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FAN5331 Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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FAN5331 Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 10 page 7 www.fairchildsemi.com FAN5331 Rev. 1.0.1 The inductor saturation current should be rated around 1A, 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 in the range 4.7µF to 10µF, 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. A feed forward capacitor CF, is required for stability. The recom- mended value (R1 x CF) is around 18µS. Some capacitors are suggested in the table below. Table 2: Recommended Capacitors 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. 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 four layer PCB with at least one ground plane connected to the pin 2 of the IC is recommended. This ground plane acts as an electromagnetic shield to reduce EMI and parasitic coupling between components. Figure 5. Recommended Layout Application Examples 1. LED Driver One or more serial LED strings can be driven with a constant current, set by the series resistor, given by Figure 6. Low Noise Boost LED Driver Inductor Value Vendor Part Number Comment 10µH Panasonic ELL6GM100M Lower Profile (1.6mm) 10µH Murata LQS66SN100M03L Highest Efficiency 10µH Coilcraft DO1605T-103Mx Small Size Capacitor Value Vendor Part Number 4.7µF Panasonic ECJ3YB1C475K 4.7µF Murata GRM31CR61C475 I LED 1.23V R1 ---------------- = SHDN VIN VOUT R1 R2 GND V IN FB SW CIN 10µH 1 3 2 4 5 4.7 µF COUT L BAT54 4.7 µF 2.7V to 5.5V ON OFF |
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