9

LTC3404

3404fb

Figure 2. Maximum Inductor Peak Current vs Duty Cycle

DUTY CYCLE (%)

0

1100

1000

900

800

700

600

80

3404 F02

20

40

60

100

OPERATIO

APPLICATIO S I FOR ATIO

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant fre-

quency architectures by preventing subharmonic oscilla-

tions at high duty cycles. It is accomplished internally by

adding a compensating ramp to the inductor current

signal at duty cycles in excess of 40%. As a result, the

maximum inductor peak current is reduced for duty cycles

> 40%. This is shown in the decrease of the inductor peak

current as a function of duty cycle graph in Figure 2.

The basic LTC3404 application circuit is shown on the first

page. External component selection is driven by the load

requirement and begins with the selection of L followed by

Inductor Value Calculation

The inductor selection will depend on the operating fre-

quency of the LTC3404. The internal nominal frequency is

1.4MHz, but can be externally synchronized from 1MHz to

1.7MHz.

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. However, oper-

ating at a higher frequency generally results in lower

efficiency because of increased internal gate charge losses.

The inductor value has a direct effect on ripple current. The

ripple current

−

⎛

⎝⎜

⎞

⎠⎟

I

fL

V

V

V

L

OUT

OUT

IN

1

1

(1)

Accepting larger values of

inductors, but results in higher output voltage ripple and

greater core losses. A reasonable starting point for setting

ripple current is

The inductor value also has an effect on Burst Mode

operation. The transition to low current operation begins

when the inductor current peaks fall to approximately

250mA. Lower inductor values (higher

to occur at lower load currents, which can cause 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 more expensive ferrite, molypermalloy,

or Kool M

size for a fixed inductor value, but it is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, 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 cur-

rent is exceeded. This results in an abrupt increase in