) .................................................................. 30V Supply Voltage (ICC < 30mA) ......................................................................... Self Limiting Output Current ............................................................................................................. ±1A Output Energy (Capacitive Load) ................................................................................ 5µJ Analog Inputs (Pins 2, 3) ........................................................................... -0.3V to +6.3V Error Amp Output Sink Current ............................................................................... 10mA Power Dissipation at TA = 25°C (DIL-8) ...................................................................... 1W Operating Junction Temperature Ceramic (Y Package) ............................................................................................ 150°C Plastic (M, DM, D, PW Packages) ........................................................................ 150°C Storage Temperature Range .................................................................... -65°C to +150°C Lead Temperature (Soldering, 10 Seconds) ............................................................ 300°C COMP VFB ISENSE RT/CT M PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA 95°C/W DM PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA 165°C/W 6 4 5 1 8 2 7 3 6 4 5 VREF VCC OUTPUT GND DM PACKAGE (Top View) COMP N.C. VFB N.C. ISENSE N.C. RT/CT 1 14 2 13 3 12 4 11 5 10 6 9 7 8 VREF N.C. VCC VC OUTPUT GND PWR GND 120°C/W 130°C/W PW PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA 3 VREF VCC OUTPUT GND D PACKAGE (Top View) Y PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA 7 COMP VFB ISENSE RT/CT D PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA 8 2 M & Y PACKAGE (Top View) Note 1. Exceeding these ratings could cause damage to the device. All voltages are with respect to Ground. Currents are positive into, negative out of the specified terminal. Pin numbers refer to DIL packages only. T H E R M A L D ATA 1 144°C/W Junction Temperature Calculation: TJ = TA + (PD x θJA). The θ JA numbers are guidelines for the thermal performance of the device/pc-board system. All of the above assume no ambient airflow N.C. N.C. COMP VFB N.C. ISENSE N.C. RT/CT N.C. N.C. 1 20 2 19 3 18 4 17 5 16 6 15 7 14 8 13 9 12 10 11 N.C. N.C. VREF N.C. VCC VC OUTPUT GND PWR GND N.C. PW PACKAGE (Top View) 2 Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N ELECTRICAL D A T A S H E E T CHARACTERISTICS (Unless otherwise specified, these specifications apply over the operating ambient temperatures for LX155xC with 0°C ≤ T A ≤ 70°C, LX155xI with -40°C ≤ TA ≤ 85°C, LX155xM with -55°C ≤ T A ≤ 125°C; VCC=15V (Note 5); RT=10K; 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 LX155xI/155xM LX155xC Units Min. Typ. Max. Min. Typ. Max. Reference Section Output Voltage Line Regulation Load Regulation Temperature Stability (Note 2 & 7) Total Output Variation Output Noise Voltage (Note 2) Long Term Stability (Note 2) Output Short Circuit V REF VN TA = 25°C, IL = 1mA 12 ≤ VIN ≤ 25V 1 ≤ IO ≤ 20mA 4.95 5.00 5.05 4.95 5.00 5.05 6 20 6 20 6 25 6 25 0.2 0.4 0.2 0.4 4.9 5.1 4.9 5.1 50 50 5 25 5 25 -30 -100 -180 -30 -100 -180 V mV mV mV/°C V µV mV mA TA = 25°C TA = 25°C, R T = 698Ω, CT = 22nF, LX1552/3 only 12 ≤ VCC ≤ 25V TMIN ≤ TA ≤ TMAX VPIN 4 peak to peak TA = 25°C, VPIN 4 = 2V VPIN 4 = 2V, TMIN ≤ TA ≤ TMAX 48.5 50.5 52.5 48.5 50.5 52.5 56 58 60 56 58 60 0.2 1 0.2 1 5 5 1.7 1.7 8.0 8.3 8.6 8.0 8.3 8.6 7.6 8.8 7.8 8.8 kHz kHz % % V mA mA VPIN 1 = 2.5V 2.45 2.50 2.55 2.45 2.50 2.55 -0.1 -1 -0.1 -0.5 65 90 65 90 0.6 0.6 60 70 60 70 2 4 2 4 -0.5 -0.8 -0.5 -0.8 5 6.5 5 6.5 0.7 1.1 0.7 1.1 V µA dB MHz dB mA mA V V Over Line, Load, and Temperature 10Hz ≤ f ≤ 10kHz, TA = 25°C TA = 125°C, t = 1000hrs ISC Oscillator Section Initial Accuracy (Note 6) Voltage Stability Temperature Stability (Note 2) Amplitude (Note 2) Discharge Current ID Error Amp Section Input Voltage Input Bias Current Open Loop Gain Unity Gain Bandwidth (Note 2) Power Supply Rejection Ratio (Note 3) Output Sink Current Output Source Current Output Voltage High Level Output Voltage Low Level IB AVOL UGBW PSRR IOL IOH V OH V OL 2 ≤ VO ≤ 4V TA = 25°C 12 ≤ VCC ≤ 25V VPIN 2 = 2.7V, VPIN 1 = 1.1V VPIN 2 = 2.3V, VPIN 1 = 5V VPIN 2 = 2.3V, RL = 15K to ground VPIN 2 = 2.7V, RL = 15K to VREF Current Sense Section Gain (Note 3 & 4) Maximum Input Signal (Note 3) Power Supply Rejection Ratio (Note 3) Input Bias Current Delay to Output (Note 2) AVOL PSRR IB TPD 3.15 2.85 1.1 0.9 VPIN 3 = 0 to 2V 3 1 70 -2 150 ISINK = 20mA ISINK = 200mA ISOURCE = 20mA ISOURCE = 200mA TA = 25°C, CL = 1nF TA = 25°C, CL = 1nF VCC = 5V, ISINK = 10mA 0.1 1.5 13.5 13.5 50 50 0.7 0.4 2.2 VPIN 1 = 5V 12 ≤ VCC ≤ 25V 2.85 0.9 -10 300 3 1 70 -2 150 3.15 1.1 0.1 1.5 13.5 13.5 50 50 0.7 0.4 2.2 -5 300 V/V V dB µA ns Output Section Output Voltage Low Level V OL Output Voltage High Level V OH Rise Time (Note 2) Fall Time (Note 2) UVLO Saturation TR TF VSAT 13 12 13 12 100 100 1.2 100 100 1.2 V V V V ns ns V ( E l e c tr i c a l Cha r a ct er i st i cs cont i nu e next pa g e.) Copyright © 1994 Rev. 1.0a 1/01 3 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P D R O D U C T I O N A T A S H E E T ELECTRICAL CHARACTERISTICS Parameter Symbol (Con't.) LX155xI/155xM LX155xC Units Min. Typ. Max. Min. Typ. Max. Test Conditions Under-Voltage Lockout Section Start Threshold VST Min. Operation Voltage After Turn-On 1552/1554 1553/1555 1552/1554 1553/1555 15 7.8 9 7.0 16 8.4 10 7.6 1552/1553 1552/1553, RT = 698Ω, CT = 22nF 1554/1555 94 96 50 48 17 9.0 11 8.2 15 7.8 9 7.0 16 8.4 10 7.6 94 96 50 48 17 9.0 11 8.2 V V V V 0 % % % % PWM Section Maximum Duty Cycle 47 47 0 Minimum Duty Cycle Power Consumption Section Start-Up Current Operating Supply Current VCC Zener Voltage IST I CC VZ ICC = 25mA 30 Notes: 2. These parameters, although guaranteed, are not 100% tested in production. 3. Parameter measured at trip point of latch with VFB = 0. ∆V 4. Gain defined as: A = ∆ V COMP ; 0 ≤ VISENSE ≤ 0.8V. ISENSE 5. Adjust VCC above the start threshold before setting at 15V. 6. Output frequency equals oscillator frequency for the LX1552 and LX1553. Output frequency is one half oscillator frequency for the LX1554 and LX1555. 150 11 35 250 17 30 150 11 35 250 17 µA mA V 7. Temperature stability, sometimes referred to as average temperature coefficient, is described by the equation: Temp Stability = V REF (max.) - VREF (min.) TA (max.) - TA (min.) V REF (max.) & V REF (min.) are the maximum & minimum reference voltage measured over the appropriate temperature range. Note that the extremes in voltage do not necessarily occur at the extremes in temperature. BLOCK DIAGRAM VCC* 34V UVLO S/R GROUND** 16V (1552/1554) 8.4V (1553/1555) 5V REF VREF 16V (1552/1554) 8.4V (1553/1555) INTERNAL BIAS VREF GOOD LOGIC RT/CT VC* OSCILLATOR T *** ERROR AMP VFB COMP ISENSE 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. *** - Toggle flip flop used only in 1554 and 1555. 4 Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM 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 Theory of Operation Section FIGURE # FIGURE # 1. OSCILLATOR FREQUENCY vs. TIMING RESISTOR 23. TYPICAL APPLICATION OF START-UP CIRCUITRY 2. MAXIMUM DUTY CYCLE vs. TIMING RESISTOR 24. REFERENCE VOLTAGE vs. TEMPERATURE 3. OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE 25. SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION 4. OSCILLATOR FREQUENCY vs. TEMPERATURE 26. DUTY CYCLE VARIATION vs. DISCHARGE CURRENT 5. OUTPUT INITIAL ACCURACY vs. TEMPERATURE 27. OSCILLATOR FREQUENCY vs. TIMING RESISTOR 6. OUTPUT DUTY CYCLE vs. TEMPERATURE 28. MAXIMUM DUTY CYCLE vs. TIMING RESISTOR 7. REFERENCE VOLTAGE vs. TEMPERATURE 29. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT 8. REFERENCE SHORT CIRCUIT CURRENT vs. TEMPERATURE 9. E.A. INPUT VOLTAGE vs. TEMPERATURE Typical Applications Section 10. START-UP CURRENT vs. TEMPERATURE FIGURE # 11. START-UP CURRENT vs. SUPPLY VOLTAGE 30. CURRENT SENSE SPIKE SUPPRESSION 12. START-UP CURRENT vs. SUPPLY VOLTAGE 31. MOSFET PARASITIC OSCILLATIONS 13. DYNAMIC SUPPLY CURRENT vs. OSCILLATOR FREQUENCY 14. CURRENT SENSE DELAY TO OUTPUT vs. TEMPERATURE 32. ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH SOFT-START 15. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT 33. EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYCHRONIZATION 16. START-UP THRESHOLD vs. TEMPERATURE 34. SLOPE COMPENSATION 17. START-UP THRESHOLD vs. TEMPERATURE 35. OPEN LOOP LABORATORY FIXTURE 18. MINIMUM OPERATING VOLTAGE vs. TEMPERATURE 36. OFF-LINE FLYBACK REGULATOR 19. MINIMUM OPERATING VOLTAGE vs. TEMPERATURE 20. LOW LEVEL OUTPUT SATURATION VOLTAGE DURING UNDERVOLTAGE LOCKOUT 21. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE 22. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE Copyright © 1994 Rev. 1.0a 1/01 5 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A CHARACTERISTIC FIGURE 1. — OSCILLATOR FREQUENCY vs. TIMING RESISTOR H E E T C U RV E S FIGURE 2. — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR 100 1000 CT = 1nF 90 CT = 3.3nF 80 100 Maximum Duty Cycle - (%) Oscillator Frequency - (kHz) S CT = 6.8nF 10 CT = 22nF CT = 47nF 1 CT = 0.1µF 0.1 0.1 VCC = 15V TA = 25°C 70 60 50 40 30 20 VCC = 15V TA = 25°C 10 10 1 0 0.1 100 1 FIGURE 3. — OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE FIGURE 4. — OSCILLATOR FREQUENCY vs. TEMPERATURE 55 VCC = 15V VPIN4 = 2V 8.40 VCC = 15V RT = 10k CT = 3.3nF 54 Oscillator Frequency - (KHz) (Id) Oscillator Discharge Current - (mA) 8.50 8.30 8.20 8.10 8.00 7.90 7.80 53 52 51 50 49 48 47 46 -50 -25 0 25 50 75 100 (TA) Ambient Temperature - (°C) 6 100 (RT) Timing Resistor - (k ) (RT) Timing Resistor - (k ) 7.70 -75 10 125 45 -75 -50 -25 0 25 50 75 100 125 (TA) Ambient Temperature - (°C) Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A CHARACTERISTIC FIGURE 5. — OUTPUT INITIAL ACCURACY vs. TEMPERATURE C U RV E S 48 LX1552 and LX1553 only VCC = 15V RT = 698W CT = 22nF 62.0 60.5 59.0 57.5 56.0 54.5 46 45 44 43 42 53.0 41 51.5 50.0 -75 -50 -25 0 25 50 75 100 40 -75 125 (TA) Ambient Temperature - (°C) (VREF) Reference Voltage - (V) (ISC) Reference Short Circuit Current - (mA) VCC = 15V IL = 1mA 5.01 5.00 4.99 4.98 4.97 4.96 4.95 -75 -50 -25 0 25 50 75 100 (TA) Ambient Temperature - (°C) -25 0 25 50 75 100 125 FIGURE 8. — REFERENCE SHORT CIRCUIT CURRENT vs. TEMPERATURE 5.03 5.02 -50 (TA) Ambient Temperature - (°C) FIGURE 7. — REFERENCE VOLTAGE vs. TEMPERATURE Copyright © 1994 Rev. 1.0a 1/01 VCC = 15V RT = 698W CT = 22nF 47 Output Duty Cycle - (%) Output Initial Accuracy - (kHz) H E E T FIGURE 6. — OUTPUT DUTY CYCLE vs. TEMPERATURE 65.0 63.5 S 125 180 165 150 135 120 105 90 75 60 45 30 -75 -50 -25 0 25 50 75 100 125 (TA) Ambient Temperature - (°C) 7 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A CHARACTERISTIC FIGURE 9. — E.A. INPUT VOLTAGE vs. TEMPERATURE H E E T C U RV E S FIGURE 10. — START-UP CURRENT vs. TEMPERATURE 2.55 250 VCC = 15V 2.54 225 (IST) Start-Up Current - (µA) 2.53 E.A. Input Voltage - (V) S 2.52 2.51 2.50 2.49 2.48 2.47 2.46 200 LX1552/LX1554 175 150 125 100 LX1553/LX1555 75 50 25 2.45 -75 -50 -25 0 25 50 75 100 0 -75 125 -50 (TA) Ambient Temperature - (°C) 25 50 75 100 125 FIGURE 12. — START-UP CURRENT vs. SUPPLY VOLTAGE 250 250 LX1553/LX1555 TA = 25°C 225 LX1552/LX1554 TA = 25°C 225 200 (IST) Start-Up Current - (µA) (IST) Start-Up Current - (µA) 0 (TA) Ambient Temperature - (°C) FIGURE 11. — START-UP CURRENT vs. SUPPLY VOLTAGE 175 150 125 100 75 50 25 200 175 150 125 100 75 50 25 0 0 0 2 4 6 8 10 12 14 16 (VCC) Supply Voltage - (V) 8 -25 18 20 0 1 2 3 4 5 6 7 8 9 10 (VCC) Supply Voltage - (V) Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A CHARACTERISTIC FIGURE 13. — DYNAMIC SUPPLY CURRENT vs. OSCILLATOR FREQUENCY H E E T C U RV E S FIGURE 14. — CURRENT SENSE DELAY TO OUTPUT vs. TEMPERATURE 30 300 TA = 25°C RT = 10k CL = 1000pF 27 24 270 (Tpd) C.S. Delay to Output - (ns) (ICC) Dynamic Supply Current - (mA) S VIN = 16V VIN = 12V VIN = 10V 21 18 15 12 9 6 240 210 180 150 120 90 60 30 3 0 -75 0 10 1000 100 FIGURE 15. — CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT -25 0 25 50 75 100 125 FIGURE 16. — START-UP THRESHOLD vs. TEMPERATURE 8.8 1.1 LX1553 LX1555 8.7 1.0 TA = 125°C 0.9 8.6 Start-Up Trheshold - (V) Current Sense Threshold - (V) -50 (TA) Ambient Temperature - (°C) Oscillator Frequency - (kHz) 0.8 TA = 25°C 0.7 0.6 TA = -55°C 0.5 0.4 0.3 8.5 8.4 8.3 8.2 8.1 8.0 0.2 7.9 0.1 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Error Amplifier Output Voltage - (V) Copyright © 1994 Rev. 1.0a 1/01 VCC = 15V VPIN3 = 0V to 2V CL = 1nF 5.0 7.8 -75 -50 -25 0 25 50 75 100 125 (TA) Ambient Temperature - (°C) 9 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A CHARACTERISTIC FIGURE 17. — START-UP THRESHOLD vs. TEMPERATURE C U RV E S 11.0 LX1552 LX1554 16.6 16.4 16.2 16.0 15.8 15.6 15.4 10.6 10.4 10.2 10.0 9.8 9.6 9.4 9.2 15.2 -50 -25 0 25 50 75 100 9.0 -75 125 FIGURE 19. — MINIMUM OPERATING VOLTAGE vs. TEMPERATURE 0 25 50 75 100 125 1.20 (VSAT) Output Saturation Voltage - (V) LX1553 LX1555 7.9 Minimum Operating Voltage - (V) -25 FIGURE 20. — LOW LEVEL OUTPUT SATURATION VOLTAGE DURING UNDER-VOLTAGE LOCKOUT 8.0 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 -50 -25 0 25 50 75 100 (TA) Ambient Temperature - (°C) 10 -50 (TA) Ambient Temperature - (°C) (TA) Ambient Temperature - (°C) 7.0 -75 LX1552 LX1554 10.8 Minimum Operating Voltage - (V) 16.8 Start-Up Trheshold - (V) H E E T FIGURE 18. — MINIMUM OPERATING VOLTAGE vs. TEMPERATURE 17.0 15.0 -75 S 125 1.08 0.96 0.84 VCC = 5V TA = -55°C TA = 25°C 0.72 0.60 TA = 125°C 0.48 0.36 0.24 0.12 0.00 0.1 1 10 Output Sink Current - (mA) Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A CHARACTERISTIC FIGURE 21. — OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE C U RV E S 6.00 VCC = 5V Sink Transistor 5.0 4.0 3.0 TA = -55°C 2.0 TA = 25°C TA = 125°C 1.0 0.00 (VSAT) Output Saturation Voltage - (V) (VSAT) Output Saturation Voltage - (V) H E E T FIGURE 22. — OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE 6.0 VCC = 15V Source Transistor 5.40 4.80 4.20 3.60 3.00 2.40 TA = 25°C 1.80 TA = -55°C 1.20 TA = 125°C 0.60 0.00 10 100 Output Sink Current - (mA) Copyright © 1994 Rev. 1.0a 1/01 S 1000 10 100 1000 Output Source Current - (mA) 11 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A S H E E T THEORY OF OPERATION IC DESCRIPTION The LX1552/3/4/5 series of current mode PWM controller IC's are designed to offer substantial improvements in the areas of startup current and oscillator accuracy when compared to the first generation products, the UC184x series. While they can be used in most DC-DC applications, they are optimized for single-ended designs such as Flyback and Forward converters. The LX1552/ 54 series are best suited for off-line applications, whereas the 1553/55 series are mostly used in power supplies with low input voltages. The IC can be divided into six main sections as shown in the Block Diagram (page 4): undervoltage lockout and startup circuit; voltage reference; oscillator; current sense comparator and PWM latch; error amplifier; and the output stage. The operation of each section is described in the following sections. The differences between the members of this family are summarized in Table 1. TABLE 1 UVLO Start-up Voltage Hysterises Voltage (VHYS) (VST) PART # LX1552 LX1553 LX1554 LX1555 16V 8.4V 16V 8.4V 6V 0.8V 6V 0.8V MAXIMUM DUTY CYCLE <100% <100% <50% <50% The start-up capacitor (C1) is charged by current through resistor (R1) minus the start-up current. Resistor (R1) is designed such that it provides more than 250µA of current (typically 2x IST(max) ). Once this voltage reaches the start-up threshold, the IC turns on, starting the switching cycle. This causes an increase in IC operating current, resulting in discharging the start-up capacitor. During this time, the auxiliary winding flyback voltage gets rectified & filtered via (D1) and (C1) and provides sufficient voltage to continue to operate the IC and support its required supply current. The start-up capacitor must be large enough such that during the discharge period, the bootsrap voltage exceeds the shutdown threshold of the IC. Table 2 below shows a comparison of start-up resistor power dissipation vs. maximum start-up current for different devices. TABLE 2 Design Using SG384x UC384xA LX155x Max. Start-up Current Specification (IST ) 1000µA 500µA 250µA Typical Start-Up Resistor Value (RST ) 62KΩ Ω 124KΩ Ω 248KΩ Ω Max. Start-Up Resistor Power Dissipation (PR) 2.26W 1.13W 0.56W UNDERVOLTAGE LOCKOUT The LX155x undervoltage lock-out is designed to maintain an ultra low quiescent current of less than 250µA, while guaranteeing the IC is fully functional before the output stage is activated. Comparing this to the SG384x series, a 4x reduction in start-up current is achieved resulting in 75% less power dissipation in the start-up resistor. This is especially important in off-line power supplies which are designed to operate for universal input voltages of 90 to 265V AC. Figure 23 shows an efficient supply voltage using the ultra low start-up current of the LX1554 in conjunction with a bootstrap winding off of the power transformer. Circuit operation is as follows. (Resistor R1 is designed such that it provides 2X maximum start-up current under low line conditions. Maximum power dissipation is calculated under maximum line conditions. Example assumes 90 to 265VAC universal input application.) DC BUS I1 > 250µA D1 1ST < 250µA VIN REF C1 RT LX1554 VO RT/CT CT GND RS GND FIGURE 23 — TYPICAL APPLICATION OF START-UP CIRCUITRY 12 Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D S A T A H E E T T H E O R Y O F O P E R AT I O N VOLTAGE REFERENCE REF The voltage reference is a low drift bandgap design which provides +5.0V to supply charging current to the oscillator timing capacitor, as well as supporting internal circuitries. Initial accuracy for all devices are specified at ±1% max., which is a 2x improvement for the commercial product when compared to the SG384x series. The reference is capable of providing in excess of 20mA for powering any external control circuitries and has built-in short circuit protection. 5V VP IR VV RT 2.8V 1.1V TO OUTPUT STAGE RT/CT A1 S1 5.03 2 CT VCC = 15V IL = 1mA 5.02 ID = 8.3mA 1 OPEN 5.01 5.00 FIGURE 25 — SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION 4.99 4.98 4.97 4.96 4.95 -75 -50 -25 0 25 50 75 100 125 (TA) Ambient Temperature - (°C) FIGURE 24 — REFERENCE VOLTAGE vs. TEMPERATURE OSCILLATOR The oscillator circuit is designed such that discharge current and valley voltage are trimmed independently. This results in more accurate initial oscillator frequency and maximum output duty cycle, especially important in LX1552/53 applications. The oscillator is programmed by the values selected for the timing components (RT) and (CT). A simplified schematic of the oscillator is shown in Figure 25. The operation is as follows; Capacitor (CT) is charged from the 5V reference thru resistor (RT) to a peak voltage of 2.7V nominally. Once the voltage reaches this threshold, comparator (A1) changes state, causing (S1) to switch to position (2) and (S2) to (VV) position. This will allow the capacitor to discharge with a current equal to the difference between a constant discharge current (ID) and current through charging resistor (IR), until the voltage drops down to 1V nominally and the comparator changes state again, repeating the cycle. Oscillator charge time results in the output to be in a high state (on time) and discharge time sets it to a low state (off time). Since the oscillator period is the sum of the charge and discharge time, any variations in either of them will ultimately affect stability of the output frequency and the maximum duty cycle. In fact, this Copyright © 1994 Rev. 1.0a 1/01 variation is more pronounced when maximum duty cycle has to be limited to 50% or less. This is due to the fact that for longer output off time, capacitor discharge current (ID - IR) must be decreased by increasing IR. Consequently, this increases the sensitivity of the frequency and duty cycle to any small variations of the internal current source (ID), making this parameter more critical under those conditions. Because this is a desired feature in many applications, this parameter is trimmed to a nominal current value of 8.3±0.3mA at room temperature, and guaranteed to a maximum range of 7.8 to 8.8mA over the specified ambient temperature range. Figure 26 shows variation of oscillator duty cycle versus discharge current for LX155x and SG384x series devices. 100 90 Oscillator Duty Cycle - (%) (VREF) Reference Voltage - (V) S2 80 TA = 25°C VP = 2.7V V = 1V VREF = 5V Id = 9.3mA Id = 8.6mA 70 60 SG384x Upper Limit 50 Id = 8.0mA 40 LX155x Limits 30 Id = 7.5mA SG384x Lower Limit 20 600 700 800 900 1000 (RT) Timing Resistor - ( ) FIGURE 26 — DUTY CYCLE VARIATION vs. DISCHARGE CURRENT 13 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A S H E E T THEORY OF OPERATION OSCILLATOR (continued) The oscillator is designed such that many values of RT and CT will give the same frequency, but only one combination will yield a specific duty cycle at a given frequency. A set of charts as well as the timing equations are given to determine approximate values of timing components for a given frequency and duty cycle. Given: frequency ≅ f; maximum duty-cycle ≅ Dm Calculate: 1) RT = 277 (1.74) 1000 1 Dm (1.74) -1 1-Dm Dm (Ω), 0.3 ≤ Dm ≤ 0.95 -1 Oscillator Frequency - (kHz) CT = 1nF Note: RT must always be greater than 520Ω for proper operation of oscillator circuit. CT = 3.3nF 100 CT = 6.8nF 2) 10 CT = 1.81 * Dm (µf) f * RT for duty cycles above 95% use: CT = 22nF CT = 47nF 1 3) f ≈ 1.81 R TC T where RT ≥ 5kΩ CT = 0.1µF 0.1 0.1 VCC = 15V TA = 25°C 1 10 100 Example: A flyback power supply design requires the duty cycle to be limited to less than 45%. If the output switching frequency is selected to be 100kHz, what are the values of RT and C T for the a) LX1552/53, and the b) LX1554/55 ? a) LX1552/53 (RT) Timing Resistor - (k ) FIGURE 27 — OSCILLATOR FREQUENCY vs. TIMING RESISTOR Given: f = 100kHz Dm = 0.45 100 RT = 267 90 (1.74) 80 Maximum Duty Cycle - (%) (1.74) 70 CT = 60 50 1 .45 .55 .45 -1 = 669Ω -1 1.81 * 0.45 = .012 µf 100x10 3 * 669 b) LX1554/55 40 fOUT = ½ fOSC (due to internal flip flop) fOSC = 200kHz 30 20 0 0.1 select CT = 1000pf using Figure 27 or Equation 3: RT = 9.1k VCC = 15V TA = 25°C 10 1 10 100 (RT) Timing Resistor - (k ) FIGURE 28 — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR 14 Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P R O D U C T I O N D A T A S H E E T T H E O R Y O F O P E R AT I O N CURRENT SENSE COMPARATOR AND PWM LATCH ERROR AMPLIFIER Switch current is sensed by an external sense resistor (or a current transformer), monitored by the C.S. pin and compared internally with voltage from error amplifier output. The comparator output resets the PWM latch ensuring that a single pulse appears at the output for any given oscillator cycle. The LX1554/55 series has an additional flip flop stage that limits the output to less than 50% duty cycle range as well as dividing its output frequency to half of the oscillator frequency. The current sense comparator threshold is internally clamped to 1V nominally which would limit peak switch current to: VZ (1) ISP = where: ISP ≡ Peak switch current RS VZ ≡ internal zener 0.9V ≤ VZ ≤ 1.1V The error amplifier has a PNP input differential stage with access to the Inverting input and the output pin. The N.I. input is internally biased to 2.5 volts and is not available for any external connections. The maximum input bias current for the LX155XC series is 0.5µA, while LX155XI/155XM devices are rated for 1µA maximum over their specified range of ambient temperature. Low value resistor dividers should be used in order to avoid output voltage errors caused by the input bias current. The error amplifier can source 0.5mA and sink 2mA of current. A minimum feedback resistor (RF) value of is given by: Equation 1 is used to calculate the value of sense resistor during the current limit condition where switch current reaches its maximum level. In normal operation of the converter, the relationship between peak switch current and error voltage (voltage at pin 1) is given by: VE - 2VF (1) ISP = VE ≡ Voltage at pin 1 VF ≡ Diode - Forward voltage 0.7V at TA = 25°C where: 3 * RS The above equation is plotted in Figure 29. Notice that the gain becomes non-linear above current sense voltages greater than ≈ 0.95 volts. It is therefore recommended to operate below this range during normal operation. This would insure that the overall closed loop gain of the system will not be affected by the change in the gain of the current sense stage. 1.1 RFMIN = 3(1.1) + 1.8 ≈ 10K 0.5mA OUTPUT STAGE The output section has been specifically designed for direct drive of power MOSFETs. It has a totempole configuration which is capable of high peak current for fast charging and discharging of external MOSFET gate capacitance. This typically results in a rise and fall time of 50ns for a 1000pf capacitive load. Each output transistor (source and sink) is capable of supplying 200mA of continuous current with typical saturation voltages versus temperature as shown in Figures 21 & 22 of the characteristic curve section. All devices are designed to minimize the amount of shoot-thru current which is a result of momentary overlap of output transistors. This allows more efficient usage of the IC at higher frequencies, as well as improving the noise susceptibility of the device. Internal circuitry insures that the outputs are held off during VCC ramp-up. Figure 20, in the characteristic curves section, shows output sink saturation voltage vs. current at 5V. Current Sense Threshold - (V) 1.0 TA = 125°C 0.9 0.8 TA = 25°C 0.7 0.6 TA = -55°C 0.5 0.4 0.3 0.2 0.1 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Error Amplifier Output Voltage - (V) FIGURE 29 — CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT Copyright © 1994 Rev. 1.0a 1/01 15 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM 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 Unless otherwise specified, pin numbers refer to 8-pin package. FIGURE 30. — CURRENT SENSE SPIKE SUPPRESSION VCC FIGURE 31. — MOSFET PARASITIC OSCILLATIONS DC BUS VCC DC BUS 7 7 Q1 LX155x LX155x 6 5 Q1 R1 6 IPK 3 C IPK(MAX) = RS RS 1.0V RS 5 The RC low pass filter will eliminate the leading edge current spike caused by parasitics of Power MOSFET. FIGURE 32. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH SOFT-START VCC A resistor (R1) in series with the MOSFET gate reduces overshoot & 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.) FIGURE 33. — EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYNCHRONIZATION VIN 8 7 8 RA 8 4 7 4 Q1 2 LX155x 6 RB 6 IPK LX155x 555 TIMER 3 4 1N4148 1 R2 3 MPSA63 R1 C IPK = 5 R1 V CS Where: VCS = 1.67 RS tSOFTSTART = -ln 1 - ( R +R ) and V 1 VEAO - 1.3 5( R1 R 1+R2 ) 2 ( R1 R2 R1+R2 VCS 2 RS 0.01 5 = 1V (Typ.) C.S.MAX )C f = (R 1.44 + 2RB)C A 1 5 To other LX155x devices R f = R + B2R A B where; VEAO ≡ voltage at the Error Amp Output under minimum line and maximum load conditions. Soft start and adjustable peak current can be done with the external circuitry shown above. 16 Precision duty cycle limiting as well as synchronizing several parts is possible with the above circuitry. Copyright © 1994 Rev. 1.0a 1/01 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM 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 (continued) FIGURE 34. — SLOPE COMPENSATION VCC LX155x DC BUS 7(12) VO 5V 8(14) UVLO S R 5V REF RT INTERNAL BIAS 2.5V 2N222A VREF GOOD LOGIC RSLOPE 7(11) 4(7) OSCILLATOR From VO 2(3) CF RF 6(10) C.S. COMP 2R Ri Rd Q1 CT 1V ERROR AMP R 5(8) PWM LATCH R 3(5) 1(1) 5(9) C RS Due to inherent instability of fixed frequency current mode converters running above 50% duty cycle, slope compensation should be added to either the current sense pin or the error amplifier. Figure 34 shows a typical slope compensation technique. Pin numbers inside parenthesis refer to 14-pin package. FIGURE 35. — OPEN LOOP LABORATORY FIXTURE VREF RT 4.7K 100K 1K ERROR AMP ADJUST 4.7K 1 COMP VREF 8 2 VFB VCC 7 5K ISENSE ADJUST 3 ISENSE OUTPUT 6 4 RTCT GROUND 5 CT VCC A LX155x 2N2222 0.1µF 0.1µF 1K OUTPUT GROUND High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected 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. Copyright © 1994 Rev. 1.0a 1/01 17 PRODUCT DATABOOK 1996/1997 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM P D R O D U C T I O N A T A S H E E T TYPICAL APPLICATION CIRCUITS (continued) FIGURE 36. — OFF-LINE FLYBACK REGULATOR TI 4.7Ω 1W 1N4004 220µF 250V 1N4004 4.7kΩ 2W 3600pF 400V 250kΩ 1/2W AC INPUT 1N4004 MBR735 5V 2-5A 4700µF 10V 1N4935 1N4004 1N4935 16V 150kΩ 3.6kΩ VCC VFB 1 COMP 8 VREF 4 RT/CT 10µF 20V 0.01µF 27kΩ OUT 6 100pF CUR 3 SEN 10kΩ 0.01µF .0022µF 820pF 1N4935 2 7 GND 5 2.5kΩ IRF830 LX1554 20kΩ 1kΩ 470pF 0.85kΩ ISOLATION BOUNDARY SPECIFICATIONS Input line voltage: Input frequency: Switching frequency: Output power: Output voltage: Output current: Line regulation: Load regulation: Efficiency @ 25 Watts, VIN = 90VAC: VIN = 130VAC: Output short-circuit current: 18 90VAC to 130VAC 50 or 60Hz 40KHz ±10% 25W maximum 5V +5% 2 to 5A 0.01%/V 8%/A* * This circuit uses a low-cost feedback scheme in which the DC voltage developed from the primary-side control winding is sensed by the LX1554 error amplifier. Load regulation is therefore dependent on the coupling between secondary and control windings, and on transformer leakage inductance. 70% 65% 2.5Amp average Copyright © 1994 Rev. 1.0a 1/01