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FAN5308MPX データシート(PDF) 8 Page - Fairchild Semiconductor |
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FAN5308MPX データシート(HTML) 8 Page - Fairchild Semiconductor |
8 / 11 page 8 www.fairchildsemi.com FAN5308 Rev. 1.0.1 UVLO and Soft Start The reference and the circuit remain reset until the VIN crosses its UVLO threshold. The FAN5308 has an internal soft-start circuit that limits the in- rush current during start-up. This prevents possible voltage drops of the input voltage and eliminates the output voltage overshoot. The soft-start is implemented as a digital circuit increasing the switch current in four steps to the P-channel cur- rent limit (1500mA). Typical start-up time for a 20µF output capacitor and a load current of 800mA is 800µs. Short Circuit Protection The switch peak current is limited cycle-by-cycle to a typical value of 1500mA. In the event of an output voltage short circuit, the device operates with a frequency of 400kHz and minimum duty cycle, therefore the average input current is typically 200mA. Thermal Shutdown When the die temperature exceeds 150°C, a reset occurs and will remain in effect until the die cools to 130°C, at that time the circuit will be allowed to restart. Applications Information Setting the Output Voltage The internal reference is 0.8V (Typical). The output voltage is divided by a resistor divider, R1 and R2 to the FB pin. The out- put voltage is given by: Where R1 + R2 < 800KΩ. According to this equation, and assuming desired output volt- age of 1.5096V, and given R2 = 10K Ω, the calculated value of R1 is 8.87K Ω. If quiescent current is a key design parameter a higher value feedback resistor can be used (e.g. R2 = 100K Ω) and a small bypass capacitor of 10pF is required in parallel with the upper resistor as shown in Figure 4. Figure 4. Setting the Output Voltage Inductor Selection The inductor parameters directly related to the device’s perfor- mances are saturation current and dc resistance. The FAN5308 operates with a typical inductor value of 3.3µH. The lower the dc resistance, the higher the efficiency. For saturation current, the inductor should be rated higher than the maximum load current plus half of the inductor ripple current. This is calculated as follows: where: ∆IL = Inductor Ripple Current f = Switching Frequency L = Inductor Value Some recommended inductors are suggested in the table below: Table 1: Recommended Inductors Capacitors Selection For best performances, a low ESR input capacitor is required. A ceramic capacitor of at least 10µF, placed as close to the VIN and AGND pins of the device is recommended. The output capacitor determines the output ripple and the transient response. Table 2: Recommended Capacitors PCB Layout Recommendations The recommended PCB layout is shown in Figure 5. The inher- ently high peak currents and switching frequency of power sup- plies require a careful PCB layout design. Figure 5. Recommended PCB Layout I OUT V OUT 1V OUT V IN ⁄ () – 2Lf × × ------------------------------------------ × = 2 x 10 µF 10 µF V OUT EN SW C IN C OUT 1.2V (800mA) 1 2 3 6 5 4 FB PGND VIN NC P1 (AGND) C f 3.3 µH R1 10K Ω 5K Ω R2 Inductor Value Vendor Part Number 3.3µH Panasonic ELL6PM3R3N 3.3µH Murata LQS66C3R3M04 Capacitor Value Vendor Part Number 10µF Taiyo Yuden JMK212BJ106MG JMK316BJ106KL TDK C2012X5ROJ106K C3216X5ROJ106M Murata GRM32ER61C106K ∆I L V OUT 1V OUT VIN ⁄ () – Lf × ----------------------------------------- × = |
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