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LTC1430AIGN Datasheet(PDF) 11 Page - Linear Technology |
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LTC1430AIGN Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  11 LTC1430A S APPLICATI I FOR ATIO are only 1.1W per device or less — large TO-220 packages and heat sinks are not necessarily required in high effi- ciency applications. Siliconix Si4410DY (in SO-8) and Motorola MTD20N03HL (in DPAK) are two small, surface mount devices with RON values of 0.03Ω or below with 5V of gate drive; both work well in LTC1430A circuits with up to 10A output current. A higher PMAX value will generally decrease MOSFET cost and circuit efficiency and increase MOSFET heat sink requirements. Inductor The inductor is often the largest component in an LTC1430A design and should be chosen carefully. Inductor value and type should be chosen based on output slew rate require- ments and expected peak current. Inductor value is prima- rily controlled by the required current slew rate. The maximum rate of rise of the current in the inductor is set by its value, the input-to-output voltage differential and the maximum duty cycle of the LTC1430A. In a typical 5V to 3.3V application, the maximum rise time will be: 90% = (VIN – VOUT) L AMPS SECOND 1.53A µs I L where L is the inductor value in µH. A 2µH inductor would have a 0.76A/ µs rise time in this application, resulting in a 6.5 µsdelayinrespondingtoa5Aloadcurrentstep.During this 6.5 µs,thedifferencebetweentheinductorcurrentand the output current must be made up by the output capaci- tor, causing a temporary droop at the output. To minimize this effect, the inductor value should usually be in the 1 µH to 5 µH range for most typical 5V to 2.xV-3.xV LTC1430A circuits. Different combinations of input and output volt- ages and expected loads may require different values. Once the required value is known, the inductor core type can be chosen based on peak current and efficiency requirements. Peak current in the inductor will be equal to the maximum output load current added to half the peak- to- peak inductor ripple current. Ripple current is set by the inductor value, the input and output voltage and the operating frequency. If the efficiency is high and can be approximately equal to 1, the ripple current is approxi- mately equal to: ∆I = DC (VIN – VOUT) (fOSC)(L) DC = VOUT VIN fOSC = LTC1430A oscillator frequency L = inductor value Solving this equation with our typical 5V to 3.3V applica- tion, we get: = 2.8AP–P (1.7)(0.66) (200kHz)(2 µH) Peak inductor current at 10A load: = 11.4A 10A + 2.8A 2 The inductor core must be adequate 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 the current may rise above this maximum level in circuits under current limit or under fault conditions in unlimited circuits; the inductor should be sized to withstand this additional current. Input and Output Capacitors A typical LTC1430A design puts significant demands on both the input and output capacitors. Under normal steady load operation, a buck converter like the LTC1430A draws square waves of current from the input supply at the switching frequency, with the peak value equal to the output current and the minimum value near zero. Most of this current must come from the input bypass capacitor, since few raw supplies can provide the current slew rate to feed such a load directly. The resulting RMS current flow in the input capacitor will heat it up, causing 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 current ratings are often based on only 2000 hours (3 months) lifetime; further derating of the input capacitor ripple current beyond the manufacturer’s specification is recommended to extend the useful life of the circuit. |
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