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LTC1474 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1474 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1504A APPLICATIONS INFORMATION 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 TP3-470 47 µH 0.55A Toroid Ferrite 2.2mm TP3-470 47 µH 0.72A Toroid Ferrite 3mm 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 LTC1504A 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 LTC1504A feedback loop stable. Transient load response of an LTC1504A circuit is controlled almost entirely by the output capacitor and the inductor. In steady load opera- tion, the average current in the inductor will match the load current. When the load current changes suddenly, the inductor is suddenly carrying the wrong current and requires a finite amount of time to correct itself—at least several switch cycles with typical LTC1504A inductor values. Even if the LTC1504A 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 cannot 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 LTC1504A 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 C of the output stage form a 2nd order roll-off with 180 ° of phase shift; the R due to ESR forms a single zero at a somewhat higher frequency that reduces the roll-off to first order and reduces the phase shift to 90 °. If the output capacitor has a relatively high ESR, the zero comes in well before the initial phase shift gets all the way to 180 ° and the loop only requires a single small capacitor from COMP to GND to remain stable (Figure 4a). If, on the other hand, the output capacitor is a low ESR type to maximize transient response, the ESR zero can increase in frequency by a decade or more and the output stage phase shift can get awfully close to 180 ° before it turns around and comes back to 90 °. Large value ceramic, OS-CON electrolytic and low impedance tantalum capacitors fall into this category. These loops require an additional zero to be inserted at the COMP pin; a series RC in parallel with a smaller C to ground will usually ensure stability. Figure 4b shows a typical compensation network which will opti- mize transient response with most output capacitors. Adjustable output parts can add a feedforward capacitor across the feedback resistor divider to further improve Kool M µ is a registered trademark of Magnetics, Inc.. |
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