Features: Two independent channels, each of which can drive up to six LED channels Independent (digital) brightness control of two channels LED current can be controlled at 0 to 20mA digital, analog and PWM brightness control 2.7V to 5V input voltage range 0.5MHz operating frequency (8MHz internal clock) Soft start Low shutdown current: ICC <1µA minimum external components required for LED short circuit protection Available in 8-lead MLP and 12-lead MLP packages

Application: Mobile Phone Handheld Computer PDA, DSC, MP3 Player Keypad Backlight LED Display Description: FAN 5608 LED driver produces a stable output current to 5V from a battery with an input voltage varying between 2.7V. The internal NMOS switch drives the external inductor, and the Schottky diode transfers the energy stored in the inductor to the load. Proprietary internal circuitry continuously monitors each LED current string and automatically adjusts the resulting output DC voltage to the minimum value of the highest digest forward voltage required by the LED's string. This adaptability of the FAN5608 ensures operation with the highest possible efficiency. A soft-start circuit prevents excessive current from being drawn from the power supply when powering up. Any number of LEDs can be connected as long as the sum of the forward voltages does not result in exceeding the specified operating output voltage range. Although each branch is not required to have the same number of LEDs in series, efficiency is highest when the number of LEDs is connected in series. In the FAN5608 device, two internal two-bit D/A converters provide independent programmable current for each output channel. Analog programming of the output current is also possible in the fan 5608. To do this, connect the "B" pin to ground, then connect a resistor between the "A" pin and the fixed supply voltage. The output current can be programmed to the desired value within the specified range. The fan5608dmpx/fan5608mpx version uses an external resistor to set the current and turn the device on and off. The FAN5608DMPX/FAN5608MPX are available in 8-lead MLP package diodes with or without internal Schottky. The FAN5608DHMPX provides an internal Schottky diode in a 12-lead MLP package.

CIRCUIT DESCRIPTION When the input voltage is connected to the VIN pin and the system is turned on, the bandgap reference obtains its nominal voltage and the soft-start cycle begins. Once "power good" is reached (0.5mA in the diode), the soft-start cycle stops and the boost voltage rises to produce the desired current selected by the input control pins. If the second channel is unchecked, its output will be up to about vin and the diode is off. The FAN5608 DC/DC converter automatically adjusts its internal duty cycle for high efficiency. It provides tight regulation of the output current of the LED. Internal circuitry determines which LED string requires the highest voltage to maintain a preset current level, and adjusts the boost regulator accordingly. To keep the regulation current at the selected value, the difference in the number of LEDs between the branches should be no more than two. If only one branch is used, the other branch should be disabled and the corresponding DAC input connected low. If the output external capacitor is shorted, the Schottky diode may be damaged and should be avoided. LED Brightness Control Control inputs are A1, B1 (for CH1) and A2, B2 (for CH2). B1 and B2 are digital inputs, so they require low (GND) and high (VCC) control signals. In analog mode, A1 and A2 are connected to ground through external resistors, and the B1 and B2 inputs. The current through the resistor is a factor of about 1000 .

Pulse width modulation (pwm) control variable duty cycle (delta) can modulate any DAC input. Care should be taken not to use too low a frequency, otherwise flickering effects may occur. The minimum range is 100Hz to 1kHz. For the maximum range of LED current, the A and B inputs can be modulated simultaneously. No-Load Protection Built-in overvoltage protection circuitry prevents the device from being damaged when it is powered up with no load. This circuit minimizes the boost converter duty cycle, thereby limiting the output voltage to a safe value when no-load condition is detected. If one of the enabled branches is accidentally disconnected, the converter continues to operate, but the branch is no longer regulated, and the actual branch current will be determined by the input voltage, inductance value, and switching frequency. However, when fully loaded, the fan 5608 can break (more than 6 LEDs, driven by 20mA) and suddenly disconnect from VOUT. To protect the fan 5608 from this unlikely event, an external 20V Zener diode can be connected between VOUT and GND. Shutdown mode is available for each branch by applying a low logic level voltage to the A and B inputs. When both branches are disabled, the fan 5608 goes into shutdown mode and the supply current is reduced to less than 1 µA.

Application Information Inductor Selection: Inductors are boost converters that store energy. The stored energy is in the inductor and transferred to the load by a regulator using pulse width modulation and pulse skipping techniques. The FAN5608 operates an inductor in discontinuous conduction mode in most cases. To ensure that the current regulator is within the full range of conditions, the inductor should be selected based on the maximum required power (POUT) and minimum input voltage (vin). where the units of l, vin and pout are micro-hours, volts and watts, and f=0.4 are coefficients depending on the fan5608 building, respectively. The above relationship applies for pout = 0.6w and L = 4.3_ or greater. At lower inductance values, the efficiency of the FET is reduced due to the resistive losses of the switching power supply. Using L = 4.3µm_ and increasing the load to 12 LEDs X 20mA (Pout = 800mW ) requires VIN > 3.5V to maintain a constant 20mA current through the LEDs. Inductor L=4.3µh ensures proper operation of 2 x 4 white LEDs 20mA at 3.5V for vehicles with VIN > 2.8V. For any lighter loads or higher VINs, the inductance can be a factor that increases the efficiency of the system. Applying it gives the maximum current rating of the inductor. For L = 4.3 µl, t max = 1.25 µs, VIN max = 4.2V, the inductor saturation current should be at least 1A. Capacitor selection should use low ESR capacitors to minimize input and output ripple voltage. Using CIN=4.7μF/6.3V and cout=4.7μF/25v type x5r/x7r multilayer ceramic capacitors are recommended. As close as possible to the fan 5608 may be required to reduce input voltage ripple in noise sensitive applications. An additional LC filter between the battery and the FAN5608 input can help further reduce battery ripple to the level required for a specific application. PCB Layout Considerations Fan 5608 has two separate die attach pads (dap) and a double dap package. In a single DAP package, DAP is connected to GND. In a dual DAP package, one dap is connected to gnd and the other to vout, so there is no need to provide any to DAPS. Due to the lower internal power dissipation, the 3x3mm and 4x4mm mlp packages are able to dissipate maximum power without providing any pcb ground pattern. When looking at a DAP unit, a single exposed metal island is visible; when looking at the bottom of a dual DAP device package, two exposed metal islands are seen that are electrically isolated.