STMicroelectronics L5985

Overview

L5985 from STMicroelectronics is a 2 A step-down switching regulator with integrated P-channel MOSFET, designed for DC-DC conversion in space-constrained power rails. It operates from 2.9 V to 18 V input, delivers adjustable output down to 0.6 V, features 250 kHz–1 MHz programmable switching frequency, and supports 100% duty cycle for low-dropout operation in battery-powered and 3.3 V bus systems.

For engineers reviewing the L5985 datasheet, L5985 pinout, L5985 application, or L5985 equivalent, key selection criteria include its voltage-mode control architecture, internal soft-start timing (7.4–9.1 ms), thermal shutdown at 150 °C with 30 °C hysteresis, and VFQFPN8 package's 60 °C/W R_thJA - critical for thermal design in consumer and industrial DC-DC modules.

Technical Context

The L5985 implements voltage-mode PWM control with a fixed-frequency sawtooth oscillator whose slope is dynamically adjusted via voltage feedforward (input-dependent) and frequency feedforward (FSW-resistor dependent) to maintain constant PWM gain across line and frequency variations. Its error amplifier provides 100 dB open-loop gain and 4.5 MHz GBWP, supporting stable Type II or Type III compensation networks.

Functional blocks include a high-side P-channel MOSFET driver (R_DS(on) = 140–220 mΩ), peak-current-limit protection with 200 ns blanking time, hiccup-mode overcurrent response during regulation, and pulse-skipping current limiting during soft-start. The INH pin enables logic-level (0.6 V/1.9 V thresholds) standby control with ≤30 µA quiescent current.

Key Specifications

Pinout & Package

VFQFPN8 (3 mm × 3 mm, 0.5 mm pitch) with exposed thermal pad - achieves R_thJA ≈ 60 °C/W on standard 4-layer demo board, enabling compact thermal design without heatsinks.

Key Features

Applications

Equivalent & Alternatives

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

Compared with TPS54202DRT and MP2315GJ-Z, the L5985 uniquely supports 2.9 V start-up, voltage feedforward for superior line regulation, and programmable soft-start - making it optimal for low-voltage, high-transient consumer and industrial rails where reliability under brownout and load surge is non-negotiable.

Availability

L5985 is available at Aetrix Electronics and suitable for consumer audio, FPGA power, DSL modem DC-DC modules, and HDD preamp rails requiring stable component supply, long-term lifecycle support, and validated thermal performance in VFQFPN8 packages.

Supply support for L5985 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, designing and manufacturing analog, microcontroller, power, and sensor solutions for industrial, automotive, and consumer markets.

The L5985 belongs to ST's high-efficiency monolithic DC-DC regulator product line, engineered specifically for compact, thermally demanding point-of-load applications where input voltage range, light-load stability, and robust protection are prioritized over synchronous rectification.

FAQ

What is the minimum input voltage required for L5985 startup and regulation?

The L5985 requires a minimum input voltage of 2.9 V to initiate startup, as defined by its V_CCON UVLO threshold. Regulation is maintained down to this level with full 2 A capability, making it suitable for single-cell Li-ion (3.0 V nominal) and 3.3 V bus applications where dropout margin is critical. Below 2.9 V, the device remains disabled.

How does the SYNCH pin function in master-slave configurations?

When two L5985 devices have their SYNCH pins connected, the unit with the higher oscillator frequency automatically becomes the master, and the other synchronizes as a slave with 180° phase shift. This interleaving reduces RMS input capacitor current by up to 30%, lowering heat and EMI. No external clock source is needed - synchronization occurs inherently via pin coupling.

Can L5985 operate without an external compensation network?

No - the L5985 requires an external Type II or Type III compensation network connected between COMP and FB pins. Its voltage-mode error amplifier has no internal compensation; omitting the network causes instability and oscillation. The choice depends on output capacitor ESR: Type II for high-ESR ceramics or electrolytics, Type III for ultra-low-ESR polymer or MLCCs.

What thermal derating applies when using the VFQFPN8 package on a 2-layer PCB?

On a standard 2-layer board with minimal copper pour, R_thJA rises from the documented 60 °C/W (4-layer demo board) to approximately 95 °C/W. At 2 A load and 12 V input, junction temperature may exceed 125 °C without thermal relief. ST recommends adding ≥2 cm² of 2-oz copper connected to the exposed pad and using thermal vias to inner ground planes to maintain safe operation.