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CTX50-1 Datasheet(PDF) 7 Page - Linear Technology |
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CTX50-1 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LTC1504A Ideally, the LTC1504A requires a low impedance bypass right at the chip and a larger reservoir capacitor that can be located somewhat farther away. This requirement usually can be met with a ceramic capacitor right next to the LTC1504A and an electrolytic capacitor (usually 10 µF to 100 µF,dependingonexpectedloadcurrent)locatedsome- where nearby. In certain cases, the bulk capacitance requirement can be met by the output bypass of the input supply. Applications running at very high load currents or at input supply voltages greater than 6V may require the local ceramic capacitor to be 1 µF or greater. In some cases, both the low impedance and bulk capacitance requirements can be met by a single capacitor, mounted very close to the LTC1504A. Low ESR organic semicon- ductor (OS-CON) electrolytic capacitors or surge tested surface mount tantalum capacitors can have low enough impedance to keep the LTC1504A happy in some circuits. Often the RMS current capacity of the input bypass capaci- tors is more important to capacitor selection than value. Buck converters like the LTC1504A are hard on input capacitors, since the current flow alternates between the full load current and near zero during every clock cycle. In the worst case (50% duty cycle or VOUT = 0.5VIN) the RMS current flow in the input capacitor is half of the total load current plus half the ripple current in the inductor— perhaps 300mA in a typical 500mA load current applica- tion. This current flows through the ESR of the input bypass capacitor, heating it up and shortening its life, sometimes dramatically. Many ordinary electrolytic ca- pacitors that look OK at first glance are not rated to withstand such currents—check the RMS current rating before you specify a device! If the RMS current rating isn’t specified, it should not be used as an input bypass capaci- tor. Again, low ESR electrolytic and surge tested tantalums usually do well in LTC1504A applications and have high RMS current ratings. The local ceramic bypass capacitor usually has negligible ESR, allowing it to withstand large RMS currents without trouble. Table 1 shows typical surface mount capacitors that make acceptable input bypass capacitors in LTC1504A applications. Inductor The LTC1504A requires an external inductor to be con- nected from the switching node SW to the output node APPLICATIONS INFORMATION Table 1. Representative Surface Mount Input Bypass Capacitors PART VALUE ESR MAX RMS TYPE HEIGHT AVX TPSC226M016R0375 22 µF 0.38Ω 0.54A Tantalum 2.6mm TPSD476M016R0150 47 µF 0.15Ω 0.86A Tantalum 2.9mm TPSE107M016R0125 100 µF 0.13Ω 1.15A Tantalum 4.1mm 1206YC105M 1 µF Low >1A X7R Ceramic 1.5mm 1210YG106Z 10 µF Low >1A Y5V Ceramic 1.7mm Sanyo 16SN33M 33 µF 0.15Ω 1.24A OS-CON 7mm 16SN68M 68 µF 0.1 Ω 1.65A OS-CON 7mm 16CV100GX 100 µF 0.44Ω 0.23A* Electrolytic 6mm 16CV220GX 220 µF 0.34Ω 0.28A* Electrolytic 7.7mm Sprague 593D476X0016D2W 47 µF 0.17Ω 0.93A Tantalum 2.8mm 593D107X0016E2W 100 µ 0.15Ω 1.05A Tantalum 4mm *Note: Use multiple devices in parallel or limit output current to prevent capacitor overload. where the load is connected. Inductor requirements are fairly straightforward; it must be rated to handle continu- ous DC current equal to the maximum load current plus 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 LTC1504A 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 LTC1504A 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 mate- rials are small and don’t radiate much energy, but gener- ally cost more than powdered iron rod core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC1504A requires to operate. Table 2 shows some typical surface mount inductors that work well in LTC1504A applications. |
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