Allegro MicroSystems A3977KED-T

Overview

A3977KED-T from Allegro MicroSystems is a bipolar stepper motor driver IC with integrated translator, supporting full-, half-, quarter-, and eighth-step microstepping modes. It delivers ±2.5 A output current at up to 35 V supply, features fixed off-time PWM current regulation with mixed/fast/slow decay control, and is used in precision motion control systems such as industrial automation actuators and medical infusion pumps.

For engineers reviewing the A3977KED-T datasheet, A3977KED-T pinout, A3977KED-T application, or A3977KED-T equivalent, key selection criteria include its 28-pin TSSOP-LP package with exposed thermal pad, translator-based step sequencing via STEP/DIR inputs, R_DS(on) of 0.28 Ω (source) / 0.22 Ω (sink), and built-in thermal shutdown, UVLO, and crossover-current protection.

Technical Context

The A3977KED-T integrates dual N-channel DMOS H-bridges with on-chip translator logic that converts STEP/DIR/MS1/MS2 commands into precise DAC-controlled phase currents. Its fixed off-time PWM regulator uses external RC timing components (RC1/RC2) and internal blanking (t_BLANK ≈ 950 ns typical) to manage current decay dynamically per step transition.

It implements automatic decay mode selection-slow decay for rising DAC steps, and PFD-voltage–controlled fast/mixed decay for falling steps-enabling reduced audible noise and improved step accuracy. Synchronous rectification (enabled by SR input) replaces body-diode conduction with low-R_DS(on) MOSFET paths during decay, cutting power dissipation without external Schottky diodes.

Key Specifications

Pinout & Package

Package: 28-pin TSSOP with exposed thermal pad (LP suffix), <1.2 mm profile, lead-free with 100% matte tin plating. Designed for surface-mount assembly on 4-layer PCBs with thermal vias under the pad for R_θJA = 28°C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with TB6600HG and DRV8825, the A3977KED-T uniquely combines translator-based simplicity, 2.5 A drive strength, and programmable decay control in a thermally optimized LP package-making it optimal for embedded motion systems demanding both precision and autonomy.

Availability

A3977KED-T is available at Aetrix Electronics and suitable for industrial CNC positioning, medical infusion pumps, automated laboratory analyzers, and 3D printer extruder control requiring stable component supply across multi-year production cycles.

Supply support for A3977KED-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 sensors and power ICs, specializing in motion control, current sensing, and motor driver solutions since 1989.

The A3977 product line delivers fully integrated stepper drivers with on-chip translators for applications where minimal MCU intervention, low acoustic noise, and robust thermal protection are essential.

FAQ

What is the maximum step frequency supported by the A3977KED-T?

The A3977KED-T supports a minimum guaranteed step frequency of 500 kHz, enabling high-speed microstepping operation. While higher frequencies may be achievable depending on layout, trace inductance, and motor inductance, Allegro specifies 500 kHz as the warranted maximum for reliable translator and current-regulator performance. The A3977KED-T achieves this using fast logic propagation (200 ns setup/hold times) and low-latency DAC stepping synchronized to the STEP input edge.

How does the PFD pin affect current decay behavior in the A3977KED-T?

The PFD pin on the A3977KED-T sets the percent fast decay threshold: voltages below 0.21×VDD select fast decay, above 0.6×VDD select slow decay, and intermediate values enable mixed decay. During a step that reduces phase current, the A3977KED-T uses this voltage to determine how much of the fixed off-time period is spent in fast decay before switching to slow decay-optimizing current waveform shape and minimizing distortion caused by motor back-EMF.

Can the A3977KED-T drive unipolar stepper motors?

No, the A3977KED-T is designed exclusively for bipolar stepper motors. Its dual full-bridge architecture (OUT1A/OUT1B and OUT2A/OUT2B) provides bidirectional current control per winding, which is required for bipolar operation. Unipolar motors require center-tapped windings and single-ended switching, incompatible with the A3977KED-T's H-bridge topology and current-sense architecture.

What is the purpose of the HOME output on the A3977KED-T?

The HOME output on the A3977KED-T is an active-low logic signal that indicates when the internal translator is in its predefined initial state-occurring at power-up, reset, or after a RESET pulse. This signal enables system-level homing verification and ensures deterministic motor phase alignment before motion begins, critical for repeatable positioning in automated equipment using the A3977KED-T.

Does the A3977KED-T require external Schottky diodes for flyback protection?

No, the A3977KED-T does not require external Schottky diodes due to its integrated synchronous rectification feature. When enabled (SR = low), the IC actively turns on low-R_DS(on) MOSFETs during current decay to bypass body diodes-reducing power dissipation and eliminating reverse recovery losses. External diodes are only needed if synchronous rectification is disabled (SR = high) for specialized power transfer requirements.