feature

Parallel LED driver supports all forward voltages Adaptive VOUT adjustment to highest diode voltage Internally matched LED current source No external components required to set LED current Built-in charge pump has three modes of operation: – Linear regulation VIN > 4.2V – 3 /2 DC-DC converter and regulated 3.6V

application

Mobile phone handheld computer PDA, digital camera, MP3 player keyboard backlight LED display

illustrate

The fan 5609 runs from a battery with an input voltage between 2.7 volts and 5.5 volts. Switch reconfiguration and fractional switching techniques are used to achieve high efficiency across the input voltage range. Proprietary internal circuitry monitors each LED current loop and automatically adjusts the resulting output DC voltage to the minimum value required for the LED with the highest forward voltage. This adaptive nature of the Fan 5609 eliminates the need for LED preselection (matching) and ensures high efficiency operation. When the input voltage is high enough to maintain the programmed current level in the LED, the FAN5609 is reconfigured to run the regulator in a linear fashion, and the DC-DC converter is turned off. An inline dual digital-to-analog converter provides programmable output current. Only two 0.1µF barrel capacitors and two 4.7µF input/output capacitors are required for operation. A soft-start circuit prevents the power from being turned on. The unit has built-in short-circuit protection.

Circuit Description: The FAN5609's switched capacitor DC/DC converter automatically configures its internal switches for high efficiency - and provides tightly regulated output current LEDs. An analog detector determines which diode requires the highest voltage to maintain a preset current level and adjusts the pump regulator accordingly. Each diode has its own linear current regulator. In addition, the voltage regulator controls the output voltage to provide the maximum possible efficiency when the battery voltage is within the linear regulation range. If the battery voltage is too low to maintain the diode current in linear mode, the fractional 3:2 charge pump is enabled. When the battery voltage drops further, this mode is no longer sufficient for normal operation and the pump automatically reconfigures to operate in 2:1 mode. As the battery discharges the voltage decays and the fan 5609 switches between modes to maintain a constant current through the LEDs throughout the battery life. This transition has a hysteresis to prevent switching. Supply Voltage The device's internal supply voltage is automatically selected from the VIN or VOUT pins, whichever is higher. Soft start is when the device is initially started and enabled. The reference voltage controls the rate at which the output voltage rises to its final value. Typical startup time is 1 ms. The rate at which the output voltage ramps up is controlled by an internally generated slow ramp with an internal variable resistor limiting the input current.

Shutdown and Short-Circuit Current Limit sets both DAC inputs low to shut down the device. Built-in short-circuit protection limits supply current to a maximum of 50mA. A digitally controlled digital-to-analog converter (DAC) allows the following modes to be selected: Off, 7mA, 14mA, 20mA, per diode. By opening and closing the input B pin, the current can be modulated between 7 and 20 mA to any average value (pulse width modulation). In PWM mode, the modulation frequency must be set high enough to avoid flickering effects (100 Hz to 1 Hz). PWM-Based Digital Control Any input can be modulated by a pulse train of variable duty cycle (delta). By turning the DAC input A or B current on and off, it can be continuously modulated to any average value between 1 and 19 mA. For the maximum range of LED current, A and B can be modulated simultaneously.

notes:

2. Select the duty cycle proportionally to obtain a typical LED current between 1mA and 6mA.

3. If input A or B is continuously high, the other input can be modulated at a maximum rate of 30kHz. Otherwise the maximum modulation rate should be limited to 1kHz.

4. Select duty cycle proportionally to achieve typical LED current between 8mA and 19mA.

Application Information Selecting Capacitors Select the appropriate capacitor type and value for the FAN5609. These capacitors determine parameters such as power efficiency, maximum charge pump continuous load current, input and output ripple and start-up time. To reduce ripple, both CIN and COUT should be low ESR capacitors. Increasing the COUT capacitor can reduce the output ripple voltage. But this increases power time. The CIN value controls the input ripple. If desired, this energized fan 5609 can further reduce the ripple through a very small series inductor filter, as shown in Figure 3. The current capability of AP1 and CAP2 to control the charge affects the overall efficiency of the system. A lower value will improve efficiency, but it may limit the LED's current at low input voltages. A 100nF capacitor is an optimal 2.7V to 4.2V for a 4×20mA load over the entire input voltage range. Save space and cost and improve efficiency - when the load is less than 4 x 7 mA. Pulse Width Modulation (PWM) mode transition errors are minimized, and the best LED-to-LED matched settings are achieved over the entire average current range, and LED currents between zero and maximum when modulated using PWM brightness control ( A=1, B=1). PC Board Layout For best performance, it is recommended to use a solid ground plane on the backside of the printed circuit board. CIN and COUT should be connected together, close to the IC