STMicroelectronics L6668TR

Overview

L6668TR from STMicroelectronics is a current-mode primary-side PWM controller IC for offline flyback converters, featuring load-dependent multi-mode operation (fixed-frequency at heavy load, frequency foldback at medium load, burst-mode at no-load), on-board 700 V high-voltage start-up generator, ±800 mA totem-pole gate driver with 15 V clamp, and integrated PFC-stop interface. It targets energy-efficient AC-DC adapters for notebooks and LCD monitors.

For engineers reviewing the L6668TR datasheet, L6668TR pinout, L6668TR application, or L6668TR equivalent, key selection criteria include its 100 kHz typical switching frequency, 2 mA quiescent current, programmable soft-start, slope compensation support, and latched OVP/OTP disable - all critical for meeting Blue Angel and Energy Star standby power requirements.

Technical Context

The L6668TR implements a dual-chip BCD-based architecture: a high-voltage 700 V start-up generator (pin 1) charges the Vcc capacitor until 13.5 V, then disables itself; the low-voltage control die manages PWM regulation via COMP pin feedback, current sensing at ISEN (pin 12) with 200 ns blanking, and slope compensation via S-COMP (pin 15). Its oscillator frequency is set by RCT (pin 16) and modulated by STBY (pin 13) during light-load foldback.

It supports three distinct operating modes: fixed-frequency (95–107 kHz) under full load; linear frequency reduction as VCOMP falls below 3 V; and burst-mode shutdown when VCOMP drops 50 mV below SKIPADJ (pin 9) voltage - enabling sub-300 mW no-load consumption. The PFC_STOP (pin 14) open-drain output toggles between 0.1 V (sinking 2 mA) and high-impedance states based on VCOMP thresholds (2.2 V fall / 2.7 V rise).

Key Specifications

Pinout & Package

SO-16 narrow package (ECOPACK® 1.1 compliant), surface-mount, 1.27 mm pitch, 10.3 × 7.5 mm body size. Pin 2 (HVS) is an internal NC spacer for creepage compliance; Pin 6 (N.C.) must be tied to GND.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar primary-side PWM controller applications.

Compared with ICE2QS03G, L6668TR offers superior light-load efficiency via multi-mode control and PFC coordination; versus UCC28600DR, it provides higher power capability (via external MOSFET) and greater flexibility in transformer design, albeit with two extra components.

Availability

L6668TR is available at Aetrix Electronics and suitable for high-efficiency AC-DC adapters, LCD monitor power supplies, and digital TV standby converters requiring stable component supply, long-term lifecycle support, and RoHS-compliant packaging.

Supply support for L6668TR includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, designing and manufacturing analog, microcontroller, power, and sensor solutions for industrial, automotive, and consumer markets.

L6668TR belongs to ST's SMART PRIMARY CONTROLLER product line, engineered specifically for energy-efficient offline flyback converters in regulated AC-DC power supplies targeting Blue Angel, Energy Star, and EU Code of Conduct compliance.

FAQ

What is the purpose of the HVS pin (Pin 2) on the L6668TR?

Pin 2 (HVS) is an internal no-connect spacer designed solely to increase creepage distance between the HV (Pin 1) and adjacent pins on the SO-16 package. It is not bonded internally and must remain unconnected on the PCB to meet safety isolation standards for mains-connected equipment.

How does the L6668TR achieve compliance with Energy Star standby power limits?

The L6668TR meets Energy Star requirements through three coordinated features: (1) ultra-low 2 mA quiescent current, (2) burst-mode operation that reduces switching frequency to hundreds of Hz at no-load, and (3) PFC_STOP pin that disables the pre-regulator under light load - collectively enabling <250 mW total input power at 230 VAC/0.5 W output.

Can the L6668TR drive a 600 V MOSFET directly without external gate resistors?

Yes - the L6668TR's ±800 mA totem-pole driver and 15 V output clamp allow direct connection to standard 600 V MOSFET gates. However, a small series gate resistor (5–10 Ω) is recommended to damp ringing and limit dV/dt-induced shoot-through during hard switching transitions in high-power designs.

What happens to the PFC_STOP pin during latch-off events like OVP or OTP?

During any latched shutdown event (e.g., DIS >2.2 V or ISEN >1.5 V), the PFC_STOP pin goes into high-impedance (open) state regardless of VCOMP level. This ensures the PFC controller remains disabled until full power recycle, preventing unsafe reactivation of the pre-regulator during fault conditions.