LED Driver with Adaptive Charge Pump DC/DC Converter# Technical Documentation: FAN5609MPX LED Driver
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component Type : Monolithic, Constant-Current, Step-Up DC/DC Converter for LED Driving
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## 1. Application Scenarios
### Typical Use Cases
The FAN5609MPX is a specialized switching regulator designed primarily for driving series-connected white or blue LEDs in small to medium-sized backlighting applications. Its core function is to provide a stable, regulated constant current to an LED chain, independent of variations in input voltage or LED forward voltage.
* Primary Function : It operates as a step-up (boost) converter, making it ideal for situations where the required LED string voltage exceeds the available battery or input supply voltage. A common example is powering 4 series-connected white LEDs (each with a typical forward voltage of 3.2V to 3.6V, requiring ~13-15V) from a single-cell Lithium-ion battery (2.8V to 4.2V).
* Current Regulation : The device uses an external sense resistor to set a precise LED current. This constant-current drive is critical for maintaining uniform LED brightness and color temperature, as LED intensity is directly proportional to forward current.
* Dimming Control : It typically features a dedicated pin for brightness control, compatible with either a DC voltage (analog dimming) or a Pulse-Width Modulation (PWM) signal. PWM dimming allows for a wide dimming range (e.g., 1000:1) without shifting the LED's chromaticity.
### Industry Applications
1. Mobile Device Displays : The primary application is backlighting for monochrome or color LCD displays in feature phones, PDAs, portable media players, and handheld GPS units.
2. Portable Instrumentation : Used in handheld medical devices, industrial scanners, and test equipment where a bright, even backlight is required for readability.
3. Consumer Electronics : Suitable for auxiliary lighting, indicator bars, or small panel lighting in various battery-operated gadgets.
4. Automotive Clusters : Can be employed in secondary interior lighting or small display backlights, though it must be evaluated against the stringent automotive electrical and environmental standards.
### Practical Advantages and Limitations
Advantages:
* High Efficiency (Typically >80%) : Minimizes power loss and extends battery life in portable devices. Efficiency is maximized by the synchronous rectifier architecture (if present in this specific variant).
* Compact Solution : Integrating the power switch and control logic into a single package reduces the total component count and PCB footprint.
* Excellent Line/Load Regulation : Maintains consistent LED brightness despite battery discharge or input voltage ripple.
* Integrated Protection : Usually includes features like open-LED protection (output over-voltage clamp) and cycle-by-cycle current limiting for the internal switch.
Limitations:
* Limited Output Power : Designed for driving LED strings with moderate current (e.g., up to 25-30mA per string). It is not suitable for high-power LED applications like general lighting or automotive headlights.
* Step-Up Only Topology : Cannot be used in applications where the input voltage may always be higher than the required LED voltage (a buck or linear driver would be more efficient).
* External Component Dependency : Performance and stability rely on the proper selection of external passive components (inductor, capacitors, sense resistor).
* Thermal Management : In compact enclosures or at high ambient temperatures, the package's thermal resistance can limit the maximum sustainable output current.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Inductor Selection Causing Instability or Poor Efficiency.
* Solution : Choose an inductor with a saturation
