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LM3557 数据表(PDF) 9 Page - National Semiconductor (TI) |
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LM3557 数据表(HTML) 9 Page - National Semiconductor (TI) |
9 / 11 page Application Information (Continued) 20131626 20131627 t ON: Internal Power FET On-Time. T S: Switching Period of Operation. D: Duty Cycle for DCM Operation. V OUT: Output Voltage. V IN: Input Voltage. I OUT: White LED Current/Load Current. Fs: Switching Frequency. Eff: Efficiency of the LM3557. L: Inductor Value/Inductance Magnitude. INDUCTOR SELECTION In order to maintain inductance, an inductor used with the LM3557 should have a saturation current rating larger than the peak inductor current of the particular application. Induc- tors with low DCR values contribute decreased power losses and increased efficiency. The peak inductor current can be computed for both modes of operation: CCM (continuous current mode) and DCM (discontinuous current mode). The cycle-by-cycle peak inductor current for CCM operation can be computed with: 20131628 20131629 V IN: Input Voltage. Eff: Efficiency of the LM3557. Fs: Switching Frequency. I OUT: White LED Current/Load Current. L: Inductance Magnitude/Inductor Value. D: Duty Cycle for CCM Operation. I Peak: Peak Inductor Current. ∆i L: Inductor Ripple Current. I L(avg): Average Inductor Current. The cycle-by-cycle peak inductor current for DCM operation can be computed with: 20131630 V IN: Input Voltage. Fs: Switching Frequency. L: Inductance Magnitude/Inductor Value. D: Duty Cycle for DCM Operation. I Peak: Peak Inductor Current. Some recommended inductor manufacturers are as follows: Coilcraft [www.coilcraft.com] Coiltronics [www.cooperet.com] TDK [www.tdk.com] CAPACITOR SELECTION Multilayer ceramic capacitors are the best choice for use with the LM3557. Multilayer ceramic capacitors have the lowest equivalent series resistance (ESR). Applied voltage or DC bias, temperature, dielectric material type (X7R, X5R, Y5V, etc), and manufacturer component tolerance have an affect on the true or effective capacitance of a ceramic capacitor. Be aware of how your application will affect a particular ceramic capacitor by analyzing the aforemen- tioned factors of your application. Before selecting a capaci- tor always consult the capacitor manufacturer’s data curves to verify the effective or true capacitance of the capacitor in your application. INPUT CAPACITOR SELECTION The input capacitor serves as an energy reservoir for the inductor. In addition to acting as an energy reservoir for the inductor the input capacitor is necessary for the reduction in input voltage ripple and noise experienced by the LM3557. The reduction in input voltage ripple and noise helps ensure the LM3557’s proper operation, and reduces the effect of the LM3557 on other devices sharing the same supply voltage. To ensure low input voltage ripple, the input capacitor must have an extremely low ESR. As a result of the low input voltage ripple requirement multilayer ceramic capacitors are the best choice. A minimum capacitance of 2.0 µF is required for normal operation, consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance re- quirement is going to be achieved for a particular application. OUTPUT CAPACITOR SELECTION The output capacitor serves as an energy reservoir for the white LED load when the internal power FET switch (Figure 2: N1) is ON or conducting current. The requirements for the output capacitor must include worst case operation such as when the load opens up and the LM3557 operates in over- voltage protection (OVP) mode operation. A minimum ca- pacitance of 0.5 µF is required to ensure normal operation. Consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance requirement is going to be achieved for a particular application. Some recommended capacitor manufacturers are as fol- lows: TDK [www.tdk.com] Murata [www.murata.com] Vishay [www.vishay.com] DIODE SELECTION To maintain high efficiency it is recommended that the aver- age current rating (I F or IO) of the selected diode should be larger than the peak inductor current (I Lpeak). To maintain diode integrity the peak repetitive forward current (I FRM) must be greater than or equal to the peak inductor current (I Lpeak). Diodes with low forward voltage ratings (VF) and low www.national.com 9 |
类似零件编号 - LM3557 |
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类似说明 - LM3557 |
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