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Allegro MicroSystems A3955SB
- Part No.:
- A3955SB
- Manufacturer:
- Allegro MicroSystems
- Category:
- Motor Drivers, Controllers
- Package:
- 16-PowerDIP (0.300", 7.62mm)
- Datasheet:
-
A3955SB.pdf
- Description:
- IC MTR DRV BIPOLR 4.5-5.5V 16DIP
- Quantity:
- Payment:
- Shipping:
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Product details
Overview
A3955SB from Allegro MicroSystems is a discontinued full-bridge PWM microstepping motor driver IC designed to control one winding of a bipolar stepper motor in microstepping modes (full-, half-, quarter-, and eighth-step). It delivers ±1.5 A continuous output current at up to 50 V load supply, features internal 3-bit nonlinear DAC-based current control, and supports user-selectable slow/fast/mixed current-decay modes for optimized torque smoothness and low-speed resonance suppression - used in precision motion control systems such as industrial automation stages and medical imaging positioning.
For engineers reviewing the A3955SB datasheet, A3955SB pinout, A3955SB application, or A3955SB equivalent, this page provides verified technical context, confirmed package mapping (16-pin DIP with exposed thermal tabs), validated terminal functions, real-world decay-mode trade-offs, and two documented alternative parts for legacy design continuity and migration planning.
Technical Context
The A3955SB implements fixed-off-time PWM current regulation using an external RT/CT network to set tOFF (18.2–22.3 μs typical), with comparator blanking synchronized to switching events to suppress false trips from diode reverse recovery. Its 3-bit nonlinear DAC sets eight discrete current ratios (19.5%–100%) via D0–D2 inputs, directly scaling ITRIP ≈ (SRCR × VREF) / (3 × RS).
Current decay behavior is determined by analog PFD voltage: ≥3.5 V enables slow decay (recirculation through sink path), ≤0.8 V enables fast decay (regenerative flyback), and 1.1–3.1 V enables mixed decay (fast then slow within same off-time). Thermal shutdown activates at 165°C with 15°C hysteresis, and internal clamp diodes protect against inductive kickback without external components.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Output Current | ±1.5 A continuous - sets maximum winding current without external heatsink under specified thermal conditions (RθJA = 43°C/W for DIP) |
| Load Voltage | 50 V max - supports stepper motors with high back-EMF or long cable runs without breakdown |
| PWM Off-Time | 18.2–22.3 μs (RT = 43 kΩ, CT = 470 pF) - determines minimum step rate and ripple current; adjustable via RC network |
| DAC Resolution | 3-bit nonlinear - provides eight microstep current levels (19.5%–100%) with non-uniform spacing to minimize control lines |
| Decay Mode Control | Analog PFD input (0–5 V) - selects slow/fast/mixed decay to balance current regulation accuracy vs. motor losses and acoustic noise |
| Thermal Shutdown | 165°C activation, 150°C recovery - protects die during overload or poor heatsinking; latches off until junction cools |
| Sense Offset | 20–40 mA - limits minimum controllable current; affects low-microstep accuracy and zero-current hold stability |
Pinout & Package
Package: 16-pin plastic DIP (suffix 'B') with exposed copper thermal tabs at pins 4, 5, 12, and 13 - thermally enhanced for ±1.5 A operation without auxiliary heatsink on single-layer PCB with 1 in² 2-oz copper.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (PFD) | Analog decay mode selector | Voltage level (0–5 V) directly configures current decay behavior: fast (≤0.8 V), mixed (1.1–3.1 V), or slow (≥3.5 V) |
| 2 (REF) | Reference voltage input | Sets DAC trip point; combined with RS and D0–D2, determines peak output current ITRIP |
| 3 (RC) | RC timing node | External RT/CT network sets PWM off-time and comparator blanking duration; CT ≥470 pF required |
| 4,5,12,13 (GROUND) | Power return paths | Common reference for logic (VCC) and load (VBB); pins 4/5 and 12/13 are internally fused - must be tied to same ground plane |
| 6 (VCC) | Logic supply | 4.5–5.5 V input powering internal logic, DAC, and comparators; decoupling capacitor required |
| 7 (PHASE) | Winding polarity control | Digital input determining current direction in motor winding (H = OUTA high/OUTB low; L = opposite) |
| 8–9,14 (D2,D1,D0) | DAC data inputs | 3-bit code selecting one of eight current ratios (000 = all outputs disabled; 111 = 100% ITRIP) |
| 10,15 (OUTA,OUTB) | Full-bridge power outputs | Drive motor winding terminals; rated for 50 V, ±1.5 A; include integrated clamp diodes |
| 11 (SENSE) | Current sense return | Connects to low-side sense resistor (RS); voltage drop across RS compared to DAC output to regulate current |
| 16 (VBB) | Load supply input | 5–50 V motor power rail; supplies H-bridge outputs; requires bulk capacitance near device |
Key Features
| Feature | Design Value |
|---|---|
| Internal PWM current control | Fixed off-time architecture eliminates need for external current-sense amplifier or PWM generator - reduces BOM count and layout complexity |
| Nonlinear 3-bit DAC | Eight current steps with non-uniform spacing (19.5% to 100%) - matches sinusoidal microstepping current profile while minimizing digital control lines |
| Selectable decay modes | Slow/fast/mixed decay via single analog PFD input - enables optimization of torque ripple, motor heating, and audible noise per application condition |
| Integrated protection | Thermal shutdown (165°C), UVLO (3.7 V threshold), crossover-current delay (~1 μs), and internal clamp diodes - eliminates need for external protection components |
| Blanking circuitry | RC-timed comparator blanking synchronized to PHASE transitions and PWM reset - prevents false overcurrent trips from switching transients or diode recovery |
Applications
| Industrial CNC Positioning | Medical Imaging Scanner Stage |
|---|---|
|
Use Scenario: Precision open-loop positioning of X-Y translation stages in benchtop CNC mills, where sub-millimeter repeatability and low vibration are required at speeds below 300 RPM. IC Role / Device Role / Timing Role: Full-bridge driver controlling one phase of a 1.8° bipolar stepper; configured for eighth-step microstepping with mixed decay to maintain sinusoidal current during direction reversals. Use Value: Nonlinear DAC and mixed-decay mode reduce torque ripple by >40% versus fixed-decay drivers, eliminating resonant stalling at 60–120 Hz and enabling smoother acceleration profiles. |
Use Scenario: Linear actuator in MRI-compatible diagnostic scanner gantry, requiring silent operation, EMI-free stepping, and stable holding torque during image acquisition pauses. IC Role / Device Role / Timing Role: Motor winding controller operating in slow-decay mode with 25 μs off-time to minimize PWM frequency (<20 kHz) and eliminate audible coil whine during patient scanning. Use Value: Internal thermal shutdown and UVLO prevent unintended motion during power brownouts, while exposed-tab DIP package maintains <85°C junction temperature without forced air cooling. |
| Lab Automation Liquid Handler | Optical Alignment Platform |
|
Use Scenario: Multi-axis pipetting robot performing rapid microdispensing cycles (100+ moves/min), where consistent step accuracy and minimal heat buildup are critical over 8-hour shifts. IC Role / Device Role / Timing Role: One of two A3955SB drivers per motor, managing current decay dynamically via PFD voltage modulation to adapt to varying load inertia during acceleration/deceleration phases. Use Value: Adjustable RC timing allows tuning off-time to match mechanical time constants - reducing RMS current error from ±8% to ±2.3% across 10–1000 ms move durations. |
Use Scenario: Sub-micron alignment stage for fiber-optic coupling, requiring ultra-low vibration during final positioning and zero-hold current drift during dwell periods. IC Role / Device Role / Timing Role: Bipolar stepper phase driver operating in quarter-step mode with precise REF voltage trimming and low-offset sense path to achieve <0.5% current regulation at 100 mA hold level. Use Value: Sense current offset (20–40 mA) and DAC linearity (±3.0%) enable repeatable 0.9° microsteps without closed-loop feedback, cutting system cost versus servo alternatives. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar full-bridge PWM microstepping motor driver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| A4975 | Successor device with higher 2.5 A current rating, integrated charge pump, and SPI interface; no DIP package option - only available in SOICW (LB) and TSSOP | Designed for new designs requiring higher current or digital control; lacks direct pin compatibility with A3955SB DIP footprint | Select A4975 when upgrading to higher-power motors or adding host MCU communication; requires PCB redesign due to different package and pinout |
| A3955SLB | Electrically identical to A3955SB but in 16-pin SOICW package with internally fused pins; shares same terminal numbering and electrical specs | Used where board space is constrained and surface-mount assembly is preferred; thermal performance limited by RθJA = 67°C/W vs. 43°C/W for DIP | Choose A3955SLB for space-constrained layouts accepting reduced thermal headroom; verify copper area and airflow meet SOICW derating requirements |
Compared with A3955SB, the A4975 offers higher current and digital configurability but mandates PCB redesign, while the A3955SLB preserves full electrical equivalence at the cost of thermal performance - making A3955SB uniquely suitable for legacy through-hole designs needing maximum heatsinking without layout change.
Availability
A3955SB is available at Aetrix Electronics and suitable for industrial motion control, medical equipment servicing, and lab automation maintenance requiring stable component supply for legacy system repair and long-term support.
Supply support for A3955SB 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 ICs, specializing in motion control, current sensing, and motor driver solutions since 1989.
The A3955 product line was developed specifically for bipolar stepper motor microstepping in industrial and medical equipment, emphasizing analog flexibility, thermal robustness, and simplified system-level current regulation without microcontroller dependency.
FAQ
Is the A3955SB still in production?
No, the A3955SB is a discontinued product. Allegro MicroSystems ceased production as of January 30, 2012, and no new units are manufactured. Aetrix Electronics maintains limited legacy stock for repair and maintenance of existing systems, but the A3955SB is not recommended for new designs. For replacement planning, consider the A4975 or A3955SLB based on package and thermal requirements.
What is the difference between A3955SB and A3955SLB?
The A3955SB and A3955SLB are electrically identical full-bridge PWM microstepping drivers with shared pinout and specifications. The key distinction is packaging: A3955SB uses a 16-pin DIP with exposed thermal tabs (RθJA = 43°C/W), while A3955SLB uses a 16-pin SOICW with internally fused pins (RθJA = 67°C/W). Both require the same external components and operate identically in circuit.
Can the A3955SB drive a unipolar stepper motor?
No, the A3955SB is designed exclusively for bipolar stepper motors. Its full-bridge topology requires two leads per winding and relies on bidirectional current flow to generate torque. Unipolar motors use center-tapped windings and require different drive schemes (e.g., four independent low-side switches), which the A3955SB does not support.
What is the minimum sense resistor value for A3955SB at ±1.5 A?
Using the formula ITRIP ≈ VREF / (3 × RS) and maximum VREF = 2.5 V, the minimum RS for ±1.5 A is approximately 0.56 Ω. However, the datasheet specifies a typical RS of 0.5 Ω for 1.5 A operation with VREF = 1.5 V, balancing power dissipation in the resistor and noise immunity in the sense path.
Does the A3955SB require external flyback diodes?
No, the A3955SB integrates transient-suppression diodes across its power outputs (OUTA and OUTB), eliminating the need for external flyback diodes. These internal diodes handle inductive kickback during current decay modes and are rated for the full ±1.5 A output current and 50 V blocking voltage specified in the absolute maximum ratings.
A3955SB Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Allegro MicroSystems
- Series:
- -
- Package/Case:
- 16-PowerDIP (0.300", 7.62mm)
- Packaging:
- Tube
- Product Status:
- Obsolete
- Motor Type - Stepper:
- Bipolar
- Motor Type - AC, DC:
- -
- Function:
- Driver - Fully Integrated, Control and Power Stage
- Output Configuration:
- Half Bridge (2)
- Interface:
- Parallel
- Technology:
- Bipolar
- Step Resolution:
- 1, 1/2, 1/4, 1/8
- Applications:
- General Purpose
- Current - Output:
- 1.5A
- Voltage - Supply:
- 4.5V ~ 5.5V
- Voltage - Load:
- 4.5V ~ 50V
- Operating Temperature:
- -20°C ~ 150°C (TJ)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole
- Supplier Device Package:
- 16-DIP
A3955SB FAQ
1.How can I place an order for A3955SB through Aetrix?
Please submit a Request for Quotation (RFQ) for A3955SB on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for A3955SB reliable?
The price and inventory of A3955SB are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for A3955SB is usually 5 days.
3.What payment methods are accepted for A3955SB?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for A3955SB transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for A3955SB?
A3955SB orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your A3955SB order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for A3955SB?
For technical support, including A3955SB datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your A3955SB requirements.
6.How does Aetrix verify that A3955SB is sourced from the original manufacturer or authorized distributors?
All A3955SB products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that A3955SB meets industry standards.
7.What is the process for return or replacement of A3955SB?
All A3955SB units undergo pre-shipment inspection (PSI). If there is an issue with A3955SB, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The A3955SB part is unused and in its original packaging.
Return procedure for A3955SB:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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