LTC3370 datasheet(14/26 Pages) AD

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LTC3307A

Rev. A

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disadvantages of higher switching frequencies are lower

efficiency, because of increased switching losses, and a

smaller input voltage range, because of minimum switch

on-time limitations.

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 as follows:

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

switching frequency is necessary to accommodate a high

VIN(MAX)/VOUT ratio.

The LTC3307A 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 LTC3307A uses a constant frequency peak current

mode control architecture. There are three methods to

set the switching frequency.

The first method, connecting the RT pin to VIN, sets the

switching frequency to the internal default with a nominal

value of 2MHz.

The second method is with a resistor (RT) tied from the

RT pin to ground. The frequency can be programmed

from 1MHz to 3MHz. Table 1 and the Equation 3 show

the necessary RT value for a desired switching frequency:

RT =

73.4

fsw

– 1.9

(3)

where RT is in kΩ and fSW is the desired switching fre-

quency in MHz, ranging from 1MHz to 3MHz.

APPLICATIONS INFORMATION

Table 1. RT Value vs Switching Frequency

fSW (MHz)

RT (kΩ)

1.0

71.5

1.2

59.0

1.4

49.9

1.6

43.2

1.8

38.3

2.0

34.8

2.2

30.9

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 the following equation:

L ≥

VOUT

0.9A • fSW

• 1−

VOUT

VIN MAX

(

)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟ for

VOUT

VIN MAX

(

)

≤ 0.5(4)

L ≥

0.25 • VIN MAX

(

)

0.9A • fSW

for

VOUT

VIN MAX

(

)

> 0.5

(5)

where fSW is the switching frequency, 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.

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