LTC3104IMSE#TRPBF【PDF 12/20 页】IC REG DL BCK/LINEAR SYNC 16MSOP

LTC3104

12

3104f

APPLICATIONS INFORMATION

The basic LTC3104 application circuit is shown as the

Typical Application on the front page of this data sheet.

The external component selection is determined by the

desired output voltage, output current, desired noise im-

munity and ripple voltage requirements for each particular

application. However, basic guidelines and considerations

for the design process are provided in this section.

Inductor Selection

The choice of inductor value influences both the efficiency

and the magnitude of the output voltage ripple. Larger in-

ductance values will reduce inductor current ripple and will

therefore lead to lower output voltage ripple. For a fixed DC

resistance, a larger value inductor will yield higher efficiency

by lowering the peak current to be closer to the average.

However, a larger value inductor within the same family

will generally have a greater series resistance, thereby

offsetting this efficiency advantage. Given a desired peak

to peak current ripple, I

L

(A), the required inductance

can be calculated via the following expression:

Le

V

OUT

1.2 I

L

1

V

OUT

V

IN

?/DIV>

礖

( )

A reasonable choice for ripple current is I

L

= 120mA

which represents 40% of the maximum 300mA load

current. The DC current rating of the inductor should be

at least equal to the maximum load current plus half the

ripple current in order to prevent core saturation and loss

of efficiency during operation. To optimize efficiency the

inductor should have a low series resistance. In particularly

space restricted applications it may be advantageous to

use a much smaller value inductor at the expense of larger

ripple current. In such cases, the converter will operate

in discontinuous conduction for a wider range of output

loads and efficiency will be reduced. In addition, there is a

minimum inductor value required to maintain stability of the

current loop (given the fixed internal slope compensation).

Specifically, if the buck converter is going to be utilized at

duty cycles greater than 40%, the inductance value must

be at least L

MIN

as given by the following equation:

L

MIN

e 2.5 " V

OUT

(礖)

Table 1 depicts the minimum required inductance for

several common output voltages using standard induc-

tor values.

Table 1. Minimum Inductance

OUTPUT VOLTAGE (V)

MINIMUM INDUCTANCE (礖)

0.8

2.2

1.2

3.3

2.0

5.6

2.7

6.8

3.3

8.3

5.0

15

A large variety of low ESR, power inductors are available

that are well suited to the LTC3104 converter applications.

The trade-off generally involves PCB area, application

height, required output current and efficiency. Table 2

provides a representative sampling of small surface mount

inductors that are well suited for use with the LTC3104

buck converter. The inductor specifications listed are for

comparison purposes but other values within these induc-

tor families are generally well suited to this application

as well. Within each family (i.e., at a fixed inductor size),

the DC resistance generally increases and the maximum

current generally decreases with increased inductance.

Output Capacitor Selection

A low ESR output capacitor should be utilized at the buck

output in order to minimize voltage ripple. Multilayer ce-

ramic capacitors are an excellent choice as they have low

ESR and are available in small footprints. In addition to

controlling the output ripple magnitude, the value of the

output capacitor also sets the loop crossover frequency

and therefore can impact loop stability. There is both a

minimum and maximum capacitance value required to

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