LTC3315B datasheet(13/28 Pages) AD

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LTC3311S

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

The second method to set the LTC3311S switching fre-

quency is by synchronizing the internal PLL circuit to an

external frequency 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 LTC3311S detects when the external clock is removed

and will gradually adjust its operating frequency to the

2MHz default frequency. The LTC3311S operates in forced

continuous mode when synchronized to an external clock.

The third method of setting the LTC3311S 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

4A • fSW

• 1

VOUT

VIN MAX

(

)

for

VOUT

VIN MAX

(

)

0.5

(4)

0.25 • VIN MAX

(

)

4A • 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 maxi-

mum 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 LTC3311S 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 supply 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 LTC3311S

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. Inductor Manufacturers

VENDOR

URL

Coilcraft

www.coilcraft.com

Sumida

www.sumida.com

Toko

www.toko.com

Wurth Elektronik

www.we-online.com

Vishay

www.vishay.com

XFMRS

www.xfmrs.com

Input Capacitors

Bypass the input of the LTC3311S with at least two bulk

storage 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

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