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LTC1430CS Datasheet(PDF) 8 Page - Linear Technology |
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LTC1430CS Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LTC1430 Once the threshold voltage has been selected, RON should be chosen based on input and output voltage, allowable power dissipation and maximum required output current. In a typical LTC1430 buck converter circuit operating in continuous mode, the average inductor current is equal to the output load current. This current is always flowing through either M1 or M2 with the power dissipation split up according to the duty cycle: DC M V V DC M V V VV V OUT IN OUT IN IN OUT IN () () 1 21 = =− = − () The RON required for a given conduction loss can now be calculated by rearranging the relation P = I2R: RM PM DC M I VP M VI ON MAX MAX IN MAX OUT MAX () () () () 1 1 1 1 2 2 = • = • • RM PM DC M I VP M VV I ON MAX MAX IN MAX IN OUT MAX () () () () 2 2 2 2 2 2 = • = • − ()• PMAX should be calculated based primarily on required efficiency. A typical high efficiency circuit designed for 5V in, 3.3V at 10A out might require no more than 3% efficiency loss at full load for each MOSFET. Assuming roughly 90% efficiency at this current level, this gives a PMAX value of (3.3V • 10A/0.9) • 0.03 = 1.1W per FET and a required RON of: RM VW VA RM VW VV A ON ON () . . . () . . . 1 51 1 33 10 0 017 2 51 1 53 3 10 0 032 2 2 = • • =Ω = • − ()• =Ω Note that the required RON for M2 is roughly twice that of M1 in this example. This application might specify a single 0.03 Ω device for M2 and parallel two more of the same devices to form M1. Note also that while the required RON values suggest large MOSFETs, the dissipation numbers 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 LTC1430 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 LTC1430 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 LTC1430. In a typical 5V to 3.3V application, the maximum rise time will be: 90 153 % . • − () = µ VV L AMPS SECOND A s I L IN OUT 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 3.xV LTC1430 circuits. Different combinations of input and output voltages 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 APPLICATIO S I FOR ATIO |
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