Programmable Indicator “Soft” LED Blinker with TinyWire? Single-Wire Interface # Technical Documentation: FAN5646UC00X LED Driver
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component Type : White LED Step-Up Converter with Integrated Schottky Diode
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## 1. Application Scenarios
### Typical Use Cases
The FAN5646UC00X is a compact, high-efficiency white LED driver designed for portable electronics requiring consistent backlighting. Its primary function is to drive up to 6 series-connected white LEDs from a single-cell lithium-ion battery (2.7V to 5.5V input range). The device employs a current-mode, fixed-frequency boost converter architecture with integrated power switch and Schottky diode, minimizing external component count.
Key operational modes include:
- PWM Dimming : External pulse-width modulation (0-100% duty cycle) for precise brightness control
- Analog Dimming : DC voltage adjustment via the CTRL pin for simplified dimming implementations
- Enable/Disable : Low-power shutdown mode (<1µA typical) when backlighting is not required
### Industry Applications
| Application Sector | Specific Implementations | Key Requirements Addressed |
|-------------------|--------------------------|----------------------------|
| Mobile Devices | Smartphone LCD backlights, Keypad illumination | Small footprint, High efficiency (>85%), Low noise |
| Portable Media | MP3 players, Handheld gaming displays | Extended battery life, Smooth dimming transitions |
| Industrial PDAs | Barcode scanners, Portable measurement tools | Reliable operation in varying temperatures, Robust EMI performance |
| Consumer Electronics | Digital camera displays, Portable DVD players | Consistent brightness across LED strings, Thermal protection |
### Practical Advantages and Limitations
Advantages:
- High Integration : Combines power switch, Schottky diode, and control circuitry in a 6-pin µSMD package (1.6×1.6×0.6mm)
- Excellent Efficiency : Typically 85-90% across load range, extending battery runtime
- Precision Current Regulation : ±5% LED current accuracy ensures uniform brightness
- Minimal External Components : Requires only 4 external components (inductor, capacitor, resistor, optional diode)
- Thermal Protection : Automatic shutdown at approximately 160°C junction temperature
Limitations:
- Fixed LED Count : Optimized for 4-6 series LEDs; not suitable for parallel LED configurations
- Limited Input Range : Maximum 5.5V input restricts use in multi-cell battery applications
- Package Thermal Constraints : Small package limits continuous power dissipation to approximately 400mW
- No Built-in Soft-Start : Requires external capacitor on CTRL pin for controlled startup
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
| Pitfall | Consequence | Solution |
|---------|-------------|----------|
| Inadequate Inductor Selection | Reduced efficiency, excessive ripple, potential instability | Select inductor with:
• 4.7µH to 10µH value
• Saturation current >500mA
• Low DCR (<200mΩ) |
| Improper Feedback Resistor | Incorrect LED current, reduced brightness or LED damage | Calculate using: R_FB = 0.1V / I_LED
Example: For 20mA LED current: R_FB = 0.1V / 0.02A = 5Ω |
| Insufficient Input Decoupling | Voltage spikes, unstable operation, EMI issues | Place 10µF ceramic capacitor within 5mm of VIN pin, plus 0.1µF ceramic directly adjacent |
| Poor Thermal Management | Premature thermal shutdown, reduced reliability | Ensure adequate copper area
