STMicroelectronics L4981BD

Overview

L4981BD from STMicroelectronics is a fixed-frequency average-current-mode power factor correction (PFC) controller IC designed for boost pre-regulator topologies in universal-input AC-DC power supplies. It delivers up to 0.99 power factor, limits line current distortion to <5%, integrates a ±2% precision 5.1V reference, supports 85–265V AC input, and features a totem-pole gate driver capable of >1A peak output for MOSFET/IGBT switching.

For engineers reviewing the L4981BD datasheet, L4981BD pinout, L4981BD application, or L4981BD equivalent, key selection considerations include its fixed-frequency operation (L4981A variant), programmable UVLO threshold, integrated overvoltage/overcurrent protection, soft-start timing via external capacitor, and compatibility with BCD 60II process for high-voltage robustness in industrial and server SMPS designs.

Technical Context

The L4981BD implements an analog multiplier-based average current mode control architecture that generates a sinusoidal reference current proportional to rectified line voltage (via VRMS pin) and load condition (via LFF pin), eliminating slope compensation. Its internal error amplifier regulates boost output voltage using feedback from VFEED, while the CA-OUT and MULT-OUT pins interface directly with external RC networks to set loop gain and stability.

It operates with a fixed 100 kHz nominal switching frequency (set by ROSC/COSC), includes a 100 µA soft-start current source (pin 12), and uses bipolar/DMOS totem-pole output (pin 20) to drive power switches without external buffers. Protection functions include programmable UVLO (pin 15), overvoltage (pin 3), and cycle-by-cycle overcurrent sensing (pin 2 for L4981A/L4981BD).

Key Specifications

Pinout & Package

Available in SO20 package (13.0 mm × 7.6 mm, 1.27 mm pitch), thermally rated at R_{th j-amb} = 120 °C/W. Pin numbering follows standard SOIC top-view convention.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar power factor correction controller applications.

Compared with UC3854BDW and FAN4803SJX, the L4981BD offers superior THD performance (<5% vs. ~8%) due to its dedicated LFF and VRMS inputs, tighter reference tolerance (±2% vs. ±3%), and integrated protection - making it optimal for high-reliability, space-constrained PFC stages demanding minimal external components.

Availability

L4981BD is available at Aetrix Electronics and suitable for server power supplies, industrial AC-DC adapters, medical equipment PSUs, and LED driver front-ends requiring stable component supply, long-term lifecycle support, and guaranteed traceable sourcing.

Supply support for L4981BD 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, specializing in power management, analog, microcontrollers, and automotive ICs with vertical manufacturing capability.

The L4981 series belongs to ST's high-voltage analog power IC portfolio, engineered specifically for continuous-conduction-mode (CCM) boost PFC applications in universal-input offline power supplies up to 500 W.

FAQ

What is the difference between L4981BD and L4981B?

The L4981BD is the SO20-packaged version of the L4981B, which is a frequency-modulated PFC controller. However, official ST documentation confirms L4981BD is functionally identical to L4981A (fixed-frequency), not L4981B - the 'D' suffix denotes SO20 packaging, and the device operates at fixed 100 kHz. This is verified by pin 16 functionality (SYNC, not FREQ-MOD) and absence of frequency modulation circuitry in its SO20 variant.

Can L4981BD drive a 600 V, 15 A MOSFET directly?

Yes - the L4981BD's GDRV pin (20) delivers 1.5 A source / 2 A sink peak current with 50 ns rise time into 1 nF, sufficient to drive STW15NB50 (600 V, 15 A) within safe SOA limits. Layout best practices (short gate traces, local 100 nF decoupling at VCC, Kelvin source connection) are required to avoid oscillation and ensure reliable turn-on/turn-off.

How is the soft-start time calculated for L4981BD?

Soft-start time is determined by the external capacitor on pin 12 (SS) and the internal 100 µA current source: t_SS ≈ C_SS × 5 V / 100 µA. For example, a 1 µF capacitor yields ~50 ms ramp time. The internal discharge MOSFET resets SS during UVLO or OVP events, ensuring repeatable startup behavior after faults.

Does L4981BD support discontinuous conduction mode (DCM)?

No - the L4981BD is explicitly designed for continuous conduction mode (CCM) boost PFC operation. Its average current mode architecture, lack of zero-current detection, and internal compensation structure assume CCM operation. Using it in DCM results in unstable current loop behavior and poor PF/THD performance, as confirmed in ST AN628 application note.