LTC3633_15 Datasheet, PDF(12/28 Page) Linear Technology – Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator

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LTC3633_15 Datasheet, PDF (12/28 Pages) Linear Technology – Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator

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LTC3633

APPLICATIONS INFORMATION

A general LTC3633 application circuit is shown on the

first page of this data sheet. External component selection

is largely driven by the load requirement and switching

frequency. Component selection typically begins with the

selection of the inductor L and resistor RT. Once the inductor

is chosen, the input capacitor, CIN, and the output capaci-

tor, COUT, 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, track/

soft-start, VIN UVLO, and PGOOD.

Programming Switching Frequency

Selection of the switching 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.

Connecting a resistor from the RT pin to SGND programs

the switching frequency (f) between 500kHz and 4MHz

according to the following formula:

RRT

3.2E11

where RRT is in Î© and f is in Hz.

When RT is tied to INTVCC, the switching frequency will

default to approximately 2MHz, as set by an internal

6000

5000

4000

3000

2000

1000

0 100 200 300 400 500 600 700

RT RESISTOR (kÎ©)

3633 F01

Figure 1. Switching Frequency vs RT

resistor. This internal resistor is more sensitive to pro-

cess and temperature variations than an external resistor

(see Typical Performance Characteristics) and is best used

for applications where switching frequency accuracy is

not critical.

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:

ÎIL

ï£«

ï£¬

ï£

VOUT

f â¢L

ï£¶ï£«

ï£·ï£¬1â

ï£¸ï£

VOUT

VIN

ï£¶

ï£·

ï£¸

Where ÎIL = inductor ripple current, f = operating frequency

and L = inductor value. A trade-off between component

size, efficiency and operating frequency can be seen from

this equation. Accepting larger values of âIL allows the

use of lower value inductors but results in greater inductor

core loss, greater ESR loss in the output capacitor, and

larger output voltage ripple. Generally, highest efficiency

operation is obtained at low operating frequency with

small ripple current.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). Note that the largest

ripple current occurs at the highest VIN. Exceeding 60%

of IOUT(MAX) is not recommended. To guarantee that

ripple current does not exceed a specified maximum, the

inductance should be chosen according to:

ï£«

ï£¬ï£¬

ï£

â¢

VOUT

ÎIL(MAX)

ï£¶ï£«

ï£·ï£·ï£¬ï£¬1â

ï£¸ï£

VOUT

VIN(MAX)

ï£¶

ï£·ï£·

ï£¸

The inductor ripple current also must not be so large that

its valley current level exceeds the negative current limit,

which can be as small as â1.2A. If the negative current

limit is exceeded while the part is in the forced continu-

ous mode of operation, VOUT can get charged up to above

its regulation level â until the inductor current no longer

exceeds the negative current limit. In such instances,

choose a larger inductor value to reduce the inductor

ripple current. The alternative is to reduce the inductor

3633fd

For more information www.linear.com/LTC3633

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