STMicroelectronics L5981

Overview

L5981 from STMicroelectronics is a 1.5 A (min) integrated step-down switching regulator with internal P-channel MOSFET, designed for DC-DC conversion in space-constrained power rails. It operates from 2.9 V to 18 V input, delivers ≥1 A DC output current, supports adjustable output down to 0.6 V, and features 250 kHz–1 MHz programmable switching frequency - enabling use in 3.3 V bus systems and high-efficiency point-of-load regulation for FPGA/PLD core supplies.

For engineers reviewing the L5981 datasheet, L5981 pinout, L5981 application, or L5981 equivalent, key selection criteria include its voltage-mode control architecture, VFQFPN8 thermal performance (R_thJA ≈ 60 °C/W), soft-start timing (7.4–9.1 ms), overcurrent hiccup protection, and synchronization capability via SYNCH pin - all critical for stable multi-rail industrial and consumer power designs.

Technical Context

The L5981 implements voltage-mode PWM control with a fixed-frequency sawtooth oscillator whose slope is dynamically adjusted via voltage feedforward (input-dependent ramp scaling) and frequency feedforward (FSW-resistor–adjusted ramp scaling), maintaining constant PWM gain across line and frequency variations. Its error amplifier features 100 dB open-loop gain, 4.5 MHz GBWP, and rail-to-rail output swing (0–3.3 V).

Internal protection includes cycle-by-cycle peak current limiting (1.5–2.1 A), 150 °C hysteretic thermal shutdown (30 °C hysteresis), UVLO with 2.9 V turn-on threshold and 0.175–0.3 V hysteresis, and inhibit logic with 0.6 V ON / 1.9 V OFF thresholds. The device supports zero-load operation and 100% duty-cycle low-dropout mode.

Key Specifications

Pinout & Package

The L5981 is housed in a thermally enhanced VFQFPN8 package (3 mm × 3 mm, 0.55 mm height) with exposed thermal pad, optimized for PCB heat dissipation and compact layout in high-density applications.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar step-down regulator applications.

Compared with MP2307DN-LF-Z and TPS54202DRCT, the L5981 provides tighter 0.6 V reference tolerance (±1.2%), lower quiescent current in standby (20–30 µA), and integrated voltage feedforward - making it preferable for precision, low-power, and line-sensitive industrial and consumer power rails where thermal density is critical.

Availability

L5981 is available at Aetrix Electronics and suitable for FPGA power supplies, car audio DSP rails, LCD TV backlight bias, and industrial PLC I/O modules requiring stable component supply, long-term lifecycle support, and consistent thermal performance across production batches.

Supply support for L5981 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, microcontrollers, sensors, and automotive ICs - with over 40 years of analog and power conversion expertise.

The L5981 belongs to ST's high-efficiency monolithic DC-DC regulator product line, engineered specifically for space-constrained, thermally demanding applications in consumer electronics, automotive infotainment, and industrial automation where reliability, precision, and thermal performance are non-negotiable.

FAQ

What is the minimum input voltage required for L5981 to start switching?

The L5981 has a guaranteed turn-on threshold of 2.9 V with 0.175–0.3 V hysteresis, meaning it begins regulation when V_CC rises above 2.9 V and remains active until V_CC drops below ~2.7 V. This allows direct operation from 3.3 V distributed rails without pre-buck stages, and supports cold-crank conditions in automotive 12 V systems.

Can L5981 synchronize to an external clock, and what is the required signal level?

Yes - the SYNCH pin accepts an external TTL/CMOS-compatible clock signal (high level 2–3.3 V, low level ≤1 V) to force operation at the external frequency with zero phase shift. When driven, the internal oscillator locks to the external source; the pin also outputs a 180° phase-shifted signal when floating or used in master-slave configurations.

How does the L5981 handle short-circuit conditions on the output?

Under sustained short-circuit, the L5981 enters hiccup mode: it disables the power switch for 2048 clock cycles (~8.2 ms at 250 kHz), then attempts soft-start. If overload persists, it repeats the cycle. During this, quiescent current drops to <30 µA, junction temperature is clamped at 150 °C, and recovery is fully automatic once fault clears - no latch or manual reset required.

Is an external freewheeling diode required, and what are the key selection criteria?

No - the L5981 integrates a P-channel high-side MOSFET but requires an external Schottky diode (or synchronous rectifier controller) for the low-side path. Diode selection must prioritize low forward voltage (<0.5 V @ I_OUT) and sufficient RMS current rating; typical choices include STPS1L60 (1 A, 60 V) or SB160 (1 A, 60 V), mounted close to OUT and GND pins to minimize loop inductance.