SECONDARY SUPERVISORY IC# Technical Documentation: FAN7687MX Synchronous Buck Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component Type : High-Efficiency Synchronous Buck PWM Controller IC
Primary Function : Provides control logic and drive signals for external N-channel MOSFETs in synchronous buck (step-down) DC-DC converter topologies.
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## 1. Application Scenarios
### Typical Use Cases
The FAN7687MX is designed as a versatile, high-performance PWM controller for constructing voltage regulator modules (VRMs) and point-of-load (POL) converters. Its primary use cases include:
* Intermediate Bus Voltage Conversion : Stepping down a 12V or 5V intermediate bus voltage to lower voltages (e.g., 3.3V, 2.5V, 1.8V, 1.2V, 1.0V) required by modern digital ICs.
* CPU/GPU Core Voltage Regulators : Providing the high-current, tightly regulated supply needed for processors and graphics processing units in computing equipment.
* Distributed Power Architectures : Serving as a localized POL converter on daughter cards or system boards, converting a higher system rail to specific voltages for ASICs, FPGAs, memory banks, or interface circuits.
### Industry Applications
* Computing & Servers : Motherboard VRMs, SSD power supplies, add-in card power regulation.
* Telecommunications & Networking : Power supplies for routers, switches, and baseband units, where efficiency and reliability are critical.
* Industrial Electronics : Power modules for PLCs, motor drives, and test/measurement equipment requiring stable, efficient DC power conversion.
* Consumer Electronics : High-end set-top boxes, gaming consoles, and display panels.
### Practical Advantages and Limitations
Advantages:
* High Efficiency: Utilizes synchronous rectification (using a low-side N-MOSFET instead of a diode), significantly reducing conduction losses, especially at low output voltages and high currents.
* Wide Input Range: Typically operates from input voltages as high as 24V, offering design flexibility.
* Adjustable Frequency: Allows optimization of the trade-off between efficiency (lower frequency) and component size (higher frequency).
* Integrated Features: Includes under-voltage lockout (UVLO), programmable soft-start, and over-current protection (OCP), reducing external component count and improving system robustness.
* Dual N-Channel Drive: Directly drives standard-level N-MOSFETs, simplifying gate drive circuitry.
Limitations:
* External MOSFETs Required: Performance and cost are heavily dependent on the selection and characteristics of the external power MOSFETs and inductor.
* Design Complexity: Requires careful compensation network design for stable operation across all load conditions, demanding more expertise than simple linear regulators or integrated switchers.
* Noise Sensitivity: As a high-frequency switching controller, PCB layout is critical to minimize EMI and ensure stable switching.
* Minimum Load: Some configurations may require a minimum load to maintain regulation, which can be a constraint for very low-power standby states.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Instability and Ringing:
* Pitfall: Poorly designed feedback compensation network or improper component selection (R/C) for the error amplifier.
* Solution: Meticulously calculate the Type II or Type III compensation network based on the output LC filter's characteristics and desired crossover frequency. Use the manufacturer's design tools or application notes as a guide. Always verify stability with load transient testing.
2. Excessive MOSFET Heating:
* Pitfall: Inadequate MOSFET selection (high Rds(on), poor gate charge Qg) or insufficient gate drive strength leading
