LTC3646-1_15 Datasheet, PDF(11/28 Page) Linear Technology – 40V, 1A Synchronous Step-Down Converter

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LTC3646-1_15 Datasheet, PDF (11/28 Pages) Linear Technology – 40V, 1A Synchronous Step-Down Converter

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LTC3646/LTC3646-1

APPLICATIONS INFORMATION

A general LTC3646 application circuit is shown on the first

page of this data sheet. External component selection is

largely driven by the load requirement and begins with the

selection of the inductor L. Once the inductor is chosen,

the input capacitor, CIN, the output capacitor, COUT, the

internal regulator capacitor, CVCC, and the boost capaci-

tor, CBOOST, can be selected. Next, the feedback resistors

are selected to set the desired output voltage. Finally, the

remaining optional external components can be selected for

functions such as external loop compensation, externally

programmed oscillator frequency and PGOOD.

Operating Frequency

Selection of the operating frequency is a trade-off between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger

inductance values and/or capacitance to maintain low

output ripple voltage. The operating frequency, fO, of the

LTC3646 is determined by an external resistor that is con-

nected between the RT pin and ground. The value of the

resistor sets the ramp current that is used to charge and

discharge an internal timing capacitor within the oscillator

and can be calculated by using the following equation:

RRT

9E10

where RRT is in Î© and fO is in Hz.

3000

2500

2000

1500

1000

500

100 200 300 400 500

RRT (kâ¦)

3646 F01

Connecting the RT pin to INTVCC will assert the internal

default fO = 2.25MHz; however, this switching frequency

will be more sensitive to process and temperature variations

than using a resistor on RT (see the Typical Performance

Characteristics section).

Inductor Selection

For a given input and output voltage, the inductor value and

operating frequency determine the inductor ripple current.

More specifically, the inductor ripple current decreases

with higher inductor value or higher operating frequency

according to the following equation:

âIL

ï£«

ï£¬

ï£

VOUT

fO â¢ L

ï£¶ï£«

ï£·ï£¬1â

ï£¸ï£

VOUT

VIN

ï£¶

ï£·

ï£¸

where ÎIL = inductor ripple current (A), fO = operating

frequency (Hz) and L = inductor value (H). A trade-off be-

tween component size, efficiency and operating frequency

can be seen from this equation. Accepting larger values of

ÎIL allows the use of lower value inductors but results in

greater core loss in the inductor, greater ESR loss in the

output capacitor, and larger output ripple. Generally, the

highest efficiency operation is obtained at low operating

frequency with small ripple current.

The inductor value should be chosen to give a peak-to-

peak ripple current of between 30% and 40% of IOUT(MAX),

where IOUT(MAX) equals the maximum average output

current. Note that the largest ripple current occurs at

the highest VIN. To guarantee the ripple current does not

exceed a specified maximum, the inductance should be

chosen according to:

ï£«

ï£¬

ï£

VOUT

fO â¢ âIL

ï£¶ï£«

ï£·ï£¬1â

ï£¸ï£

VOUT

VIN

ï£¶

ï£·

ï£¸

Once the value for L is known, the type of inductor must

be selected. Actual core loss is independent of core size

for a fixed inductor value, but is very dependent on the

inductance selected. As the inductance increases, core loss

decreases. Unfortunately, increased inductance requires

more turns of wire leading to increased copper loss.

Figure 1

For more information www.linear.com/LTC3646

36461fb

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