STMicroelectronics L4985B

Overview

L4985B from STMicroelectronics is a peak current-mode CCM-operated PFC controller IC for boost pre-regulators, featuring 130 kHz switching frequency, 800 V high-voltage startup with integrated AC sensing, and proprietary THD-optimized multiplier emulation enabling <1% line current THD in both CCM and DCM. It delivers active X-capacitor discharge, brown-in/brownout compliance per IEC 60601-1-2, and drives MOSFETs/IGBTs via a 0.7 A source / 1.5 A sink totem-pole gate driver - used in >1 kW medical and industrial SMPS.

For engineers reviewing the L4985B datasheet, L4985B pinout, L4985B application, or L4985B equivalent, key selection criteria include its quasi-fixed-frequency off-time modulator, HV startup architecture, dual OCP thresholds (-0.49 V / -0.75 V), 2.5 V internal reference (±1.2%), and SO8 package compatibility with safety-critical PFC designs requiring IEC 61000-3-2, JEIDA-MITI, and IEC 60601-1-2 conformance.

Technical Context

The L4985B implements fixed-off-time (FOT) peak current-mode control with a proprietary "off-time modulator" that maintains quasi-constant 130 kHz switching across input voltage (94–117 V_PK), load, and operating mode (CCM/DCM), eliminating need for external frequency compensation. Its THD-CCM optimizer injects a shaped current into the CS pin via R_{THD_CCM}, while the THD-DCM optimizer dynamically adjusts the current reference to preserve sinusoidal line current under light-load DCM.

Internal circuitry integrates AC brownout detection (630 ms debounce), feedback failure lockout (V_FB < 0.5 V), dual-level overcurrent protection (inductor saturation at -0.75 V), and HV-startup-triggered X-cap discharge (5 mA sink at 45 V residual). The transconductance error amplifier (200 µS typ.) operates with <20 Hz bandwidth to generate a DC-like COMP voltage used by the Vin-emulator block.

Key Specifications

Pinout & Package

Package: SO8 (Small Outline, 8-pin, 150 °C/W junction-to-ambient thermal resistance). Pin 7 is NC (high-voltage spacer for creepage compliance); all other pins are electrically functional and validated per DS13769 Rev 3.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar CCM boost PFC controller applications.

Compared with UCC28070 and FAN9612, the L4985B uniquely integrates HV startup, X-cap discharge, dual-mode THD optimization, and medical-grade brownout timing - reducing external component count by ≥6 and enabling single-chip compliance with IEC 60601-1-2, JEIDA-MITI, and Energy Star V2.0.

Availability

L4985B is available at Aetrix Electronics and suitable for desktop/server SMPS, high-power LED luminaries, industrial motor drives, and medical diagnostic equipment requiring stable component supply, long-term lifecycle assurance, and traceable sourcing for safety-critical designs.

Supply support for L4985B 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, specializing in power management, analog, microcontrollers, and automotive ICs - with >30 years of PFC controller innovation and ISO 9001/ISO 14001 certified manufacturing.

The L4985B belongs to ST's high-voltage analog power IC portfolio, designed specifically for energy-efficient, safety-compliant AC-DC front-ends in medical, industrial, and computing applications demanding IEC 61000-3-2, IEC 60601-1-2, and Energy Star conformance.

FAQ

What is the purpose of the NC pin (Pin 7) on the L4985B?

Pin 7 is a non-connected (NC) high-voltage spacer designed solely to increase creepage distance between the HV (Pin 8) and signal pins (Pins 1–6). It has no internal connection and must remain unconnected on the PCB to maintain IEC 62368-1 and IEC 60601-1-2 insulation requirements. Routing traces or vias to this pad violates safety spacing and invalidates certification.

How does the L4985B achieve low THD in both CCM and DCM modes?

The L4985B uses two dedicated circuits: the THD-CCM optimizer injects a shaped current into the CS pin via R_{THD_CCM} to linearize peak inductor current in CCM; the THD-DCM optimizer dynamically modifies the current reference waveform using T_ON, T_FW, and T_R timing signals to preserve sinusoidal input current in DCM - both enabled without external multipliers or mode-switching logic.

Can the L4985B replace the L4985A in an existing design?

Yes, but only with board-level modifications: the L4985B's 130 kHz frequency halves minimum off-time (0.5 µs vs. 1.0 µs), requiring recalculation of R_{THD_CCM}, boost inductor value, and gate resistor to maintain stability and THD. Also, verify V_{CS_OCP1} (-0.49 V) and soft-start timing remain compatible with the original MOSFET's SOA and inrush profile.

What happens during an X-capacitor discharge event?

When AC is removed, the HV pin detects voltage decay and activates an internal 5 mA current sink, discharging X-capacitors to ≤45 V within 77 ms (typical). This satisfies IEC 62368-1 touch-safe limits (<60 V within 1 s) without external resistors - eliminating ~1.5 W standby loss and enabling sub-0.5 W no-load consumption in certified designs.