LTC3633 datasheet(12/28 Pages) LINER

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LTC3633

3633f

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 out-

put capacitor, 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 =

3.2E

where RRT is in Ω and f is in Hz.

When RT is tied to INTVCC, the switching frequency will

default to approximately 2MHz, as set by an internal re-

sistor. This internal resistor is more sensitive to process

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:

IL =

VOUT

f• L

1– VOUT

VIN

Where ΔIL=inductorripplecurrent,f=operatingfrequency

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

size, efﬁciency and operating frequency can be seen from

this equation. Accepting larger values of ΔIL 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:

L =

VOUT

f• IL(MAX)

1– VOUT

VIN(MAX)

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

losses decrease. Unfortunately, increased inductance

requires more turns of wire, leading to increased DCR

and copper loss.

1000

2000

3000

5000

200

700

600

3633 F01

6000

4000

100

300

400

500

RT RESISTOR (kΩ)

Figure 1. Switching Frequency vs RT

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