LX1555

L I N D O C # : 1552 LX1552/3/4/5 U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM T HE I NFINITE P OWER OF I N N O VAT I O N P RODUCTION D ATA S HEET 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 dissipation. 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. K E Y F E AT U R E S s ULTRA-LOW START-UP CURRENT (150µA typ.) s TRIMMED OSCILLATOR DISCHARGE CURRENT (±2% typ.) s INITIAL OSCILLATOR FREQUENCY BETTER THAN ±4% s OUTPUT PULLDOWN DURING UVLO s 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 A P P L I C AT I O N S PRODUCT HIGHLIGHT s ECONOMY OFF-LINE FLYBACK OR FORWARD CONVERTERS s DC-DC BUCK OR BOOST CONVERTERS s LOW COST DC MOTOR CONTROL T YPICAL A PPLICATION O F L X155X U SING I TS M ICRO P OWER S TART -U P F EATURE TABLE 1 R ST Design Using SG384x UC384xA LX155x 250µA A VA I L A B L E O P T I O N S Part # LX1552 LX1553 LX1554 LX1555 PER P A RT # AC INPUT I ST VCC Max. Start-up Current 1000µA 500µA Specification (IST) Typical Start-Up Resistor Value (RST) 62KΩ Start-Up Hysteresis Max. Duty Voltage Cycle 16V 8.4V 16V 8.4V 6V 0.8V 6V 0.8V