Adjustable, 1.6MHz Boost Regulator with 25V Integrated FET Switch# Technical Documentation: FAN5331SX White LED Step-Up Converter
Manufacturer : FSC (Fairchild Semiconductor)
Component Type : White LED Step-Up DC/DC Converter
Document Version : 1.0
---
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN5331SX is a high-efficiency, fixed-frequency step-up DC/DC converter specifically designed for driving series-connected white LEDs. Its primary use cases include:
* Backlighting for Small to Medium LCD Displays : Powering 2 to 5 series-connected white LEDs commonly found in mobile phones, PDAs, digital cameras, and handheld gaming devices.
* Constant Current LED Drivers : Providing a stable, regulated current (up to 30mA typical) to LED strings, ensuring uniform brightness and protecting LEDs from current fluctuations.
* Battery-Powered Portable Devices : Efficiently boosting a low input voltage from a single-cell Li-Ion battery (2.5V to 5.5V) to the higher voltage required by the LED string, maximizing battery life.
### 1.2 Industry Applications
* Consumer Electronics : Dominant in smartphones, feature phones, MP3 players, and portable media players for display and keypad backlighting.
* Industrial Handhelds : Used in barcode scanners, portable measurement tools, and handheld terminals where readable displays in various lighting conditions are critical.
* Automotive Aftermarket/Portable : Found in portable GPS units and dashboard-mounted devices for backlighting, though not typically in primary automotive clusters due to wider temperature and reliability specifications required.
### 1.3 Practical Advantages and Limitations
Advantages:
* High Efficiency (>80%) : Utilizes a 1.2MHz fixed-frequency PWM control scheme with a low-resistance internal N-channel MOSFET switch, minimizing power loss and heat generation, which is crucial for battery runtime and compact form factors.
* Compact Solution : Requires minimal external components (inductor, diode, capacitors, and a sense resistor), saving valuable PCB space.
* Excellent Line/Load Regulation : Maintains consistent LED brightness despite variations in battery voltage or forward voltage (`Vf`) differences between LEDs.
* Integrated Features : Includes under-voltage lockout (UVLO) and cycle-by-cycle current limiting, enhancing system robustness.
* Dimming Capability : Brightness can be controlled via a low-frequency PWM signal applied to the `SHDN`/`PWM` pin, enabling dynamic power saving and user interface effects.
Limitations:
* Fixed Output Configuration : Designed specifically for driving LEDs in a constant current mode. It is not a general-purpose adjustable voltage regulator.
* Limited Current Drive : Maximum output current is suitable for backlighting but not for high-power illumination or flash applications.
* Frequency-Sensitive Layout : The 1.2MHz switching frequency requires careful PCB layout to minimize noise and ensure stable operation.
* Component Dependency : Overall efficiency and performance are dependent on the correct selection of external passive components, particularly the inductor and Schottky diode.
---
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
* Pitfall 1: Inductor Saturation .
* Problem : Using an inductor with a saturation current rating lower than the peak switch current can cause drastic efficiency drops and potential device failure.
* Solution : Select an inductor with a saturation current rating significantly higher than the calculated peak inductor current (`Ipeak`). Use the formula `Ipeak = Iled + (ΔIL / 2)`, where `ΔIL` is the inductor ripple current.
* Pitfall 2: Excessive Output Ripple/Noise .
* Problem : In
