LT3580_15 datasheet(8/28 Pages) LINER

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LT3580

3580fg

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:

FB =

OUT − 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:

MAX =

P − Min Off Time)

• 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:

≅

OUT − VIN + VD

OUT + VD − VCESAT

For the SEPIC or dual inductor inverting topology (see

Figures 1 and 2):

≅

D + |VOUT |

IN + |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

saturating. 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 fraction 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 inductor.

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. Thorough lab

evaluation is recommended to verify that the following

guidelines properly suit the final application.

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. Choose an inductance that is high

enough to meet both of these requirements.

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

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