TL3577 Datasheet, PDF (14/21 Pages) Texas Instruments – 100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR

TL3577
100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR
SLVS633 – OCTOBER 2006
www.ti.com
B. Identify Inductor Value
1. From Figure 6, identify the inductor code for the region indicated by the intersection of E • T and I_IND,DC. This
code gives the inductor value in microhenries. The L or H prefix signifies whether the inductor is rated for a
maximum E • T of 90 Vµs (L) or 250 Vµs (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:
If L_min is smaller than the inductor values found in step B1, go on to step C. Otherwise, the inductor value
found in step 1, above, is too low; an appropriate inductor code should be obtained from Figure 6 as follows:
a. Find the lowest-value inductor that is greater than L_min.
b. 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 Table 2 which cross references the inductor codes to the part numbers of the three
different manufacturers. The inductors listed in Table 2 have the following characteristics:
AIE (ferrite, pot-core inductors): Benefits of this type are low electromagnetic interference (EMI), small
physical size, and very low power dissipation (core loss).
Pulse (powdered iron, toroid core inductors): Benefits 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 inductors generate more EMI than the other types, and this
may interfere with signals sensitive to noise.
200 _H2200
150
H1500 H1000 H680
H470
H330
H220
100
90
80
70
L680
60
50
45
40 ^L470
L330
35
30
25
L220
L150
H150
L100
L68
L47
20
0.3 0.35 0.4 0.45 0.5 0.6 0.7 0.8 0.9 1.0
1.5
2.0 2.5 3.0
I_IND,DC (A)
A. 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 voltage. 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.
Figure 6. Inductor Selection Graph
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