LTC3312SAAVPBF datasheet(17/28 Pages) AD

17 / 28 page

LTC3312SA

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Table 1. RT Value vs Switching Frequency

fSW (MHz)

RT (kΩ)

1.0

71.5

1.2

59.0

1.4

51.1

1.6

44.2

1.8

39.2

2.0

34.8

2.2

31.6

2.4

28.7

2.6

26.1

2.8

24.3

3.0

22.6

The third method to set the switching frequency is by syn-

chronizing the internal PLL circuit to an external square

wave clock applied to the MODE/SYNC pin. The synchro-

nization frequency range is 1MHz to 3MHz. The square

wave amplitude should have valleys that are below 0.4V

and peaks above 1.2V. High and low pulse widths should

both be at least 40ns.

Inductor Selection and Maximum Output Current

Considerations in choosing an inductor are inductance,

RMS current rating, saturation current rating, DCR, and

core loss.

Select the inductor value based on Equation 4 and

Equation 5.

≈

VOUT

1.8A

•fSW

• 1−

VOUT

PVIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟ for

VOUT

PVIN(MAX)

≤0.5 (4)

≈

0.25

•PVIN(MAX)

1.8A

•fSW

for

VOUT

PVIN(MAX)

>0.5

(5)

where fSW is the switching frequency and PVIN(MAX) is the

maximum input voltage.

To avoid overheating of the inductor choose an inductor

with an RMS current rating that is greater than the maxi-

mum expected output load of the application. Overload

and short circuit conditions may need to be taken

into consideration.

In addition, ensure that the saturation current rating (typi-

cally labeled ISAT) of the inductor is higher than the maxi-

mum expected load current plus half the inductor ripple

current (Equation 6).

ISAT >ILOAD MAX

(

) +

∆IL

(6)

where ILOAD(MAX) is the maximum output load current for

a given application and ΔIL is the inductor ripple current

calculated by Equation 7.

∆IL =

VOUT

L • fSW

• 1–

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

(7)

A more conservative choice would be to use an inductor

with an ISAT rating higher than the maximum current limit

of the LTC3312SA.

To keep the efficiency high, choose an inductor with the

lowest series resistance (DCR). The core material should

be intended for high frequency applications. Table 2 shows

recommended inductors from several manufacturers.

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