LT3581IMSE-TRPBF 데이터 시트보기 (PDF) - Linear Technology

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LT3581IMSE-TRPBF

3.3A Boost/Inverting DC/DC Converter with Fault Protection

Linear Technology 

LT3581

APPENDIX

Inductor Current Rating

Inductors must have a rating greater than their peak

operating current, or else they could saturate and hence

contribute to losses in efficiency. The maximum inductor

current (considering start-up and steady-state conditions)

is given by:

IL _ PEAK

= ILIM

+

VIN

•

TMIN _ PROP

L

where:

IL_PEAK = Peak Inductor Current in L1 for a Boost

Topology, or the Peak of the sum of the

Inductor Currents in L1 and L2 for Dual

Inductor Topologies.

ILIM** = 3.3A with SW1 and SW2 Tied Together,

or 1.9A with just SW1 (This assumes

usage of an inductor whose core

material soft-saturates such as

powdered iron core).

TMIN_PROP = 100ns (Propagation Delay through the

Current Feedback Loop).

**If using an inductor whose core material saturates

hard (e.g., ferrite), then pick ILIM to be 5.4A with SW1

and SW2 tied together, or 3A when just SW1 is used.

Note that these equations offer conservative results for

the required inductor current ratings. The current ratings

could be lower for applications with light loads, if the SS

capacitor is sized appropriately to limit inductor currents

at start-up.

Multilayer ceramic capacitors are an excellent choice, as

they have extremely low ESR and are available in very

small packages. X5R or X7R dielectrics are preferred, as

these materials retain their capacitance over wide voltage

and temperature ranges. A 10μF to 22μF output capacitor

is sufficient for most applications, but systems with very

low output currents may need only 2.2μF to 10μF. Always

use a capacitor with a sufficient voltage rating. Many

ceramic capacitors, particularly 0805 or 0603 case sizes,

have greatly reduced capacitance at the desired output

voltage. Tantalum Polymer or OS-CON capacitors can be

used, but it is likely that these capacitors will occupy more

board area than a ceramic, and will have higher ESR with

greater output ripple.

INPUT CAPACITOR SELECTION

Ceramic capacitors make a good choice for the input

decoupling capacitor, and should be placed such that it is

in close proximity to the VIN of the chip as well as to the

inductor connected to the input of the power path. If it is

not possible to optimally place a single input capacitor,

then use two separate capacitors—use one at the VIN of

the chip (see equation for CVIN in Tables 1, 2 and 3) and

one at the input to the power path (see equation for CPWR

in Tables 1, 2 and 3) A 4.7μF to 20μF input capacitor is

sufficient for most applications.

Table 6 shows a list of several ceramic capacitor man­

ufacturers. Consult the manufacturers for detailed infor­

mation on their entire selection of ceramic parts.

DIODE SELECTION

Schottky diodes, with their low forward voltage drops and

fast switching speeds, are recommended for use with the

LT3581. Choose a Schottky diode with low parasitic capaci-

tance to reduce reverse current spikes through the power

switch of the LT3581. The Central Semiconductor Corp.

CMMSH2-40 diode is a very good choice with a 40V reverse

voltage rating and an average forward current of 2A.

OUTPUT CAPACITOR SELECTION

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Table 6: Ceramic Capacitor Manufacturers

AVX

www.avxcorp.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

PMOS SELECTION

An external PMOS, controlled by the LT3581’s GATE pin,

can be used to facilitate input or output disconnect. The

GATE pin turns on the PMOS gradually during start-up

(see Soft-Start of External PMOS in the Operation section),

and turns the PMOS off when the LT3581 is in shutdown

or in fault.

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For more information www.linear.com/LT3581

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