Allegro MicroSystems A4949GLJTR-6-T

Overview

A4949GLJTR-6-T from Allegro MicroSystems is a three-phase sensorless sinusoidal fan driver IC with PWM speed input, 30 kHz motor PWM frequency, 1.6 A overcurrent limit, and 8-pin SOICN package with exposed thermal pad. It delivers low-noise, high-efficiency motor control for medium-power cooling fans in industrial and computing systems.

For engineers reviewing the A4949GLJTR-6-T datasheet, A4949GLJTR-6-T pinout, A4949GLJTR-6-T application, or A4949GLJTR-6-T equivalent, key selection criteria include PWM-based speed control interface, fast-ramp startup (31 ms current step), 4–18 V supply range, FG tach output, and thermal protection with 150°C shutdown threshold.

Technical Context

The A4949GLJTR-6-T implements trapezoidal startup followed by adaptive sinusoidal commutation without position sensors, using back-EMF detection on phase A to determine rotor position. Its internal 9-bit speed demand converter translates PWM duty cycle (7–10.5% ON threshold) into modulation profiles applied to three motor outputs with 120° phase offset.

It integrates soft-start with fast-ramp profile (50 mA step every 31 ms), lock-detection auto-restart (8 s timeout), overcurrent limiting at 1.6 A, and thermal shutdown at 165°C typical. The LJ package enables 62°C/W junction-to-ambient thermal resistance on 2-layer PCBs with thermal vias.

Key Specifications

Pinout & Package

Package: 8-pin SOICN with exposed thermal pad (LJ package); RoHS-compliant, 100% matte-tin leadframe plating; 4.90 mm × 3.90 mm body, 1.75 mm height, thermal pad improves power dissipation.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar three-phase sensorless fan driver applications.

Compared with A4949GLJTR-T and A5947KLPTR-T, the A4949GLJTR-6-T offers optimized startup dynamics for time-critical thermal response, retains full compatibility with existing A4949 layouts, and supports cost-sensitive G-temp industrial deployments without upgrading to integrated-FET architecture.

Availability

A4949GLJTR-6-T is available at Aetrix Electronics and suitable for server cooling, industrial PLC ventilation, network switch chassis fans, and medical diagnostic equipment requiring stable component supply, consistent thermal performance, and long-term production continuity.

Supply support for A4949GLJTR-6-T 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

Allegro MicroSystems is a U.S.-based designer of high-performance magnetic sensing and power IC solutions, specializing in motion control, energy-efficient power conversion, and robust motor driver architectures.

The A4949 belongs to Allegro's fan driver product line, engineered specifically for low-noise, high-efficiency three-phase BLDC fan control in computing, industrial, and communications infrastructure - emphasizing sensorless operation, integrated protection, and thermal resilience.

FAQ

What is the startup behavior difference between A4949GLJTR-6-T and standard A4949GLJTR-T?

The A4949GLJTR-6-T implements a fast-ramp startup profile: it takes a 50 mA current step every 31 ms and uses 40 mV BEMF hysteresis at startup. In contrast, the A4949GLJTR-T uses a slower 128 ms step interval and 100 mV hysteresis. This makes the A4949GLJTR-6-T better suited for applications needing rapid fan acceleration, such as hot-swap modules or burst-cooling scenarios. Both share identical pinout, protection features, and steady-state operation.

Does A4949GLJTR-6-T require external current-sense resistors for overcurrent protection?

No, the A4949GLJTR-6-T integrates high-side current sensing and limits output current to 1.6 A typical without any external components. Overcurrent protection triggers when the internal sense circuit detects phase current exceeding the IOCL threshold, immediately disabling the affected driver for the remainder of that PWM cycle. This eliminates BOM cost and layout area associated with discrete sense resistors and amplifiers.

Can A4949GLJTR-6-T drive fans at supply voltages below 5 V?

Yes, the A4949GLJTR-6-T operates down to 4 V supply voltage, enabling compatibility with low-voltage systems such as USB-powered thermal management or battery-backed instrumentation. At 4 V, total driver RDS(on) increases to 1.5–1.8 Ω (vs. 1.1–1.4 Ω at 12 V), so designers should verify motor torque and efficiency meet requirements. The IC maintains full functionality including PWM speed control, FG output, and protection features across the entire 4–18 V range.

How is thermal management handled on the A4949GLJTR-6-T LJ package?

The A4949GLJTR-6-T uses an 8-pin SOICN package with an exposed thermal pad (LJ) that must be soldered to a PCB ground plane. For optimal thermal performance, Allegro specifies ≥4 thermal vias under the pad connecting to inner-layer ground planes. On a 2-layer board with 0.8 in² copper per side, RθJA is 62°C/W - enabling continuous 1.6 A output without external heatsinks. Layout notes emphasize placing CVREF and CVBB capacitors close to the IC and routing the thermal pad directly to GND.

What does the FG output signal represent, and how is it used?

The FG pin on the A4949GLJTR-6-T is an open-drain output that pulses once per electrical revolution of the motor - not per mechanical revolution. For a 2-pole motor, this equals one pulse per full rotation; for a 4-pole motor, it yields two pulses per rotation. System controllers read FG with a pullup resistor (e.g., 20 kΩ to VBB) to derive RPM: RPM = (60 × f_FG) / P, where P is pole count. The output sinks up to 10 mA and has <0.3 V saturation voltage at 5 mA load.