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FAN5332A Scheda tecnica(PDF) 7 Page - Fairchild Semiconductor |
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FAN5332A Scheda tecnica(HTML) 7 Page - Fairchild Semiconductor |
7 / 9 page 7 www.fairchildsemi.com FAN5332A Rev. 1.0.1 inductor size, cost and overall efficiency is needed to make the optimum choice. The inductor saturation current should be rated around 1.5A, which is the threshold of the internal current limit circuit. This limit is reached only during the start-up and with heavy load condition; when this event occurs the converter can shift over in discontinuous conduction mode due to the automatic turn-off of the switching transistor, resulting in higher ripple and reduced efficiency. Some recommended inductors are suggested in the table below: Table 1: Recommended Inductors Capacitors Selection For best performance, low ESR input and output capacitors are required. Ceramic capacitors of CIN = 10µF and COUT = 2.2µF placed as close to the IC pins, are recommended for the lower input and output ripple. The output capacitor voltage rating should be according to the VOUT setting. Some capacitors are suggested in the table below. Table 2: Recommended Capacitors A feed forward capacitor CF, is required for stability. The recom- mended value (R1 x CF) is around 13µS. Diode Selection The external diode used for rectification is usually a Schottky diode. Its average forward current and reverse voltage maxi- mum ratings should exceed the load current and the voltage at the output of the converter respectively. A barrier Schottky diode such as BAT54 is preferred, due to its lower reverse current over the temperature range. Care should be taken to avoid any short circuit of VOUT to GND, even with the IC disabled, since the diode can be instantly dam- aged by the excessive current. 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. Heavy Load Limitations In heavy load applications (POUT >1.5W) the power dissipated in steady state at high ambient temperature can increase the die temperature to the point where the thermal shutdown kicks in. To prevent this, a limited time operation should be consid- ered in applications such as a flash LED driver. An operation time of 0.4Sec. at maximum load, followed by a 3.6Sec. OFF time ensures stable operation even in the worst case, provided that the IC heat dissipation is optimized to lower the junction to ambient thermal resistance. The external passive components and the layout of the PCB play an important role in reaching the highest efficiency of the boost converter. For best results, the data sheet recommenda- tions regarding external components and PCB layout should be followed closely. PCB Layout Recommendations The inherently high peak currents and switching frequency of power supplies require careful PCB layout design. Therefore, use wide traces for high current paths and place the input capacitor, the inductor, and the output capacitor as close as possible to the integrated circuit terminals. The resistor divider that sets the output voltage should be routed away from the inductor to avoid RF coupling. A two layer PCB with the bottom layer as ground plane connected to the pin 2 of the IC is recom- mended. This ground plane acts as an electromagnetic shield to reduce EMI and parasitic coupling between components. Figure 5. Recommended PCB Layout Inductor Value Vendor Part Number Comment 6.8µH TDK SLF7028T6R8M1R3 Low Profile 6.8µH MURATA LQ566C6R8M04 High Efficiency 6.8µH COOPER SD414-6R8 Small Size Capacitor Value Vendor Part Number 10µF Murata GRM31CR70J106KA01B 2.2µF Murata GRM32RR71E225KC01B |
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