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LT3580IMS8E-PBF Datasheet(PDF) 9 Page - Linear Technology |
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LT3580IMS8E-PBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  LT3580 9 3580fc Setting Output Voltage The output voltage is set by connecting a resistor (RFB) from VOUT to the FB pin. RFB is determined from the following equation: RFB = |VOUT VFB | 83.3μA where VFB is 1.215V (typical) for non-inverting topologies (i.e., boost and SEPIC regulators) and 5mV (typical) for inverting topologies (see the Electrical Characteristics). Power Switch Duty Cycle In order to maintain loop stability and deliver adequate current to the load, the power NPN (Q1 in the Block Dia- gram) cannot remain “on” for 100% of each clock cycle. The maximum allowable duty cycle is given by: DCMAX = (TP MinOffTime) TP • 100% where TP is the clock period and Min Off Time (found in the Electrical Characteristics) is typically 60ns. The application should be designed so that the operating duty cycle does not exceed DCMAX. Duty cycle equations for several common topologies are given below, where VD is the diode forward voltage drop and VCESAT is typically 300mV at 1.5A. For the boost topology: DC VOUT VIN + VD VOUT + VD VCESAT For the SEPIC or dual inductor inverting topology (see Figures 1 and 2): DC VD+|VOUT | VIN + |VOUT | + VD VCESAT The LT3580 can be used in configurations where the duty cycle is higher than DCMAX, but it must be operated in the discontinuous conduction mode so that the effective duty cycle is reduced. APPLICATIONS INFORMATION Inductor Selection General Guidelines: The high frequency operation of the LT3580 allows for the use of small surface mount inductors. For high efficiency, choose inductors with high frequency core material, such as ferrite, to reduce core losses. To improve efficiency, choose inductors with more volume for a given inductance. The inductor should have low DCR (copper wire resistance) to reduce I2R losses, and must be able to handle the peak inductor current without saturat- ing. Note that in some applications, the current handling requirements of the inductor can be lower, such as in the SEPIC topology, where each inductor only carries a frac- tion of the total switch current. Molded chokes or chip inductors usually do not have enough core area to sup- port peak inductor currents in the 2A to 3A range. To minimize radiated noise, use a toroidal or shielded induc- tor. Note that the inductance of shielded types will drop more as current increases, and will saturate more easily. See Table 1 for a list of inductor manufacturers. Table 1.Inductor Manufacturers Coilcraft DO3316P, MSS7341 and LPS4018 Series www.coilcraft.com Coiltronics DR, LD and CD Series www.coiltronics.com Murata LQH55D and LQH66S Series www.murata.com Sumida CDRH5D18B/HP, CDR6D23MN, CDRH6D26/HP, CDRH6D28, CDR7D28MN and CDRH105R Series www.sumida.com TDK RLF7030 and VLCF4020 Series www.tdk.com Würth WE-PD and WE-PD2 Series www.we-online.com Minimum Inductance: Although there can be a tradeoff with efficiency, it is often desirable to minimize board space by choosing smaller inductors. When choosing an inductor, there are two conditions that limit the minimum inductance; (1) providing adequate load current, and (2) avoidance of subharmonic oscillation. Adequate Load Current: Small value inductors result in increased ripple currents and thus, due to the limited peak switch current, decrease the average current that can be provided to a load (IOUT). In order to provide adequate load current, L should be at least: L > DC • VIN 2(f) ILIM |VOUT |• IOUT VIN • |
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