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LT1507CN8 Datasheet(PDF) 10 Page - Linear Technology |
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LT1507CN8 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LT1507 APPLICATIONS INFORMATION be due to a radiated magnetic field coupling into PC board traces. But why were some boards bad and others good? In a moment of desperation (or divine inspiration) I unsoldered a “bad” inductor, rotated it 180 ° and resoldered it. Problem fixed!! It turns out that the inductor was symmetrical in all regards except that the polarity of the magnetic field reversed when the unit was rotated 180 ° because current flowed in the opposite direction in the coil. In one direction, the magnetically induced ripple in the board traces added to output ripple. Rotating the induc- tor caused the induced field to reduce output ripple. Unfortunately the inductor had no physical package assymmetry to indicate rotation, including part mark- ing, so we had to visually examine the winding in each unit before soldering it to the boards. This little horror story should not preclude the use of open core induc- tors, but it emphasizes the need to carefully check the effect these seductively small, low cost inductors may have on regulator or system performances. 4. Look for an inductor (see Table 1) which meets the requirements of core shape, peak current (to avoid saturation), average current (to limit heat) and fault current (if the inductor gets too hot, wire insulation will melt and cause turn-to-turn shorts). Keep in mind that all good things like high efficiency, surface mounting, low profile and high temperature operation will increase cost, sometimes dramatically. 5. After making an initial choice, consider secondary things like output voltage ripple, second sourcing, etc. Use the experts in the Linear Technology Applications Depart- ment if you feel uncertain about the final choice. They have experience with a wide range of inductor types and can tell you about the latest developments in low profile, surface mounting, etc. Table 1. Representative Surface Mount Units VALUE DC CORE SERIES HEIGHT MANUFACTURER ( µH) (A) TYPE ( Ω) CORE (mm) Coiltronics CTX5-1 5 2.3 Tor 0.027 KM µ 4.2 CTX10-1 10 1.9 Tor 0.039 KM µ 4.2 CTX5-1P 5 1.8 Tor 0.021 52 4.2 CTX10-1P 10 1.6 Tor 0.030 52 4.2 Sumida CDRH64 10 1.7 SC 0.084 Fer 4.5 CDRH73 10 1.7 SC 0.055 Fer 3.4 CD73 10 1.4 Open 0.062 Fer 3.5 CD104 10 2.4 Open 0.041 Fer 4.0 Gowanda SM20-102K 10 1.3 Open 0.038 Fer 7 Dale IHSM-4825 10 3.1 Open 0.071 Fer 5.6 IHSM-5832 10 4.3 Open 0.053 Fer 7.1 SC = Semi-closed geometry Fer = Ferrite core material 52 = Type 52 powdered iron core material KM µ = Kool Mµ OUTPUT CAPACITOR The output capacitor is normally chosen by its effective series resistance (ESR), because that is what determines output ripple voltage. At 500kHz any polarized capacitor is essentially resistive. To get low ESR takes volume ; physi- cally larger capacitors have lower ESR. The ESR range needed for typical LT1507 applications is 0.05 Ω to 0.5Ω. A typical output capacitor is an AVX type TPS, 100 µF at 10V, with a guaranteed ESR less than 0.1 Ω. This is a “D” size surface mount solid tantalum capacitor. TPS capaci- tors are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. The value in microfarads is not particularly critical and values from 22 µF to greater than 500µF work well, but you cannot cheat mother nature on ESR. If you find a tiny 22 µF solid tantalum capacitor, it will have high ESR and output ripple voltage will be terrible. The chart in Table 2 shows some typical solid tantalum surface mount capacitors. |
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