Eliminating Rectifier Heat: LT4320 / LT4320-1 9V-72V Ideal Diode Bridge Controller Deep Dive

Executive Summary: What is the LT4320 / LT4320-1?

The LT4320 / LT4320-1 is an ideal diode bridge controller designed to drive four external N-channel MOSFETs, replacing a traditional full-wave diode bridge rectifier to significantly reduce power dissipation and voltage drop. By eliminating the standard 0.7V to 1.2V drop of silicon or Schottky diodes, it enables smaller power supply designs and higher efficiency.

• Market Position: High-performance thermal management and power efficiency solution for industrial and networking sectors.

• Top Features: Wide 9V to 72V operating range, support for up to 600Hz (LT4320-1), and an integrated charge pump for all N-channel designs.

• Primary Audience: Ideal for PoE hardware designers, industrial power engineers, and high-fidelity audio manufacturers.

• Supply Status: Active (Widely available through authorized distributors).

1. Technical Specifications & Performance Analysis

1.1 Core Architecture (Ideal Diode Control)

The LT4320 functions by sensing the voltage across the external MOSFETs and turning them on or off to emulate the behavior of a diode, but with the ultra-low resistance ($R_{DS(ON)}$) of a transistor. Unlike passive bridges, it includes an internal charge pump that generates the necessary gate-drive voltage to allow the use of cost-effective N-channel MOSFETs for both the high-side and low-side of the bridge.

1.2 Key Electrical Characteristics

• Operating Voltage: 9V to 72V, making it suitable for 12V, 24V, and 48V nominal systems.

• Quiescent Current: Typically 1.5mA, ensuring minimal overhead in battery-backed or energy-sensitive systems.

• Voltage Drop: Effectively reduced to $I_{load} \times R_{DS(ON)}$, often resulting in millivolt-level drops compared to the nearly 1V drop of standard bridges.

• Frequency Support: The LT4320 supports DC to 60Hz (standard AC mains), while the LT4320-1 variant supports up to 600Hz for specialized power distribution.

1.3 Interfaces and Connectivity

While the LT4320 is an analog controller, its "interface" consists of high-speed gate drivers capable of switching large MOSFETs. It is designed for "set and forget" hardware implementation without the need for SPI/I2C configuration.

2. Pinout, Package, and Configuration

2.1 Pin Configuration Guide

The device is streamlined for ease of layout, typically grouping input and output pins to minimize high-current loop areas.

• IN1 / IN2: AC or DC input pins.

• OUTP / OUTN: Positive and Negative rectified DC outputs.

• TG1 / TG2: Top-side N-channel MOSFET gate drives.

• BG1 / BG2: Bottom-side N-channel MOSFET gate drives.

2.2 Naming Convention & Ordering Codes

• LT4320: Optimized for DC to 60Hz (Standard AC line rectification).

• LT4320-1: Optimized for DC to 600Hz (Higher frequency AC or faster transient response).

• Suffixes (I/H): "I" denotes industrial temperature range (-40°C to 85°C); "H" denotes high temperature (-40°C to 125°C).

2.3 Available Packages

3. Design & Integration Guide (For Engineers & Makers)

3.1 Hardware Implementation

• Bypass Capacitors: Place a 1µF (minimum) ceramic capacitor across OUTP and OUTN close to the IC to stabilize the internal charge pump.

• PCB Layout: Use thick copper pours for the traces connecting the MOSFET drains and sources, as these will carry the full load current.

• Thermal Management: While the IC stays cool, the external MOSFETs still generate some heat based on $I^2R$ losses. However, most designs can eliminate bulky heatsinks entirely.

3.2 Common Design Challenges

• Issue: High-Frequency Rectification: The LT4320 is not a high-speed synchronous rectifier for 100kHz SMPS.

• Fix: Use it only for input polarity protection or 50/60/400Hz mains rectification.

• Issue: Voltage Spikes: AC lines often have transients exceeding 72V.

• Fix: Always install a TVS (Transient Voltage Suppressor) diode across the output (OUTP to OUTN) to clamp spikes.

4. Typical Applications & Use Cases

4.1 Real-World Example: PoE Powered Devices

In Power over Ethernet (PoE) applications, the input voltage can come from either polarity on the Ethernet pairs. Using an LT4320 instead of a diode bridge allows the device to run much cooler, which is critical in the small, sealed enclosures typical of IP Security Cameras.

5. Alternatives and Cross-Reference Guide

• Direct Replacements: There are few pin-compatible "ideal bridge" controllers, but the Onsemi FDMQ8205A (GreenBridge) offers an integrated MOSFET approach for lower current levels.

• Discrete Alternatives: Using four TI LM74670-Q1 "Smart Diode" controllers can achieve a similar result but requires four separate ICs, significantly increasing complexity.

• Cost-Effective Options: Standard Schottky Bridge Rectifiers (e.g., GBU8J) are significantly cheaper but require large heatsinks and waste 2W-10W of power in high-current designs.

6. Frequently Asked Questions (FAQ)

• Q: What is the difference between LT4320 and LT4320-1?  A: The LT4320 is for standard 50/60Hz AC, while the LT4320-1 supports up to 600Hz, making it suitable for 400Hz airborne power systems.

• Q: Can LT4320 be used in automotive 12V systems?  A: Yes, it is excellent for reverse polarity protection, but ensure you have adequate TVS protection against "load dump" transients.

• Q: Does it require a separate power supply?  A: No, it is self-powered from the input voltage (9V-72V).

• Q: Can I use any N-channel MOSFET?  A: You should select MOSFETs with a gate threshold voltage ($V_{GS(th)}$) compatible with the LT4320’s gate drive (typically ~7V-10V above the rail).

7. Resources

• Datasheet: Available via Analog Devices website.

• Simulation: Supported in LTspice for thermal and efficiency modeling.

• Evaluation Boards: DC1823A (for DFN) or DC1823B (for MSOP).