Overview
The FAN7688SJX is an advanced Pulse Frequency Modulated (PFM) controller designed for LLC resonant converters with Synchronous Rectification (SR). It is produced by onsemi and offers best-in-class efficiency for isolated DC/DC converters. The controller employs a current mode control technique based on charge control, combining the triangular waveform from the oscillator with integrated switch current information to determine the switching frequency. This approach simplifies the feedback loop design and allows for true input power limit capability. Additionally, it features closed-loop soft-start, which prevents error amplifier saturation and ensures monotonic rising of the output voltage regardless of the load condition.
Key Specifications
| Parameter | Min | Typ | Max | Unit |
|---|---|---|---|---|
| VDD Pin Supply Voltage to GND | 0 | - | 20 | V |
| Operating Ambient Temperature | -40 | - | 125 | °C |
| Startup Supply Current (VDD = 9 V) | - | 80 | 115 | μA |
| Operating Current (VCOMP = 0.1 V) | - | 2.8 | - | mA |
| Switching Frequency Range | 39 kHz | - | 690 kHz | - |
| UVLO (Under-Voltage Lockout) | - | 8.5 | - | V |
| Soft-Start Capacitor Charge Current | 8.4 | 10.5 | 12.6 | μA |
Key Features
- Secondary Side PFM Controller for LLC Resonant Converter with Synchronous Rectifier Control
- Charge Current Control for Better Transient Response and Easy Feedback Loop Design
- Adaptive Synchronous Rectification Control with Dual Edge Tracking
- Closed Loop Soft-Start for Monotonic Rising Output
- Wide Operating Frequency (39 kHz ~ 690 kHz)
- Green Functions to Improve Light-Load Efficiency:
- Symmetric PWM Control at Light-Load to Limit the Switching Frequency while Reducing Switching Losses
- Disabling SR at Light-Load Condition
- Protection Functions with Auto-Restart:
- Over-Current Protection (OCP)
- Output Short Protection (OSP)
- NON Zero-Voltage Switching Prevention (NZS) by Compensation Cutback (Frequency Shift)
- Power Limit by Compensation Cutback (Frequency Shift)
- Overload Protection (OLP) with Programmable Shutdown Delay Time
- Over-Temperature Protection (OTP)
- Programmable Dead Times for Primary Side Switches and Secondary Side Synchronous Rectifiers
- VDD Under-Voltage Lockout (UVLO)
- Wide Operating Temperature Range -40°C to +125°C
Applications
- Desktop ATX and Desktop-Derived Server Power Supplies
- Blade and Telecom Power Supplies
- Intelligent 100 W - 2 kW+ Off-Line Power Supplies
- High Efficiency Isolated DC-DC Converters
- Large Screen Display Power (e.g., LED TV, PDP TV, LCD TV)
- Game Console Power Supplies
- Industrial Power Supplies
Q & A
- What is the primary function of the FAN7688SJX controller?
The FAN7688SJX is a Pulse Frequency Modulated (PFM) controller for LLC resonant converters with Synchronous Rectification (SR), designed to offer high efficiency for isolated DC/DC converters.
- What control technique does the FAN7688SJX employ?
The FAN7688SJX employs a current mode control technique based on charge control, combining the triangular waveform from the oscillator with integrated switch current information to determine the switching frequency.
- What are the key benefits of the closed-loop soft-start feature?
The closed-loop soft-start prevents error amplifier saturation and ensures monotonic rising of the output voltage regardless of the load condition.
- What is the operating frequency range of the FAN7688SJX?
The operating frequency range is from 39 kHz to 690 kHz.
- What protection functions are included in the FAN7688SJX?
The controller includes Over-Current Protection (OCP), Output Short Protection (OSP), NON Zero-Voltage Switching Prevention (NZS), Power Limit by Compensation Cutback, Overload Protection (OLP) with programmable shutdown delay time, and Over-Temperature Protection (OTP).
- How does the FAN7688SJX improve light-load efficiency?
The FAN7688SJX improves light-load efficiency through symmetric PWM control at light-load to limit the switching frequency while reducing switching losses and by disabling SR at light-load conditions.
- What is the operating temperature range of the FAN7688SJX?
The operating temperature range is from -40°C to +125°C.
- What are some typical applications of the FAN7688SJX?
Typical applications include desktop ATX and server power supplies, blade and telecom power supplies, intelligent off-line power supplies, high efficiency isolated DC-DC converters, and large screen display power supplies.
- How does the adaptive synchronous rectification control work?
The adaptive synchronous rectification control uses dual edge tracking to minimize the body diode conduction time, thus maximizing efficiency.
- What is the purpose of the programmable dead times for primary and secondary side switches?
The programmable dead times allow for customization of the switching times for both primary side switches and secondary side synchronous rectifiers to optimize performance.
