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UC2577TDTR-ADJ Datasheet(PDF) 5 Page - Texas Instruments |
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UC2577TDTR-ADJ Datasheet(HTML) 5 Page - Texas Instruments |
5 / 14 page 2. Inductor Selection (L) A. Preliminary Calculations To select the inductor, the calculation of the following three parameters is necessary: Dmax, the maximum switch duty cycle (0 ≤ D ≤ 0.9): Dmax = VOUT + VF − VINmin VOUT + VF − 0.6V where typically VF = 0.5V for Schottky diodes and VF = 0.8V for fast recovery diodes. E • T, the product of volts • time that charges the induc- tor: E • T = Dmax • (VINmin − 0.6V)106 52,000Hz (V• µs) IIND, DC, the average inductor current under full load: IIND, DC = 1.05 • ILOADmax 1 − Dmax B. Identify Inductor Value: 1. From Figure 4, identify the inductor code for the region indicated by the intersection of E • T and IIND, DC. This code gives the inductor value in microhenries. The L or H prefix signifies whether the inductor is rated for a maxi- mum E • T of 90Vµs (L) or 250Vµs (H). 2. If D < 0.85, go to step C. If D ≥ 0.85, calculate the minimum inductance needed to ensure the switching regulator’s stability: 0.3 0.4 0.45 0.35 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2.0 2.5 3.0 20 30 35 25 40 45 50 60 70 80 90 100 200 150 L47 L68 L100 L150 L220 L330 H2200 L680 H1500 H1000 H680 H470 H330 H220 H150 L470 IIND, DC (A) Figure 4. Inductor Selection Graph If Lmin is smaller than the inductor values found in step B1, go on to step C. Otherwise, the inductor value found in step B1 is too low; an appropriate inductor code should be obtained from the graph as follows: 1. Find the lowest value inductor that is greater than Lmin . 2. Find where E • T intersects this inductor value to determine if it has an L or H prefix. If E • T intersects both the L and H regions, select the inductor with an H prefix. C. Inductor Selection Select an inductor from the table of Figure 5 which cross references the inductor codes to the part numbers of the three different manufacturers. The inductors listed in this table have the following characteristics: AIE (ferrite, pot-core inductors): Benefits of this type are low etectromagnetic interference (EMI), small physical size, and very low power dissipation (core loss). Pulse (powdered iron, toroid core inductors): Bene- fits are low EMI and ability to withstand E • T and peak current above rated value better than ferrite cores. Renco (ferrite, bobbin-core inductors): Benefits are low cost and best ability to withstand E • T and peak current above rated value. Be aware that these in- ductors generate more EMI than the other types, and this may interfere with signals sensitive to noise. UC2577-ADJ Note: This chart assumes that the inductor ripple current inductor is approximately 20% to 30% of the average inductor current (when the regulator is under full load). Greater ripple current causes higher peak switch currents and greater output ripple volt- age. Lower ripple current is achieved with larger value inductors. The factor of 20% to 30% is chosen as a convenient balance between the two extremes. APPLICATIONS INFORMATION (cont.) 5 |
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