Features

95 % Efficiency, Synchronous Operation Adjustable Output Voltage Options: 0.7V to 0.8V IN 2.5V to 5.5V Input Voltage Range Customization Duty to Air Low Dropout Operation Soft Start Dynamic Output Voltage Positioning 15µA Quiescent Current Excellent Load Transient Response 5-Lead SOT-23 Package 6-Lead MLP 3x3mm Package

application

Handheld Computers Cell Phones Battery Powered Portable Devices Digital Cameras Low Power Digital Signal Processor Power Supplies

illustrate

The FAN5307 is a high efficiency, low noise synchronous PWM current mode and pulse skip (power saving) mode DC-DC converter designed for battery powered applications. It provides up to 300mA of output current over a wide input range of 2.5V to 5.5V. The output voltage can be fixed internally or externally through an external voltage divider. Custom output voltage is also possible. Contact a Fairchild sales representative for custom output voltage options. At moderate and light loads, pulse skip modulation applies dynamic voltage positioning and the output voltage is 0.8% higher than nominal, for increased headroom during load transients at higher position loads, the system automatically switches to the current mode PWM control, operating frequency is 1 MHz. Current A mode control loop with fast transient response ensures good line and load regulation in power saving mode, reducing quiescent current to 15µA for high efficiency and long battery life. In shutdown mode, the supply current drops below 1 µA. The device is available in 5-wire SOT-23 and 6-wire MLP 3x3mm packages.

Detailed operating instructions: Fan 5307 is a buck converter that switches at 1MHz in a typical current mode PFM/PWM configuration. For moderately heavy loads, the converter operates in pulse width modulation (PWM) mode. At light loads, the converter enters a power-saving mode (PFM pulse skipping) to keep efficiency high. PWM Mode In PWM mode, the device operates at a fixed frequency of 1 MHz. At the beginning of each clock cycle, the Pchannel transistor is turned on. The inductor current is monitored by internal circuitry. When current is sensed, the P-channel switch closes when the output voltage is in regulation or the inductor current reaches the current limit (set internally to 520mA). After the minimum dead time, the N-channel transistor is turned on and the inductor current increases sharply downward. When the clock cycle ends, the N-channel switch is turned off and the next clock cycle begins. FM (power saving) mode As the load current decreases and the peak current of the inductor no longer reaches 80mA, the inverter enters the Pulse Frequency Modulation (PFM) mode.

In PFM mode, the device operates with variable frequency and constant peak current, reducing the quiescent current to a minimum and maintaining high efficiency at light loads. Once the output voltage falls below the threshold, set above the nominal value, the P-channel transistor turns on and the inductor current rises. The P channel switches off and the N channel turns on as a peak to reach the inductor current (140mA typical). N-channel transistors become negative at the inductor current. At this point, the Pchannel is turned on again to start the next pulse. The converter continues these pulses until the threshold is reached (typically 1.6% higher than the nominal value). Additional headroom for voltage drop during higher output voltage loads in PFM mode transitions from light to full load. The voltage overshoots during this period due to the activation of the N-channel rectifier when the regulator switches on power up. The device remains in sleep mode until the output voltage falls below the low threshold. The fan 5307 goes into PFM for a longer time once the output voltage cannot be regulated in constant peak current.

100% Duty Cycle Operation When the input voltage approaches the output voltage, the duty cycle exceeds the typical 90% duty cycle, and the converter keeps the P-channel transistor energized. In this mode, the output voltage is equal to the negative input voltage, and the voltage drop across the P-channel transistor: VOUT=VIN–ILOAD×(RDSON+RL), where (1) RDSON=P-channel on-resistance ILOAD=output current RL=inductance The DC Resistor Soft Start Fan 5307 has an internal soft start circuit that limits inrush current at startup. This prevents input voltage and eliminates output voltage overshoot. The soft-start is implemented as a digital circuit, increasing the switch current in four steps up to the P-channel current limit (520 mA). Typical start-up time for a 10µF output capacitor and 200mA load current is 500µs. The short-circuit protection switch peak current is limited to 520mA typical. In an output voltage short circuit, the device operates with a minimum duty cycle; therefore, the average input current is typically 100mA.

Application Information Adjustable Output Voltage The output voltage of the adjustable version is divided by an external resistor as follows:

To reduce noise sensitivity, R1+R2 should not exceed 1 megohm

Inductor selection The inductance parameter performance directly related to the device is saturation current and DC resistance. The typical inductance value for the fan 5307 is 10 μH. The lower the DC resistance, the higher the efficiency. For saturation current, the inductance is the rated load greater than the maximum load current, plus half the inductor ripple current, calculated as:

Input Capacitor Selection For optimum performance, low ESR input capacitors are required. Placing a ceramic capacitor of at least 4.7µF as close as possible to the input pins of the device is recommended. The output capacitor selection FAN5307 has a switching frequency of 1MHz, allowing the application of low ESR value ceramic capacitors from 10µF to 22µF. This provides low output voltage ripple. In power save mode, the output voltage ripple is independent of the output capacitor value and the ripple is determined by the internal comparator threshold. Typical output voltage ripple at light load is rated output voltage.

PCB Layout Recommendations The inherently high peak currents and switching power frequencies require careful PCB layout design. Use wide traces for high current paths and place input capacitors, inductors, and output capacitors as close as possible to the IC circuit terminals. For the adjustable version, the resistor divider should be kept away from the inductor to avoid EMI. The 6-lead MLP version of the FAN5307 grounds the high current referenced to ground; therefore, being more tolerant of PCB layout shows better performance.