Datasheet LT3580 (Analog Devices) - 8

LT3580
APPLICATIONS INFORMATION Setting Output Voltage Inductor Selection
The output voltage is set by connecting a resistor (RFB) General Guidelines: The high frequency operation of the from VOUT to the FB pin. RFB is determined from the LT3580 allows for the use of small surface mount inductors. following equation: For high efficiency, choose inductors with high frequency | V − V | core material, such as ferrite, to reduce core losses. To R = OUT FB FB improve efficiency, choose inductors with more volume 83.3µA for a given inductance. The inductor should have low where V DCR (copper wire resistance) to reduce I2R losses, and FB is 1.215V (typical) for non-inverting topologies (i.e., boost and SEPIC regulators) and 5mV (typical) for must be able to handle the peak inductor current without inverting topologies (see the Electrical Characteristics). saturating. Note that in some applications, the current handling requirements of the inductor can be lower, such
Power Switch Duty Cycle
as in the SEPIC topology, where each inductor only carries a fraction of the total switch current. Molded chokes or chip In order to maintain loop stability and deliver adequate inductors usually do not have enough core area to sup- current to the load, the power NPN (Q1 in the Block Dia- port peak inductor currents in the 2A to 3A range. To gram) cannot remain “on” for 100% of each clock cycle. minimize radiated noise, use a toroidal or shielded inductor. The maximum allowable duty cycle is given by: Note that the inductance of shielded types will drop more −MinOff Time) as current increases, and will saturate more easily. See DC = (TP • 100% MAX Table 1 for a list of inductor manufacturers. Thorough lab TP evaluation is recommended to verify that the following where TP is the clock period and Min Off Time (found in guidelines properly suit the final application. the Electrical Characteristics) is typically 60ns.
Table 1.Inductor Manufacturers
The application should be designed so that the operating Coilcraft DO3316P, MSS7341 and LPS4018 www.coilcraft.com duty cycle does not exceed DCMAX. Series Coiltronics DR, LD and CD Series www.coiltronics.com Duty cycle equations for several common topologies are given below, where V Murata LQH55D and LQH66S Series www.murata.com D is the diode forward voltage drop and V Sumida CDRH5D18B/HP, CDR6D23MN, www.sumida.com CESAT is typically 300mV at 1.5A. CDRH6D26/HP, CDRH6D28, For the boost topology: CDR7D28MN and CDRH105R Series TDK RLF7030 and VLCF4020 Series www.tdk.com V − V + V DC ≅ OUT IN D Würth WE-PD and WE-PD2 Series www.we-online.com V + V − V OUT D CESAT Minimum Inductance: Although there can be a tradeoff with For the SEPIC or dual inductor inverting topology (see efficiency, it is often desirable to minimize board space by Figures 1 and 2): choosing smaller inductors. When choosing an inductor, V +| V | there are two conditions that limit the minimum inductance; DC ≅ D OUT (1) providing adequate load current, and (2) avoidance of V + | V | + V − V IN OUT D CESAT subharmonic oscillation. Choose an inductance that is high enough to meet both of these requirements. The LT3580 can be used in configurations where the duty cycle is higher than DCMAX, but it must be operated in the Adequate Load Current : Small value inductors result in discontinuous conduction mode so that the effective duty increased ripple currents and thus, due to the limited peak cycle is reduced. switch current, decrease the average current that can be 3580fg 8