LTC3313JVPBF datasheet(14/28 Pages) AD

14 / 28 page

LTC3313

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The second method to set the LTC3313 switching frequency

is by synchronizing the internal PLL circuit to an external fre-

quency applied to the MODE/SYNC pin. The synchronization

frequency range is 0.5MHz to 2.25MHz.

The internal PLL starts up at the 2MHz default frequency.

After detecting an external clock on the first rising edge

of the MODE/SYNC pin, the internal PLL gradually adjusts

its operating frequency to match the frequency and phase

of the MODE/SYNC signal.

The LTC3313 detects when the external clock is removed

and will gradually adjust its operating frequency to the

2MHz default frequency. The LTC3313 operates in forced

continuous mode when synchronized to an external clock.

The third method of setting the LTC3313 switching fre-

quency is to use the internal nominal 2MHz default clock.

See Table 4 for pin configuration.

Inductor Selection and Maximum Output Current

Considerations in choosing an inductor are inductance,

RMS current rating, saturation current rating, DCR, and

core loss.

A good first choice for the inductor value is:

≥

VOUT

4.5A

• fSW

• 1−

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟ for

VOUT

VIN(MAX)

≤ 0.5 (4)

≥

0.25

• VIN(MAX)

4.5A

• fSW

for

VOUT

VIN(MAX)

> 0.5

(5)

where fSW is the switching frequency in MHz, VIN is the

input voltage, and L is the inductor value in μH.

To avoid overheating of the inductor, choose an inductor with

an RMS current rating that is greater than the maximum

expected output load of the application. Overload and short

circuit conditions may need to be taken into consideration.

In addition, the saturation current (ISAT) rating of the

inductor must be higher than the load current plus 1/2 of

the inductor ripple current:

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 as:

∆IL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

(7)

where VIN(MAX) is the maximum application input voltage.

To keep the efficiency high, choose an inductor with the

lowest series resistance (DCR). The core material should

be intended for high frequency applications.

The LTC3313 limits the peak switch current in order to

protect the switches and the system from overload faults.

The inductor value must then be sufficiently large to sup-

ply the desired maximum output current, IOUT(MAX), which

is a function of the switch current limit, ILIM, and the

ripple current.

IOUT(MAX) = ILIM – ΔIL

(8)

Therefore, the maximum output current that the LTC3313

will deliver depends on the switch current limit, the induc-

tor value, and the input and output voltages. The inductor

value may have to be increased if the inductor ripple cur-

rent does not allow sufficient maximum output current

(IOUT(MAX)) given the switching frequency, and maximum

input voltage used in the desired application. Table 2 pro-

vides a list of recommended inductor manufacturers.

Table 2. Inductor Manufacturers

VENDOR

URL

Coilcraft

www.coilcraft.com

Sumida

www.sumida.com

TDK

www.tdk.com

Toko

www.toko.com

Vishay

www.vishay.com

Wurth Elektronik

www.we-online.com

XFMRS

www.xfmrs.com

Input Capacitors

Bypass the input of the LTC3313 with at least two bulk stor-

age ceramic capacitors close to the part, one on each side

from VIN to PGND. These capacitors should be 0603 or

0805 in size. See layout section for more detail. X7R or X5R

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