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FAN5336MPX Fiches technique(PDF) 7 Page - Fairchild Semiconductor |
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FAN5336MPX Fiches technique(HTML) 7 Page - Fairchild Semiconductor |
7 / 9 page FAN5336 Rev. 1.0.0 7 © 2004 Fairchild Semiconductor Corporation www.fairchildsemi.com Applications Information Setting the Output Voltage The internal feedback voltage reference (VREF) is 1.23V (typical). The output voltage is divided by a resistor divider, R1 and R2 to the FB pin. The output voltage is calculated by: The maximum output current depends on the output volt- age settings. Table 1 provides the recommended voltage for several steady-state configurations: Table 1: Recommended Voltages Inductor Selection The inductor parameters directly related to device perfor- mance are saturation current and DC resistance. The FAN5336 operates with a typical inductor value of 6.8µH. The lower the DC resistance, the higher the efficiency. Balancing inductor size, cost, and overall efficiency allows 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 conditions. When this occurs, the con- verter can shift into discontinuous conduction mode due to the automatic turn-off of the switching transistor, resulting in higher ripple and reduced efficiency. Table 2: Recommended Inductors Capacitors Selection For best performance, low-ESR input and output capaci- tors are required. Ceramic capacitors of CIN = 10µF and COUT = 4.7µF, placed as close as possible to the IC pins, are recommended for the lower input and output ripple. The output capacitor voltage rating should be selected according to the VOUT setting. Table 3: Recommended Capacitors A feedforward capacitor (CF) is required for stability. The recommended value is around 180pF. Diode Selection The external diode used for rectification is usually a Schottky diode. Its average forward current should exceed the load current and its reverse voltage maxi- mum ratings should exceed the voltage at the output of the converter. A barrier Schottky diode, such as BAT54, is preferred due to its low 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 damaged by the excessive current. Flash LED Driver The FAN5336 may be used to drive a serial flash LED with up to 100mA current at 21V for a maximum of 400ms on-time and 10% duty cycle. Thermal Shutdown When the die temperature exceeds 150°C, a reset occurs and remains in effect until the die cools to 130°C, when the circuit is allowed to restart. PCB Layout Recommendations The inherently high peak currents and switching fre- quency of power supplies require careful PCB layout design. For best results, 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 Pin 2 of the IC, is recommended. The ground plane acts as an electromagnetic shield to reduce EMI and parasitic coupling between components. Figure 11. Recommended PCB Layout VOUT (V) ILOAD Max. (mA) R1 (kΩ) for R2 = 10kΩ VIN Range (V) 9 50 63.2 2.7 to 5.5 9 170 63.2 2.9 to 4.7 12 125 87.5 2.7 to 5.5 21 75 160 3.6 to 5.5 33 50 258 4.0 to 5.5 33 10 258 2.7 to 5.5 Inductor Value Vendor Part Number Comment 6.8µH TDK SLF7028T6R8M1R3 Lower Profile 6.8µH Murata LQS66C6R8M04 Higher Efficiency V OUT V REF 1 R 1 R 2 ------- + ⎝⎠ ⎜⎟ ⎜⎟ ⎛⎞ = EQ 1 Cap. Value Vendor Part Number 10µF Murata GRM31CR70J106KA01B 4.7µF Murata GRM32RR61E475KC31B |
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