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TPS61085DGKR Datasheet(PDF) 10 Page - Texas Instruments |
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TPS61085DGKR Datasheet(HTML) 10 Page - Texas Instruments |
10 / 30 page IN L S V D I f L × D = × ( ) 2 1 OUT L SW peak I I I D D = + - ( ) (max) (min) 1 2 L OUT LIM I I I D D æ ö = - × - ç ÷ è ø 1 IN S V D V h × = - TPS61085 SLVS859B – JUNE 2008 – REVISED DECEMBER 2014 www.ti.com 9.2.2 Detailed Design Procedure 9.2.2.1 Design Procedure The first step in the design procedure is to verify that the maximum possible output current of the boost converter supports the specific application requirements. A simple approach is to estimate the converter efficiency, by taking the efficiency numbers from the provided efficiency curves or to use a worst case assumption for the expected efficiency, e.g. 90%. 1. Duty cycle, D: (1) 2. Maximum output current, IOUT(max) : (2) 3. Peak switch current in application, ISW(peak) : (3) with the inductor peak-to-peak ripple current, ΔIL (4) and VIN Minimum input voltage VS Output voltage ILIM(min) Converter switch current limit (minimum switch current limit = 3.2 A) fS Converter switching frequency (typically 1.2 MHz or 650 kHz) L Selected inductor value η Estimated converter efficiency (please use the number from the efficiency plots or 90% as an estimation) The peak switch current is the steady state peak switch current that the integrated switch, inductor and external Schottky diode has to be able to handle. The calculation must be done for the minimum input voltage where the peak switch current is the highest. 9.2.2.2 Inductor Selection The TPS61085 is designed to work with a wide range of inductors. The main parameter for the inductor selection is the saturation current of the inductor which should be higher than the peak switch current as calculated in the Design Procedure section with additional margin to cover for heavy load transients. An alternative, more conservative, is to choose an inductor with a saturation current at least as high as the maximum switch current limit of 3.2 A. The other important parameter is the inductor DC resistance. Usually, the lower the DC resistance the higher the efficiency. It is important to note that the inductor DC resistance is not the only parameter determining the efficiency. Especially for a boost converter where the inductor is the energy storage element, the type and core material of the inductor influences the efficiency as well. At high switching frequencies of 1.2 MHz inductor core losses, proximity effects and skin effects become more important. Usually, an inductor with a larger form factor gives higher efficiency. The efficiency difference between different inductors can vary between 2% to 10%. For the TPS61085, inductor values between 3 μH and 6 μH are a good choice with a switching frequency of 1.2 MHz, typically 3.3 μH. At 650 kHz inductors between 6 μH and 13 μH, typically 6.8 μH are recommended. Possible inductors are shown in Table 3. Typically, it is recommended that the inductor current ripple is below 35% of the average inductor current. Therefore, the following equation can be used to calculate the inductor value, L: 10 Submit Documentation Feedback Copyright © 2008–2014, Texas Instruments Incorporated Product Folder Links: TPS61085 |
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