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FAN5346 Folha de dados(PDF) 11 Page - Fairchild Semiconductor |
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FAN5346 Folha de dados(HTML) 11 Page - Fairchild Semiconductor |
11 / 14 page © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5346 • Rev. 1.0.1 11 Application Information The reference schematic diagram is shown in Figure 29. FAN5346 is able to drive up to eight LEDs with input voltage equal to or greater than 2.9V (VIN ≥ 2.9V). However, the number of LEDs that can be used FAN5346 depends on forward voltage. It is recommended that the forward voltage (VF) of the white LED be no greater than 3.2V and the maximum LED current be 20mA. FAN5345 can be also used as a boost convertor by connect the VOUT point to the load directly. The return trace of the load should also return to GND through a sense resistor (R1). Figure 29. Reference Application Schematic Diagram Component Placement and PCB Recommendations FAN5346 switches at 1.2MHz to boost the output voltage. Component placement and PCB layout need to be carefully taken into consideration to ensure stable output and to prevent generation of noise. Figure 30 is a portion of the evaluation board layout. The critical layout elements are: the L1, CIN, CIN return trace, COUT, and the COUT return trace. Figure 30. Reference PCB Layout Input Capacitor and Return Trace The input capacitor is the first priority in a switching buck or boost regulator PCB layout. A stable input source (VIN) enables a switching regulator to deliver its best performance. During the regulator’s operation, it is switching at a high frequency, which makes the load of CIN change dynamically since it is trying to make the input source vary at the same switching frequency as the regulator. To ensure a stable input source, CIN needs to hold enough energy to minimize the variation at the input pin of the regulator. For CIN to have a fast response of charge / discharge, the trace from CIN to the input pin of the regulator and the return trace from GND of the regulator to CIN should be as short and wide as possible to minimize trace resistance, inductance, and capacitance. During operation, the current flow from CIN through the regulator to the load and back to CIN contains high-frequency variation due to switching. Trace resistance reduces the overall efficiency due to I 2R loss. Even a small trace inductance could effectively yield ground variation to add noise on VOUT. The input capacitor should be placed close to the VIN and GND pins of the regulator and traces should be as short as possible. Avoid routing the return trace through different layers because vias have strong inductance effect at high frequencies. If routing to other PCB layers is unavoidable, place vias next to the VIN and GND pins of the regulator to minimize the trace distance. Output Capacitor and Return Trace The output capacitor serves the same purpose as the input capacitor, but also maintains a stable output voltage. As explained above, the current travels to the load and back to the COUT GND terminal. COUT should be placed close to the VOUT pin. The traces of COUT to L1, VOUT, and the return |
Nº de peça semelhante - FAN5346 |
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Descrição semelhante - FAN5346 |
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