FULL BRIDGE LCD BACKLIGHT CONTROLLER# Technical Documentation: FAN7315 Ballast Control IC
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN7315 is a high-performance, resonant half-bridge controller IC specifically designed for electronic ballast applications . Its primary function is to drive two power MOSFETs in a half-bridge configuration to generate a high-frequency AC voltage, which is then used to power fluorescent lamps (typically CFLs - Compact Fluorescent Lamps) and other gas-discharge lighting systems.
Core Operational Sequence:
1. Preheat: The IC initiates a low-frequency, high-current phase to gently warm the lamp filaments, extending lamp life.
2. Ignition: It then drives the resonant tank circuit (formed by an inductor and capacitor) into resonance, generating a high-voltage pulse (typically 600V to 1kV) across the lamp to ionize the gas and initiate the arc.
3. Run: Once ignited, the IC stabilizes operation at a higher frequency, regulating lamp current for steady-state illumination.
4. Protection: It continuously monitors for fault conditions like lamp failure, open filaments, or excessive current.
### 1.2 Industry Applications
* Residential & Commercial Lighting: Primary application is in self-ballasted Compact Fluorescent Lamps (CFLs) , replacing incandescent bulbs.
* Linear Fluorescent Lamp Ballasts: Used in electronic ballasts for T5, T8, and T12 tube lights in offices, factories, and retail spaces.
* Specialty Lighting: Can be adapted for other cold-cathode fluorescent lamps (CCFLs) used in backlighting applications (though largely superseded by LEDs in this field).
### 1.3 Practical Advantages and Limitations
Advantages:
* High Integration: Combines oscillator, preheat/ignition timing, dead-time control, and multiple protection features (over-current, open-lamp, filament failure) in one 8-pin package, reducing external component count.
* Reliable Ignition: Dedicated ignition timer ensures a consistent, controlled high-voltage strike for reliable lamp starting.
* Robust Protection: Integrated protections enhance system reliability and safety, preventing catastrophic failure from lamp faults.
* Efficiency: Resonant switching topology minimizes switching losses in the power MOSFETs, leading to high electrical efficiency.
Limitations:
* Application Specific: Designed almost exclusively for fluorescent ballasts. Not suitable for driving LEDs directly without significant external redesign.
* Frequency-Dependent Control: Lamp power regulation is achieved via frequency modulation, which can make EMI filtering more challenging compared to fixed-frequency PWM controllers.
* Legacy Technology: The component and its primary application (CFLs) are in decline due to the global market shift towards LED lighting, which offers higher efficiency and longer life.
* High-Voltage Handling: Requires external high-voltage half-bridge MOSFETs and careful handling of high voltages present at the lamp connections.
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## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
| Pitfall | Consequence | Solution |
| :--- | :--- | :--- |
| Incorrect Resonant Tank (Lr, Cr) Values | Failed ignition (voltage too low) or excessive ignition voltage stressing components. | Precisely calculate resonant frequency and impedance based on lamp specifications. Use the formula `f_res = 1/(2π√(Lr*Cr))` and ensure the Q-factor provides sufficient voltage gain. |
| Poor Thermal Management for MOSFETs | Overheating and premature failure of the power switches. | Use MOSFETs with adequate current rating and low Rds(on). Ensure sufficient PCB copper area (heatsinking) for the MOSFET packages. Consider a small heatsink for higher
