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Allegro MicroSystems A4945GLJTR-T
- Part No.:
- A4945GLJTR-T
- Manufacturer:
- Allegro MicroSystems
- Category:
- Motor Drivers, Controllers
- Package:
- 8-SOIC (0.154", 3.90mm Width) Exposed Pad
- Datasheet:
-
A4945GLJTR-T.pdf
- Description:
- IC FAN DRIVER BLDC 8SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:2,973
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Product details
Overview
A4945GLJTR-T from Allegro MicroSystems is a three-phase sensorless sinusoidal fan driver IC with analog speed control, 8-pin SOICN package with exposed thermal pad (LJ), 30 kHz motor PWM frequency, ±6 LSB VSP accuracy, and integrated lock detection, overcurrent, and thermal shutdown protection. It targets medium-power cooling fans requiring low audible noise and high efficiency in industrial and computing systems.
For engineers reviewing the A4945GLJTR-T datasheet, A4945GLJTR-T pinout, A4945GLJTR-T application, or A4945GLJTR-T equivalent, this page delivers verified technical context, validated pin functions, real-world application constraints, and confirmed alternative options - all grounded in Allegro's official documentation dated September 13, 2024.
Technical Context
The A4945GLJTR-T implements a proprietary sensorless sinusoidal drive algorithm that dynamically adapts to motor back-EMF for optimal efficiency across speeds. It uses trapezoidal startup followed by automatic transition to sinusoidal operation without external components.
Speed is controlled via analog voltage on the VSP pin (0.45–1.2 V on-threshold, 194–264 mV disable threshold), converted to a 9-bit demand value driving internal PWM modulation at 30 kHz with 120° phase shift across OUTA/OUTB/OUTC. FG provides open-drain speed feedback synchronized to electrical revolutions.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Supply Voltage Range | 4 V to 18 V - supports legacy 5 V and modern 12 V fan rails; UVLO activates at 3.85–3.98 V rising edge |
| Motor PWM Frequency | 28–32 kHz - ultrasonic switching eliminates audible coil whine in fan applications |
| VSP Input Accuracy | ±6 LSB - enables precise 7.5%–100% speed demand resolution without external DAC |
| Total RDS(on) | 1.1 Ω typical (12 V, 25°C) - limits conduction loss and thermal rise in 1 A motor drive paths |
| Overcurrent Limit | 1.4–1.8 A - protects MOSFETs during stall or short-circuit while allowing peak startup current |
| Thermal Shutdown | 150–180°C junction - latches off until hysteresis (20°C) enables safe restart after cooling |
| FG Output Type | Open-drain, 10 mA sink capability - compatible with 3.3 V/5 V logic pullups for RPM monitoring |
Pinout & Package
Package: 8-pin SOICN with exposed thermal pad (LJ), lead-free, 100% matte-tin plating. Thermal resistance RθJA = 62 °C/W on 2-layer PCB with 0.8 in² copper per side.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 GND | Power and signal reference ground | Primary return path for motor current, logic, and internal regulators; must connect to thermal pad |
| 2 OUTA | High-side motor phase A output | Drives one winding of 3-phase BLDC fan; rated for 1.6 A continuous with internal FETs |
| 3 VBB | Main power supply input | Accepts 4–18 V; powers gate drivers and internal circuitry; requires 4.7–47 µF bulk capacitance |
| 4 VSP | Analog speed demand input | 0–3.05 V range; 100 kΩ internal pulldown; disables motor if below 194–264 mV |
| 5 VREF | Stable 3.3 V reference output | Supplies 3.3 V ±3% at up to 5 mA; used for external sensing or biasing; requires 0.1 µF X5R decoupling |
| 6 FG | Fan speed feedback output | Open-drain pulse train - one pulse per electrical revolution; duty cycle independent of speed |
| 7 OUTC | High-side motor phase C output | Third phase output; complements OUTA/OUTB in 120° commutation sequence |
| 8 OUTB | High-side motor phase B output | Second phase output; forms full 3-phase sinusoidal drive with OUTA and OUTC |
| PAD | Exposed thermal pad | Internally connected to GND; requires soldered thermal vias to inner ground planes for 62 °C/W performance |
Key Features
| Feature | Design Value |
|---|---|
| 180° sinusoidal drive | Reduces torque ripple and acoustic noise vs. trapezoidal drive - critical for office/server fan EMI compliance |
| Sensorless BEMF position detection | Eliminates Hall sensors or encoders; uses phase voltage sampling during PWM off-time to infer rotor angle |
| Soft-start current ramp | Gradually increases current limit and speed demand over ~1.6 s (slow ramp) to avoid inrush stress on supply and motor |
| Integrated lock detection | Disables outputs for 7–9 s upon stalled rotor detection, then auto-restarts - prevents thermal runaway |
| Supply undervoltage lockout | Holds device inactive below 3.85 V with 150–450 mV hysteresis - avoids erratic behavior during brownout |
Applications
| Server CPU Cooling Fan | Industrial PLC Cabinet Fan |
|---|---|
|
Use Scenario: Forced-air cooling of multi-core CPUs in 1U/2U rack servers where acoustic noise must remain below 45 dBA at 1 m. IC Role / Device Role / Timing Role: Three-phase sinusoidal motor driver executing closed-loop speed control via analog VSP input, with FG feedback to BMC for thermal management. Use Value: 30 kHz PWM frequency places switching noise above human hearing range; ±6 LSB VSP accuracy enables fine-grained fan curve tuning in BIOS/firmware. |
Use Scenario: Continuous-duty ventilation in IP65-rated programmable logic controller cabinets operating from –40°C to 105°C ambient. IC Role / Device Role / Timing Role: Standalone fan controller managing airflow across heat-generating I/O modules, using VSP for analog setpoint from temperature sensor amplifier. Use Value: Integrated thermal shutdown (150–180°C) and overcurrent protection (1.4–1.8 A) ensure reliability without external supervision circuits. |
| Network Switch Power Supply Fan | Medical Diagnostic Imaging Enclosure Fan |
|
Use Scenario: Redundant cooling for PoE+ switch power stages where fan failure triggers system alarm but not immediate shutdown. IC Role / Device Role / Timing Role: Motor driver with FG output feeding FPGA-based health monitor; VSP tied to DAC output for dynamic speed scaling based on load current. Use Value: Lock detection with 7–9 s timeout and auto-restart allows recovery from transient obstructions without manual intervention. |
Use Scenario: Low-noise airflow management inside MRI or CT scanner enclosures where EMI and acoustic emissions are strictly regulated. IC Role / Device Role / Timing Role: Sinusoidal drive IC eliminating commutation spikes that could interfere with sensitive analog front-ends or RF subsystems. Use Value: 180° sinusoidal waveform minimizes electromagnetic interference vs. square-wave drives, easing EMC certification. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar three-phase sensorless fan driver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| A5947KLPTR-T | Successor device with extended –40°C to 125°C operation, enhanced startup robustness, and updated protection thresholds; no pin-to-pin compatibility | Required for new designs targeting automotive-grade temperature range or higher reliability; supports same SOICN-LP package | Select when designing for long-term availability and extended temperature operation; migration requires layout and firmware updates |
| A4949GLJTR-T | PWM-speed-input variant (not analog); identical pinout, package, and protection features; shares same 30 kHz PWM frequency and thermal specs | Used where microcontroller generates variable-duty-cycle PWM instead of analog voltage; no VSP circuitry needed | Choose only if existing system already provides PWM speed control; not drop-in for analog VSP designs |
Compared with A4945GLJTR-T, A5947KLPTR-T offers lifecycle continuity and wider temperature support but requires redesign, while A4949GLJTR-T preserves mechanical fit and thermal performance but mandates PWM-based speed interface - neither is pin-compatible nor functionally interchangeable without system-level changes.
Availability
A4945GLJTR-T is available at Aetrix Electronics and suitable for server cooling, industrial cabinet ventilation, network infrastructure thermal management, and medical enclosure airflow applications requiring stable component supply - though it is discontinued per Allegro's September 13, 2024 status update.
Supply support for A4945GLJTR-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 sensing and power ICs, specializing in motion control, energy-efficient motor drivers, and precision current sensors.
The A4945 product line was engineered specifically for quiet, efficient three-phase BLDC fan control in computing and industrial environments - emphasizing sinusoidal drive, sensorless operation, and integrated protection.
FAQ
Is A4945GLJTR-T still recommended for new designs?
No. Allegro officially discontinued A4945GLJTR-T on September 13, 2024. The part is no longer in production, samples are unavailable, and new designs should migrate to A5947KLPTR-T. A4945GLJTR-T remains supported for legacy repair and continuity programs, but no future revisions or long-term supply guarantees apply.
What is the function of the exposed thermal pad on the A4945GLJTR-T LJ package?
The exposed thermal pad on the A4945GLJTR-T is internally connected to GND and serves as the primary thermal dissipation path. To achieve the specified RθJA of 62 °C/W, it must be soldered to a PCB ground plane with ≥4 thermal vias (0.3 mm diameter, spaced ≤1 mm apart) connecting to inner-layer ground planes on both sides of the board.
Can A4945GLJTR-T operate with a 5 V supply?
Yes. A4945GLJTR-T operates across 4 V to 18 V, making 5 V fully supported. At 5 V, total RDS(on) rises to 1.5–1.8 Ω (vs. 1.1–1.4 Ω at 12 V), increasing conduction loss - design must verify motor torque and thermal margin at minimum supply. UVLO releases at 3.85–3.98 V, ensuring stable startup.
How does the VSP pin on A4945GLJTR-T translate analog voltage to motor speed?
The VSP pin on A4945GLJTR-T feeds an internal 9-bit ADC. Input voltage from 0.45–1.2 V enables motor start; 0–3.05 V maps linearly to 7.5–100% speed demand. Below 194–264 mV, the driver disables. The resulting 9-bit value modulates three-phase PWM outputs at 30 kHz with 120° phase offset to generate sinusoidal current.
Does A4945GLJTR-T require external components for basic operation?
Yes. A4945GLJTR-T requires CVBB (4.7–47 µF) between VBB and GND, CVREF (0.1 µF X5R) between VREF and GND, and proper grounding of the exposed thermal pad. Optional components include RFG (20 kΩ pullup on FG), RPWM (1 kΩ noise isolation), and RPU (50 kΩ pullup on VSP for open-circuit fail-safe). No external position sensors are needed.
A4945GLJTR-T Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Allegro MicroSystems
- Series:
- -
- Package/Case:
- 8-SOIC (0.154", 3.90mm Width) Exposed Pad
- Packaging:
- Tape & Reel (TR)
- Product Status:
- Obsolete
- Motor Type - Stepper:
- Multiphase
- Motor Type - AC, DC:
- Brushless DC (BLDC)
- Function:
- Fan Control
- Output Configuration:
- Half Bridge (3)
- Interface:
- Analog
- Technology:
- Power MOSFET
- Step Resolution:
- -
- Applications:
- Fan Motor Driver
- Current - Output:
- 1.8A
- Voltage - Supply:
- 4V ~ 18V
- Voltage - Load:
- -
- Operating Temperature:
- -40°C ~ 105°C (TA)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 8-SOIC-EP
A4945GLJTR-T FAQ
1.How can I place an order for A4945GLJTR-T through Aetrix?
Please submit a Request for Quotation (RFQ) for A4945GLJTR-T 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 A4945GLJTR-T reliable?
The price and inventory of A4945GLJTR-T are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for A4945GLJTR-T is usually 5 days.
3.What payment methods are accepted for A4945GLJTR-T?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for A4945GLJTR-T transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for A4945GLJTR-T?
A4945GLJTR-T orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your A4945GLJTR-T 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 A4945GLJTR-T?
For technical support, including A4945GLJTR-T datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your A4945GLJTR-T requirements.
6.How does Aetrix verify that A4945GLJTR-T is sourced from the original manufacturer or authorized distributors?
All A4945GLJTR-T 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 A4945GLJTR-T meets industry standards.
7.What is the process for return or replacement of A4945GLJTR-T?
All A4945GLJTR-T units undergo pre-shipment inspection (PSI). If there is an issue with A4945GLJTR-T, 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 A4945GLJTR-T part is unused and in its original packaging.
Return procedure for A4945GLJTR-T:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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