LTC3376 datasheet(21/28 Pages) AD

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LTC3376

Rev 0

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APPLICATIONS INFORMATION

switching frequency (fSW(MAX)) for low duty cycle applica-

tions can be calculated as follows:

fSW(MAX) =

VOUT + VBOTSW

tON(MIN) VIN(MAX) – VTOPSW + VBOTSW

(

)

(5)

where VIN(MAX) is the maximum input voltage, VOUT is

the output voltage, VTOPSW and VBOTSW are the internal

switch drops, and tON(MIN) is the minimum top switch

on-time. This equation shows that a slower switching

frequency is necessary to accommodate a very high VIN/

VOUT ratio.

For higher duty cycle applications, the minimum off-time

also imposes a max switching frequency which can be

calculated as follows:

fSW(MAX) =

VIN – VOUT – VTOPSW

tOFF(MIN) VIN + VBOTSW – VTOPSW

(

)

(6)

where tOFF(MIN) is the minimum top switch off-time. This

equation shows that a slower switching frequency is also

necessary to accommodate a very low VIN/VOUT ratio.

Inductor Selection and Maximum Output Current

Considerations in choosing an inductor are inductance

value, RMS current rating, saturation current rating, DCR,

and core loss.

If the duty cycle of operation is 50% or less, choose the

inductor based on the following equation:

L = VOUT •

1–

VOUT

VIN(MAX)

0.2 •IMAX • fSW

for

VOUT

VIN

≤ 0.5

(7)

where fSW is the switching frequency, VIN(MAX) is the max-

imum input voltage that the buck will run at, and IMAX is

1.5A times the number of power stages (the maximum

rated load current for the LTC3376). For operation at duty

cycles higher than 50%, use instead the following equa-

tion to select the inductor:

L = 1.25 •

VIN(MAX)

fSW •IMAX

for

VOUT

VIN

> 0.5

(8)

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 should also be taken into

consideration.

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

cally labeled ISAT) is higher than the maximum expected

load plus half the inductor ripple:

ISAT >ILOAD(MAX)+

ΔIL

(9)

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

∆IL is the inductor ripple current as calculated by:

ΔIL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

(10)

A more conservative choice would be to choose an induc-

tor with an ISAT rating higher than the maximum current

limit of the LTC3376 which is 3.0A per power stage.

For highest efficiency, choose an inductor with the low-

est series resistance (DCR). The core material should be

intended for high frequency applications. Table 2 shows

recommended inductors from several manufacturers.

Input Capacitors

The LTC3376 has individual input supply pins for each

buck power stage. All of these pins must be decoupled

with low ESR capacitors to their own PGND. These capaci-

tors should be placed as close to the pins as possible.

Ceramic dielectric capacitors are a good compromise

between high dielectric constant and stability versus

temperature and DC bias. Note that the capacitance of a

capacitor deteriorates at higher DC bias. It is important

to consult manufacturer data sheets and obtain the true

capacitance of a capacitor at the DC bias voltage that it will

operate at. For this reason, avoid the use of Y5V dielec-

tric capacitors. The X5R/X7R dielectric capacitors offer

good overall performance. See Table 3 for recommended

ceramic capacitor manufacturers.

Regardless of how the power stages are configured, each

input supply voltage pin, VINA-H, needs to be decoupled

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