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LTC3832 데이터시트(HTML) 15 Page  Linear Technology 

LTC3832 데이터시트(HTML) 15 Page  Linear Technology 
15 / 24 page 15 LTC3832/LTC38321 sn3832 3832fs requirements. Peak current in the inductor will be equal to the maximum output load current plus half of the peakto peak inductor ripple current. Ripple current is set by the inductor value, the input and output voltage and the operating frequency. The ripple current is approximately equal to: I VV V fL V RIPPLE IN OUT OUT OSC O IN = − () • ( ) •• fOSC = LTC3832 oscillator frequency = 300kHz LO = Inductor value Solving this equation with our typical 3.3V to 2.5V appli cation with a 1 µH inductor, we get: (. – . ) • . •• . 33 25 25 300 1 3 3 2 VV V kHz H V AP µ = P Peak inductor current at 10A load: 10A + (2A/2) = 11A The ripple current should generally be between 10% and 40% of the output current. The inductor must be able to withstand this peak current without saturating, and the copper resistance in the winding should be kept as low as possible to minimize resistive power loss. Note that in circuits not employing the current limit function, the current in the inductor may rise above this maximum under shortcircuit or fault conditions; the inductor should be sized accordingly to withstand this additional current. Inductors with gradual saturation characteristics are often the best choice. Input and Output Capacitors A typical LTC3832 design places significant demands on both the input and the output capacitors. During normal steady load operation, a buck converter like the LTC3832 draws square waves of current from the input supply at the switching frequency. The peak current value is equal to the output load current plus 1/2 the peaktopeak ripple cur rent. Most of this current is supplied by the input bypass capacitor. The resulting RMS current flow in the input capacitor heats it and causes premature capacitor failure in extreme cases. Maximum RMS current occurs with 50% PWM duty cycle, giving an RMS current value equal to IOUT/2. A low ESR input capacitor with an adequate ripple current rating must be used to ensure reliable operation. Note that capacitor manufacturers’ ripple cur rent ratings are often based on only 2000 hours (3 months) lifetime at rated temperature. Further derating of the input capacitor ripple current beyond the manufacturer’s speci fication is recommended to extend the useful life of the circuit. Lower operating temperature has the largest effect on capacitor longevity. The output capacitor in a buck converter under steady state conditions sees much less ripple current than the input capacitor. Peaktopeak current is equal to inductor ripple current, usually 10% to 40% of the total load current. Output capacitor duty places a premium not on power dissipation but on ESR. During an output load transient, the output capacitor must supply all of the additional load current demanded by the load until the LTC3832 adjusts the inductor current to the new value. ESR in the output capacitor results in a step in the output voltage equal to the ESR value multiplied by the change in load current. An 5A load step with a 0.05 Ω ESR output capacitor results in a 250mV output voltage shift; this is 10% of the output voltage for a 2.5V supply! Because of the strong relationship between output capacitor ESR and output load transient response, choose the output capaci tor for ESR, not for capacitance value. A capacitor with suitable ESR will usually have a larger capacitance value than is needed to control steadystate output ripple. Electrolytic capacitors rated for use in switching power supplies with specified ripple current ratings and ESR can be used effectively in LTC3832 applications. OSCON electrolytic capacitors from Sanyo and other manufactur ers give excellent performance and have a very high performance/size ratio for electrolytic capacitors. Surface mount applications can use either electrolytic or dry tantalum capacitors. Tantalum capacitors must be surge tested and specified for use in switching power supplies. Low cost, generic tantalums are known to have very short lives followed by explosive deaths in switching power supply applications. Other capacitors that can be used include the Sanyo POSCAP and MVWX series. A common way to lower ESR and raise ripple current capability is to parallel several capacitors. A typical APPLICATIO S I FOR ATIO 
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