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LTC3854EMSE 数据表(PDF) 15 Page - Linear Technology |
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LTC3854EMSE 数据表(HTML) 15 Page - Linear Technology |
15 / 28 page LTC3854 3854fa applicaTions inForMaTion The required ESR due to a load current step is: ESR ≤ ∆ V ∆I where ∆I is the change in current from full load to zero load (or minimum load) and ∆V is the allowed voltage deviation (not including any droop due to finite capacitance). The amount of capacitance needed is determined by the maximum energy stored in the inductor. The capacitance mustbesufficienttoabsorbthechangeininductorcurrent when a high current to low current transition occurs. The oppositeloadcurrenttransitionisgenerallydeterminedby the control loop OPTI-LOOP components, so make sure not to over compensate and slow down the response. The minimum capacitance to assure the inductors’ energy is adequately absorbed is: COUT > L ∆I ( )2 2 ∆V ( )VOUT Manufacturers such as Nichicon, United Chemi-Con and Sanyo should be considered for high performance through-hole capacitors. The OS-CON semiconductor electrolyte capacitor available from Sanyo has the lowest (ESR)(size) product of any aluminum electrolytic at a somewhat higher price. An additional ceramic capacitor in parallel with OS-CON capacitors is recommended to reduce the inductive effects. Insurfacemountapplications,ESR,RMScurrenthandling and load step specifications may require multiple capaci- tors in parallel. Aluminum electrolytic, dry tantalum and special polymer capacitors are available in surface mount packages. Special polymer surface mount capacitors offer very low ESR but have much lower capacitive density per unit volume than other capacitor types. These capacitors offeraverycost-effectiveoutputcapacitorsolutionandare an ideal choice when combined with a controller having highloopbandwidth.Tantalumcapacitorsofferthehighest capacitance density and are often used as output capaci- tors for switching regulators having controlled soft-start. Several excellent surge-tested choices are the AVX TPS, AVX TPSV or the KEMET T51 0 series of surface mount tantalums, available in case heights ranging from 1.5mm to 4.1mm. Aluminum electrolytic capacitors can be used in cost-driven applications, provided that consideration is given to ripple current ratings, temperature and long-term reliability. A typical application will require several alumi- numelectrolyticcapacitorsinparallel.Acombinationofthe abovementionedcapacitorswilloftenresultinmaximizing performance and minimizing overall cost. Other capacitor types include Nichicon PL series, NEC Neocap, Panasonic SP and Sprague 595D series. Consult manufacturers for other specific recommendations. Like all components, capacitors are not ideal. Each capacitor has its own benefits and limitations. Combina- tions of different capacitor types have proven to be a very cost effective solution. Remember also to include high frequency decoupling capacitors. They should be placed as close as possible to the power pins of the load. Any inductance present in the circuit board traces negates their usefulness. Setting Output Voltage The LTC3854 output voltage is set by an external feed- back resistive divider carefully placed across the output, as shown in Figure 3. The regulated output voltage is determined by: VOUT = 0.8 1+ RB RA Figure 3. Feed-Forward Capacitor on FB Pin 3854 F03 LTC3854 RB CFF FB RA VOUT To improve the frequency response, a feed-forward ca- pacitor, CFF, may be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line. Fault Conditions: Current Foldback The LTC3854 includes current foldback to help limit load current when the output is shorted to ground. If the output falls below 40% of its nominal output level, the maximum |
类似零件编号 - LTC3854EMSE |
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类似说明 - LTC3854EMSE |
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