LTC3602EFE datasheet(9/20 Pages) LINER

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LTC3602

3602fb

APPLICATIONS INFORMATION

The basic LTC3602 application circuit is shown on the front

page of this data sheet. External component selection is

determined by the maximum load current and begins with

the selection of the inductor value and operating frequency

followed by CIN and COUT.

Operating Frequency

Selection of the operating frequency is a tradeoff 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 and switching losses but

requires larger inductance values and/or capacitance to

maintain low output ripple voltage. The operating frequency

of the LTC3602 is determined by an external resistor that is

connected 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:

fHz

OSC =

115 10

.•

()

–

Although frequencies as high as 3MHz are possible, the

minimum on-time of the LTC3602 imposes a minimum

limit on the operating duty cycle. The minimum on-time

is typically 90ns. Therefore, the minimum duty cycle is

equal to 100 • 90ns • f(Hz).

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

ΔIL increases with higher VIN and decreases

with higher inductance.

ΔI

L =

OUT

⎛

⎝⎜

⎞

⎠⎟

•1–

OUT

⎛

⎝⎜

⎞

⎠⎟

Having a lower ripple current reduces the ESR losses

in the output capacitors and the output voltage ripple.

Highest efﬁciency operation is achieved at low frequency

with small ripple current. This, however, requires a large

inductor.

A reasonable starting point for selecting the ripple current

ΔIL = 0.4(IMAX), where IMAX is the maximum output

current. The largest ripple current occurs at the highest

VIN. To guarantee that the ripple current stays below a

speciﬁed maximum, the inductor value should be chosen

according to the following equation:

OUT

ΔI

L(MAX)

⎛

⎝

⎜

⎞

⎠

⎟ •1–

OUT

IN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

The inductor value will also have an effect on Burst Mode

operation. The transition from low current operation

begins when the peak inductor current falls below a level

set by the burst clamp. Lower inductor values result in

higher ripple current which causes this to occur at lower

load currents. This causes a dip in efﬁciency in the upper

range of low current operation. In Burst Mode operation,

lower inductance values will cause the burst frequency

to increase.

Inductor Core Selection

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

be selected. High efﬁciency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of the more expensive ferrite cores. Actual

core loss is independent of core size for a ﬁxed inductor

value but it is very dependent on the inductance selected.

As the inductance increases, core losses decrease. Un-

fortunately, increased inductance requires more turns of

wire and therefore copper losses will increase.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” which means that

inductance collapses abruptly when the peak design current

is exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Different core materials and shapes will change the

size/current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy ma-

terials are small and do not radiate energy but generally

cost more than powdered iron core inductors with similar

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