LTC3370 datasheet(13/20 Pages) AD

13 / 20 page

LTC3302

Rev. 0

For more information www.analog.com

The minimum on-time of the buck regulator imposes a

minimum operating duty cycle. The highest switching

frequency (fSW(MAX)) for a given application can be cal-

culated with Equation 2.

fSW MAX

(

) =

VOUT

tON MIN

(

) • VIN MAX

(

)

(2)

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

output voltage, and tON(MIN) is the minimum top switch

on-time. This equation shows that a slower switch-

ing frequency is necessary to accommodate a high

VIN(MAX)/VOUT ratio.

The LTC3302 is capable of a maximum duty cycle of

100%, therefore, the VIN-to-VOUT dropout is limited by

the RDS(ON) of the top switch, the inductor DCR, and the

load current.

Setting the Switching Frequency

The LTC3302 uses a constant frequency peak current

mode control architecture. The switching frequency is

factory programmed to one of four values: 2MHz, 4MHz,

6MHz, or 8MHz. Higher frequencies increase switching

losses and reduce efficiency, but could potentially allow

for smaller external components.

The LTC3302’s internal oscillator can be synchronized

to an external frequency by applying a square wave

clock signal to the MODE/SYNC pin. The synchronization

frequency range is ±20% of the nominal, factory pro-

grammed frequency.

When the rising edge at the MODE/SYNC pin is aligned

to the internal clock, the LTC3302 switches over to the

external clock. If the external clock is removed for more

than 1.5 clock cycles, the clock instantly reverts back to

the internal clock.

Inductor Selection and Maximum Output Current

Considerations in choosing an inductor are inductance,

RMS current rating, saturation current rating, DCR, and

core loss.

APPLICATIONS INFORMATION

Select the inductor value based on Equation 3

and Equation 4.

VOUT

0.6A • fSW

• 1

VOUT

VIN MAX

(

)

for

VOUT

VIN MAX

(

)

0.5

≈

(3)

0.25 • VIN MAX

(

)

0.6A • fSW

for

VOUT

VIN MAX

(

)

> 0.5

≈

(4)

where fSW is the switching frequency, VOUT is the output

voltage, and VIN(MAX) is the maximum input voltage.

To avoid overheating of the inductor choose an induc-

tor with an RMS current rating that is greater than the

maximum expected output load of the application.

Overload and short-circuit conditions need to be taken

into consideration.

In addition, ensure that the saturation current rating

(typically labeled ISAT) of the inductor is higher than the

maximum expected load current plus half the inductor

ripple current given by Equation 5.

ISAT >ILOAD MAX

(

) +

∆IL

(5)

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

a given application and ΔIL is the inductor ripple current

calculated with Equation 6.

∆IL =

VOUT

L • fSW

• 1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

(6)

A more conservative choice would be to use an inductor

with an ISAT rating higher than the maximum current limit

of the LTC3302.

To keep the efficiency high, choose an inductor with

the lowest series resistance (DCR). The core mate-

rial should be intended for high frequency applica-

tions. Table 1 shows recommended inductors from

several manufacturers.

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