Allegro MicroSystems A3979SLP-T

Overview

A3979SLP-T from Allegro MicroSystems is a bipolar stepper motor driver IC with integrated translator, supporting full-, half-, quarter-, and 1/16-step microstepping modes, ±2.5 A output current, 35 V load supply, and fixed off-time PWM current regulation with Slow/Fast/Mixed decay control. It operates in industrial motion control systems requiring precise positioning without complex microprocessor intervention.

For engineers reviewing the A3979SLP-T datasheet, A3979SLP-T pinout, A3979SLP-T application, or A3979SLP-T equivalent, key selection considerations include its 28-pin TSSOP package with exposed thermal pad, AGND/PGND external tie requirement, 3.0–5.5 V logic supply range, and automatic decay mode detection based on DAC step transitions.

Technical Context

The A3979SLP-T implements dual full-bridge DMOS output stages controlled by a fixed off-time PWM regulator, where trip current is set via external sense resistors (RS1/RS2) and VREF using ITRIPmax = VREF/(8×RS). Decay mode-Slow, Fast, or Mixed-is dynamically selected per bridge based on the sign of the DAC output change between successive microsteps.

Its translator core accepts STEP/DIR/MS1/MS2 logic inputs to sequence 16-phase current waveforms without external sequencing tables; HOME output signals initial microstep position, and synchronous rectification (enabled via SR pin) reduces power dissipation by replacing body-diode conduction with low-RDS(On) MOSFET paths during current decay.

Key Specifications

Pinout & Package

The A3979SLP-T is supplied in a 28-pin TSSOP (LP suffix) with exposed thermal pad (height ≤1.20 mm), Pb-free construction, and 100% matte tin leadframe plating. AGND and PGND must be connected together externally at a single-point star ground under the thermal pad.

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 A3979SLP-T uniquely combines 2.5 A drive strength, integrated homing indication, and automatic decay mode adaptation - making it optimal for industrial motion systems requiring reliability, low noise, and minimal host processor involvement.

Availability

A3979SLP-T is available at Aetrix Electronics and suitable for 3D printer motion control, CNC milling axis drivers, and automated laboratory equipment requiring stable component supply, long-term production continuity, and RoHS-compliant packaging.

Supply support for A3979SLP-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 and manufacturer of high-performance magnetic sensing and power IC solutions, founded in 1989 and headquartered in Worcester, Massachusetts.

The A3979SLP-T belongs to Allegro's DMOS microstepping driver with translator product line, engineered specifically for cost-effective, low-noise, and highly integrable stepper motor control in industrial automation and precision motion systems.

FAQ

What is the maximum allowable voltage on the SENSE1 and SENSE2 pins of the A3979SLP-T?

The A3979SLP-T specifies an absolute maximum sense voltage of 0.5 V on both SENSE1 and SENSE2 pins. Exceeding this value risks permanent damage to the current-sense comparator circuitry. For full-step operation, VREF may be applied up to VDD because peak sense voltage is 0.707 × VREF/8; in all other microstep modes, VREF must not exceed 4 V to keep sense voltage within limit. The A3979SLP-T datasheet defines RS = 0.5/ITRIPmax as the recommended sense resistor value to ensure compliance.

How does the A3979SLP-T determine whether to use Slow, Fast, or Mixed decay mode?

The A3979SLP-T selects decay mode per bridge based on the direction of DAC output change between consecutive microsteps and the voltage level on the PFD pin. If the next DAC output is lower than the previous, decay mode depends on PFD: >0.6×VDD → Slow; <0.21×VDD → Fast; between → Mixed. If the DAC output increases or stays equal, Slow decay is used. This behavior is intrinsic to the A3979SLP-T's internal control logic and requires no external configuration.

Can the A3979SLP-T operate with a 3.3 V logic supply?

Yes, the A3979SLP-T supports a logic supply voltage range of 3.0 V to 5.5 V, making it fully compatible with 3.3 V microcontrollers. Input thresholds are defined as VIN(1) ≥ 0.7×VDD and VIN(0) ≤ 0.3×VDD, so at VDD = 3.3 V, valid high is ≥2.31 V and valid low is ≤0.99 V. All logic inputs (STEP, DIR, MS1, MS2, ENABLE, etc.) meet these levels with standard 3.3 V CMOS outputs, and the A3979SLP-T draws only 10 mA typical in active operation at this voltage.

What is the purpose of connecting AGND and PGND together externally for the A3979SLP-T?

AGND (analog ground) and PGND (power ground) must be connected together externally - typically at a single-point star ground located under the A3979SLP-T's exposed thermal pad - to prevent noise coupling from high-current switching paths into the sensitive current-sense circuitry. Separating them introduces IR drops across PCB traces that corrupt VREF-based current regulation. The A3979SLP-T datasheet explicitly mandates this tie, and failure to implement it results in inaccurate current limiting and potential motor step loss.

Does the A3979SLP-T require external Schottky diodes for flyback protection?

No, the A3979SLP-T does not require external Schottky diodes due to its integrated synchronous rectification feature. When SR is low, the A3979SLP-T actively turns on appropriate low-RDS(On) MOSFETs during current decay, shorting out body diodes and reducing power dissipation. External diodes are only needed if SR is held high (disabled mode), typically for specialized thermal management - but the A3979SLP-T's design eliminates this requirement for most applications.