Allegro MicroSystems A3958SB

Overview

A3958SB from Allegro MicroSystems is a DMOS full-bridge PWM motor driver IC designed for bidirectional DC motor control with ±2 A continuous output current, 50 V load supply rating, and programmable mixed/fast/slow current-decay modes via 3-wire serial interface. It integrates synchronous rectification, thermal shutdown with 15°C hysteresis, and internal charge pump for high-side gate drive - deployed in industrial automation motion controllers requiring precise current-regulated torque.

For engineers reviewing the A3958SB datasheet, A3958SB pinout, A3958SB application, or A3958SB equivalent, key selection criteria include its DIP-24 package with exposed thermal tabs, indeterminate substrate ground resistance between pins 6/7/18/19, fixed-off-time PWM timing, and compatibility with external 2.9–6.1 MHz oscillator inputs.

Technical Context

The A3958SB implements a fixed-off-time PWM current-control architecture with programmable blank time (4/f_OSC to 24/f_OSC) and off-time (1.75–63.75 μs at 4 MHz), enabling precise regulation of motor winding current using an external sense resistor and analog reference voltage. Its serial port (CLOCK/DATA/STROBE) configures decay mode, sync rect mode, sleep state, and VREF range division (×5 or ×10).

Internal circuitry includes dual DMOS H-bridge outputs with 270 mΩ typical r_DS(on), charge pump (CP/CP1/CP2) for high-side gate bias, VREG regulator for sink-side drivers, and fault protection covering UVLO (4.2 V enable threshold), thermal shutdown (165°C trip), and crossover-current prevention. Ground terminals are split into logic (pins 5,8) and substrate (pins 6,7,18,19) with indeterminate resistance between them - requiring external shorting per datasheet.

Key Specifications

Pinout & Package

Package: 24-pin plastic DIP with exposed thermal tabs (suffix 'B'); Pb-free, 100% matte tin leadframe; requires external connection between logic grounds (pins 5,8) and substrate grounds (pins 6,7,18,19) due to indeterminate resistance.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar full-bridge PWM motor driver applications.

Compared with TB6612FNG and DRV8876N, the A3958SB uniquely delivers ±2 A bidirectional current control in a single full-bridge with fully programmable decay timing, synchronous rectification, and thermal management - making it optimal for mid-power industrial motion systems requiring deterministic current regulation without microcontroller overhead.

Availability

A3958SB is available at Aetrix Electronics and suitable for industrial CNC axis control, automated guided vehicle (AGV) drive systems, and precision lab equipment stages requiring stable component supply and long-term obsolescence planning.

Supply support for A3958SB 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, headquartered in Worcester, Massachusetts.

The A3958SB belongs to Allegro's DMOS full-bridge motor driver product line, engineered specifically for robust, current-regulated DC motor control in industrial automation and motion-critical applications.

FAQ

What is the purpose of the indeterminate resistance between substrate grounds (pins 6, 7, 18, 19) and logic grounds (pins 5, 8) in the A3958SB?

The indeterminate resistance reflects internal isolation between the DIP package's substrate ground plane and logic ground leads. Per Allegro's datasheet, pins 5 and 8 must be externally shorted to pins 6, 7, 18, and 19 to ensure consistent ground reference and prevent latch-up or erratic current regulation. This requirement is unique to the A3958SB DIP variant and does not apply to the SOIC LB version. Failure to implement this external short may cause unstable A3958SB operation under load.

How does the A3958SB achieve current regulation without an external current-sense amplifier?

The A3958SB integrates a precision current-sense comparator that monitors voltage across an external sense resistor (RS) connected to the SENSE pin. The trip threshold is set by VREF divided by 5 or 10 (via RANGE pin and D16 bit), yielding ITRIP = VREF/(5×RS) or VREF/(10×RS). When sensed current exceeds this threshold, the comparator resets the source-enable latch, initiating the fixed-off-time PWM cycle. This closed-loop regulation is self-contained within the A3958SB and requires no external amplification.

Can the A3958SB operate with an external PWM signal applied to the ENABLE pin, and how does it interact with serial-programmed decay modes?

Yes - the A3958SB supports external PWM on the ENABLE pin for speed control. Bit D13 in the serial register selects fast or slow decay during ENABLE chopping, while D17/MODE determines internal PWM decay behavior. When using external ENABLE PWM, D13 overrides internal decay selection for the duration of the chop cycle. This allows independent control of motor speed (via ENABLE duty cycle) and current decay characteristics (via D13), giving designers flexibility unattainable with fixed-decay drivers like the A3958SB's alternatives.

What thermal design considerations are critical for reliable A3958SB operation at ±2 A continuous current?

To sustain ±2 A continuous output, the A3958SB in its DIP-24 'B' package requires a minimum 1 in² copper area on the PCB for heatsinking, achieving R_θJA = 40°C/W. The exposed thermal tab must be soldered directly to this copper plane. Additionally, VREG and CP decoupling capacitors (0.22 μF each) must be placed within 5 mm of their respective pins to maintain stable internal regulation. Without these measures, junction temperature can exceed 150°C even at ambient 25°C, triggering thermal shutdown and disrupting A3958SB functionality.

Is the A3958SB compatible with 3.3 V logic systems, and what are the voltage-level constraints for its digital inputs?

The A3958SB's digital inputs (PHASE, ENABLE, DATA, CLOCK, STROBE, OSC, MODE, RANGE) accept logic-high signals ≥2.0 V and logic-low ≤0.8 V, making them compatible with both 5 V and 3.3 V CMOS/TTL sources. However, VDD must be regulated at 4.5–5.5 V (typical 5.0 V) to power internal logic and avoid UVLO activation. Therefore, while interface signals may originate from 3.3 V systems, the A3958SB itself requires a dedicated 5 V supply - a constraint confirmed in the Electrical Characteristics table and critical for stable A3958SB serial communication and PWM timing.