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LM3557 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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LM3557 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 19 page [VIN * D] [L * Fs] IPeak | [IOUT] [(1 - D) * Eff] + [VIN * D] [2 * L * Fs] IPeak | IPeak IL (avg) + | 'iL 2 LM3557 www.ti.com SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013 (12) where • VIN is the input voltage • VOUT is the output voltage • Eff is the efficiency of the LM3557 • Fs is the switching frequency • IOUT is the white LED current/load current • L is the inductance magnitude/inductor value • D is the duty cycle for CCM operation • ΔiL is the inductor ripple current • IL(avg) is the average inductor current (13) The cycle-by-cycle peak inductor current for DCM operation can be computed with: where • VIN is the input voltage • VOUT is the output voltage • Eff is the efficiency of the LM3557 • Fs is the switching frequency • IOUT is the white LED current/load current • L is the inductance magnitude/inductor value • D is the duty cycle for CCM operation • ΔiL is the inductor ripple current • IL(avg) is the average inductor current (14) Some recommended inductor manufacturers are: 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 aforementioned factors of your application. Before selecting a capacitor 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 requirement is going to be achieved for a particular application. Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3557 |
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