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LTC1504CS8 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1504CS8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1504 APPLICATIONS INFORMATION half the ripple current and its value should be chosen based on the desired ripple current and/or the output current transient requirements. Large value inductors lower ripple current and decrease the required output capacitance, but limit the speed that the LTC1504 can change the output current, limiting output transient re- sponse. Small value inductors result in higher ripple currents and increase the demands on the output capaci- tor, but allow faster output current slew rates and are often smaller and cheaper for the same DC current rating. A typical inductor used in an LTC1504 application might have a maximum current rating between 500mA and 1A and an inductance between 33 µH and 220µH. Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron rod core inductors with similar electrical characteristics. The choice of which style inductor to use oftendependsmoreonthepricevssizerequirementsandany radiated field/EMI requirements than on what the LTC1504 requires to operate. Table 2 shows some typical surface mount inductors that work well in LTC1504 applications. Table 2. Representative Surface Mount Inductors CORE CORE PART VALUE MAX DC TYPE MATERIAL HEIGHT CoilCraft DT3316-473 47 µH 1A Shielded Ferrite 5.1mm DT3316-104 100 µH 0.8A Shielded Ferrite 5.1mm DO1608-473 47 µH 0.5A Open Ferrite 3.2mm DO3316-224 220 µH 0.8A Open Ferrite 5.5mm Coiltronics CTX50-1 50 µH 0.65A Toroid KoolM µ® 4.2mm CTX100-2 100 µH 0.63A Toroid KoolM µ 6mm CTX50-1P 50 µH 0.66A Toroid Type 52 4.2mm CTX100-2P 100 µH 0.55A Toroid Type 52 6mm Sumida CDRH62-470 47 µH 0.54A Shielded Ferrite 3mm CDRH73-101 100 µH 0.50A Shielded Ferrite 3.4mm CD43-470 47 µH 0.54A Open Ferrite 3.2mm CD54-101 100 µH 0.52A Open Ferrite 4.5mm Output Capacitor The output capacitor affects the performance of the LTC1504 in a couple of ways: it provides the first line of defense during a transient load step and it has a large effect on the compensation required to keep the LTC1504 feed- back loop stable. Transient load response of an LTC1504 circuit is controlled almost entirely by the output capacitor and the inductor. In steady load operation, the average current in the inductor will match the load current. When the load current changes suddenly, the inductor is sud- denly carrying the wrong current and requires a finite amount of time to correct itself—at least several switch cycles with typical LTC1504 inductor values. Even if the LTC1504 had psychic abilities and could instantly assume the correct duty cycle, the rate of change of current in the inductor is still related to its value and will not change instantaneously. Until the inductor current adjusts to match the load cur- rent, the output capacitor has to make up the difference. Applications that require exceptional transient response (2% or better for instantaneous full-load steps) will re- quire relatively large value, low ESR output capacitors. Applications with more moderate transient load require- ments can often get away with traditional standard ESR electrolytic capacitors at the output and can use larger valued inductors to minimize the required output capaci- tor value. Note that the RMS current in the output capacitor is slightly more than half of the inductor ripple current— much smaller than the RMS current in the input bypass capacitor. Output capacitor lifetime is usually not a factor in typical LTC1504 applications. Large value ceramic capacitors used as output bypass capacitors provide excellent ESR characteristics but can cause loop compensation difficulties. See the Loop Com- pensation section. Loop Compensation Loop compensation is strongly affected by the output capacitor. From a loop stability point of view, the output inductor and capacitor form a series RLC resonant circuit, with the L set by the inductor value, the C by the value of the output capacitor and the R dominated by the output capacitor’s ESR. The amplitude response and phase shift due to these components is compensated by a network of Rs and Cs at the COMP pin to (hopefully) close the feedback loop in a stable manner. Qualitatively, the L and Kool M µ is a registered trademark of Magnetics, Inc.. |
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