Allegro MicroSystems A4985SETTR-T

Overview

A4985SETTR-T from Allegro MicroSystems is a DMOS bipolar stepper motor driver IC with integrated translator, supporting full-, half-, quarter-, and eighth-step microstepping modes. It delivers ±1 A output current at up to 35 V supply, features automatic mixed/slow decay mode selection, synchronous rectification, and thermal/overcurrent protection. It is used in precision motion control systems such as 3D printer axes, CNC engravers, and automated lab equipment.

For engineers reviewing the A4985SETTR-T datasheet, A4985SETTR-T pinout, A4985SETTR-T application, or A4985SETTR-T equivalent, this page provides verified technical context, real-world design meaning for key specs, validated pin functions, and two confirmed alternative parts for stepper driver selection in embedded motion systems.

Technical Context

The A4985SETTR-T implements fixed off-time PWM current regulation with automatic decay mode switching: Mixed decay (31.25% fast + 68.75% slow) when winding current decreases; Slow decay when current increases or remains stable. Its translator eliminates phase sequencing logic - a single STEP pulse advances one microstep, controlled by DIR and MS1/MS2 inputs.

Internal protection includes undervoltage lockout (2.8 V VDDUVLO), thermal shutdown (165 °C), crossover-current prevention, short-to-ground latching, and shorted-load repetitive fault cycling. The ET package adds NSNF compliance via no-connect pins isolating critical outputs, sense, and supply terminals.

Key Specifications

Pinout & Package

Package: 32-contact QFN with exposed thermal pad (5 mm × 5 mm × 0.90 mm, ET package). Exposed pad must be soldered to PCB ground plane for thermal and electrical integrity. NSNF-compliant layout with no-connect pins between high-risk terminals.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar stepper driver applications.

Compared with TMC2209-LA and DRV8825PWPR, the A4985SETTR-T uniquely combines translator simplicity, NSNF safety, and automatic decay - making it optimal for medical, lab, and industrial motion where reliability and ease of integration are prioritized over raw current or acoustic performance.

Availability

A4985SETTR-T is available at Aetrix Electronics and suitable for 3D printer motion control, CNC positioning systems, and automated laboratory instrumentation requiring stable component supply, long-term lifecycle assurance, and traceable sourcing.

Supply support for A4985SETTR-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 semiconductor company specializing in magnetic sensing and power ICs, with over 40 years of automotive and industrial motion control expertise.

The A4985 product line was designed for robust, translator-integrated stepper motor control in safety-critical embedded systems - emphasizing fault resilience, thermal stability, and simplified system-level integration.

FAQ

What is the maximum motor supply voltage supported by the A4985SETTR-T?

The A4985SETTR-T supports a motor supply voltage (VBB) range of 8 V to 35 V. Absolute maximum rating is 35 V - exceeding this may cause permanent damage. Operation below 8 V disables current regulation; for 12 V or 24 V systems, ensure input capacitance meets datasheet recommendations (100 µF bulk + 0.1 µF ceramic per VBB pin). The A4985SETTR-T maintains ±1 A output capability across this full range when thermally managed.

How does the A4985SETTR-T implement automatic current decay mode selection?

The A4985SETTR-T monitors DAC output transitions: if the new DAC level is lower than the previous level, it selects Mixed decay (31.25% fast + 68.75% slow); if equal or higher, it selects Slow decay. This occurs independently per bridge during each STEP event. The behavior is intrinsic to the A4985SETTR-T's analog control architecture - no configuration register or external signal is needed. ROSC pin grounding forces 100% Mixed decay for low-speed stability.

What is the purpose of the NC (no-connect) pins on the A4985SETTR-T ET package?

The NC pins (2, 4, 21, 23, 26, 28, 29, 31) on the A4985SETTR-T ET package are physically present but electrically unconnected. They isolate high-risk terminals - such as VBB, SENSE, and OUT - to prevent adjacent-pin shorts from causing thermal runaway. This design satisfies No-Smoke-No-Fire (NSNF) requirements, ensuring the A4985SETTR-T fails safely (e.g., open-circuit) rather than igniting during manufacturing defects or field stress.

Can the A4985SETTR-T operate with a 3.3 V logic supply?

Yes, the A4985SETTR-T accepts VDD from 3.0 V to 5.5 V, making it fully compatible with 3.3 V microcontrollers. Logic inputs (STEP, DIR, MS1, MS2, etc.) have 5 V tolerance and defined thresholds at 70% and 30% of VDD - so at 3.3 V, VIH ≈ 2.31 V and VIL ≈ 0.99 V. Input leakage stays below ±1 µA, ensuring reliable interfacing without level shifters. The A4985SETTR-T's internal VREG (7 V) and charge pump remain functional regardless of VDD value.

What happens during a short-to-ground fault on an A4985SETTR-T output?

When a short-to-ground occurs on OUT1A, OUT1B, OUT2A, or OUT2B, the A4985SETTR-T detects overcurrent via its SENSE comparator and latches the affected bridge OFF. The device remains in this protected state until either the SLEEP pin is pulsed high or VDD is cycled. This latching behavior is specific to short-to-ground - unlike shorted-load faults, which trigger repetitive fixed-off-time cycling. The A4985SETTR-T's fault response is hardware-automated and requires no firmware intervention.