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LMG3624 in Practice: Slew-Rate Tuning, Shuntless Sensing, and GaN Design Pitfalls

  • Contents

Quick-Reference Card: LMG3624 at a Glance

Attribute Detail
Component Type 700V GaN Power FET with Integrated Driver
Manufacturer Texas Instruments
Key Spec 155mΩ RDS(on) with 700V Drain-to-Source Voltage
Supply Voltage See datasheet for VCC/AUX range
Package Options 38-pin VQFN (8mm x 5.3mm)
Lifecycle Status Active
Best For High-efficiency 65W+ USB-C PD adapters and chargers

LMG3624 product photo in VQFN package


1. What Is the LMG3624? (Definition + Architecture)

The LMG3624 is a 700V, 155mΩ GaN power FET from Texas Instruments that integrates a high-speed gate driver, protection circuitry, and current-sense emulation into a single package. By combining the power stage with its own driver, TI has minimized the parasitic inductance that typically plagues discrete GaN designs, allowing for much higher switching frequencies without the usual stability headaches.

1.1 Core Architecture & Design Philosophy

Unlike traditional silicon MOSFETs, the LMG3624 uses a Gallium Nitride (GaN) HEMT architecture. The "intelligence" of this chip lies in its integrated driver. Instead of the engineer struggling with gate resistor selection to prevent Miller-effect turn-on, the LMG3624 handles the gate drive internally. This ensures the FET is always driven at the optimal voltage for efficiency while protecting the fragile GaN gate structure from overvoltage.

1.2 Where It Fits in the Signal Chain / Power Path

In a typical AC/DC flyback or quasi-resonant converter, the LMG3624 serves as the primary-side power switch. It is driven by a PWM controller (like a UCC series controller) and directly drives the primary winding of the transformer. Because it includes current-sense emulation, it eliminates the need for a bulky, heat-generating shunt resistor in the high-current path.


2. Electrical Characteristics: The Numbers That Matter

2.1 Power Supply & Consumption Profile

The LMG3624 features a low quiescent current of approximately 240μA (AUX) and a standby current of 50μA. For designers, this is critical for meeting "No-Load" power consumption standards (like CoC Tier 2 or DoE Level VI), where every milliwatt saved on the primary side counts toward compliance.

2.2 Performance Specs (Speed, Accuracy, or Efficiency)

The 155mΩ RDS(on) is optimized for the 65W to 100W power bracket. While lower resistance parts exist, this value provides a "sweet spot" for balancing conduction losses against the cost of the die. The 700V VDS max rating provides a comfortable 100V+ safety margin over the typical 500V-600V reflected voltages seen in universal input (85-265VAC) flyback designs.

2.3 Absolute Maximum Ratings — What Will Kill It

  • Drain-to-Source Voltage (VDS): 700V. Exceeding this during a lightning surge or due to excessive leakage inductance ringing will cause immediate catastrophic failure.
  • Operating Junction Temperature: 125°C. GaN devices can fail gracefully but exceeding this limit significantly degrades the R-on and can lead to thermal runaway.
  • Drain Current (ID max): 6A. This is a hard limit; the integrated overcurrent protection (OCP) is designed to trip before this is reached, but layout-induced spikes can still be dangerous.

3. Pinout & Package Guide

3.1 Pin-by-Pin Functional Groups

Pin Group Pins Function
Power Drain, Source High-voltage power path
Supply VCC / AUX Internal logic and driver supply
Control PWM Input signal from the controller
Configuration R_Slew Resistor-to-ground to set switching speed
Status FLT Open-drain fault indicator (OTP/OCP)
Sensing CS Current-sense emulation output

3.2 Package Variants & Soldering Notes

Package Pitch Thermal Pad? Soldering Method
VQFN (8x5.3mm) 0.5mm Yes (Bottom) Reflow Only

Design Note: The 0.5mm pitch requires precise solder paste stencil design to avoid bridging. The bottom-side thermal pad is the primary cooling path; it must be stitched to internal ground planes with multiple vias to prevent localized hotspots.

3.3 Part Number Decoder

A typical TI code for this series follows: LMG3624 [X] [Y] * LMG: GaN Power Stage Family. * 3624: Specific model (155mΩ, 700V). * Suffix: Usually denotes packaging (e.g., "RWY" for the VQFN package) and tape/reel quantity.


4. Known Issues, Errata & Real-World Pain Points

4.1 EMI and Ringing vs. Switching Loss Trade-off

Problem: GaN switches so fast that it can create significant EMI (Electromagnetic Interference) and voltage ringing on the switching node, which may fail FCC/CE testing. Root Cause: High dV/dt (voltage change over time) interacting with PCB trace inductance. Recommended Fix: Utilize the programmable turn-on slew rate feature. By adjusting the resistor on the R_Slew pin, you can "soften" the turn-on edge. Start with a slower slew rate to pass EMI, then decrease resistance to improve efficiency until you hit the EMI limit.

4.2 Thermal Management in High-Density Adapters

Problem: In ultra-small 65W chargers, the LMG3624 may hit the Overtemperature Protection (OTP) limit. Root Cause: Concentrated heat on the small VQFN footprint and lack of airflow. Recommended Fix: Connect the low-side thermal pad to a large copper pour on the PCB. Since the LMG3624 uses current-sense emulation, you no longer have the heat from a 1206-sized shunt resistor adding to the thermal budget—use that saved board space for extra copper cooling.


5. Application Circuits & Integration Examples

5.1 Typical Application: 65W USB-C PD Adapter

In this scenario, the LMG3624 acts as the main switch in a Quasi-Resonant (QR) flyback. The CS pin provides a voltage signal proportional to the FET current back to the PWM controller, simulating a traditional shunt resistor.

5.2 Interface Example: Driving with a PWM Controller

The PWM input is compatible with standard 3.3V and 5V logic levels.

// Conceptual logic for a controller driving LMG3624
if (voltage_feedback < target) {
    set_pwm_high(GATE_PIN); // LMG3624 internal driver handles the GaN gate
}
if (read_CS_pin() > OCP_THRESHOLD) {
    set_pwm_low(GATE_PIN);  // Cycle-by-cycle protection
}

6. Alternatives, Replacements & Cross-Reference

6.1 Pin-Compatible Drop-In Replacements

Note: GaN Power ICs are highly proprietary. "Drop-in" compatibility is rare across manufacturers.

Part Number Manufacturer Key Difference Compatible?
LMG3622 Texas Instruments Lower RDS(on) for higher power ?? (Check Pinout)
InnoSwitch5-Pro Power Integrations Includes secondary-side control ? (Different Topology)

6.2 Upgrade Path (Better Performance)

For 100W+ designs, look at the LMG3622. It offers lower RDS(on) to handle higher peak currents without excessive heating.

6.3 Cost-Down Alternatives

If GaN is too expensive, the Infineon CoolMOS S7 series offers high-performance Silicon MOSFETs, though you will lose the high-frequency benefits and integrated sensing of the LMG3624.


7. Procurement & Supply Chain Intelligence

  • Lifecycle Status: Active. TI is heavily invested in the LMG series.
  • Typical MOQ & Lead Time: Standard reels are 2,500 units. Lead times have stabilized to 12-18 weeks post-2023.
  • BOM Risk Factors: Single-source technology. Because the current-sense emulation is proprietary, switching to a competitor like Navitas would require a complete PCB redesign.
  • Authorized Distributors: Digi-Key, Mouser, Arrow, and TI Direct.

8. Frequently Asked Questions

Q: What is the LMG3624 used for? A: It is primarily used as the high-voltage switch in AC/DC adapters, chargers (like 65W USB-C phone/laptop bricks), and LED drivers where high efficiency and small size are required.

Q: What are the best alternatives to the LMG3624? A: Depending on your topology, the Navitas GaNFast series or Power Integrations InnoSwitch5-Pro are the closest competitors, though they require different circuit layouts.

Q: Can the LMG3624 work with 3.3V logic? A: Yes, the PWM input pin is designed to interface directly with standard 3.3V and 5V microcontrollers or PWM controllers.


9. Resources & Tools

  • Official Datasheet: Available on the Texas Instruments LMG3624 product folder.
  • Evaluation Board: LMG3624EVM-081 (Check TI website for availability).
  • Reference Designs: PMP series designs from TI PowerLab.
  • SPICE Model: Available for TI-TINA and PSpice on the manufacturer's site.

LMG3624REQR Documents & Media

Download datasheets and manufacturer documentation for Texas Instruments LMG3624REQR.
Datasheets
datasheet

LMG3624REQR PCB Symbol, Footprint & 3D Model

Texas Instruments LMG3624REQR

Texas Instruments

Gate Drivers 650V 170mohm GaN FET with integrated driver, protection and current sensing

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