9

LTC3403

3403f

SUPPLY VOLTAGE (V)

2.5

1200

1000

800

600

400

200

0

3.0

3.5

4.0

4.5

3403 F02

5.0

5.5

Figure 2. Maximum Output Current vs Input Voltage

The basic LTC3403 application circuit is shown in Fig-

ure 1. External component selection is driven by the load

requirement and begins with the selection of L followed by

Inductor Selection

For most applications, the value of the inductor will fall in

the range of 1

µH to 4.7µH. Its value is chosen based on the

desired ripple current. Large value inductors lower ripple

current and small value inductors result in higher ripple

currents. As Equation 1 shows, a greater difference be-

Where these voltages are subject to change, the highest

current. A reasonable starting point for setting ripple

∆=









I

fL

V

V

V

L

OUT

OUT

IN

1

1

()( )

–

(1)

At output voltages below 0.6V, the switching frequency

decreases linearly to a minimum of approximately 700kHz.

This places the maximum ripple current (in forced con-

tinuous mode) at the highest input voltage and the lowest

output voltage. In practice, the resulting ouput ripple

voltage is 10mV to 15mV using the components specified

in Figure 1.

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 720mA rated

inductor should be enough for most applications (600mA

+ 120mA). For better efficiency, choose a low DC-resis-

tance inductor.

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

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

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 mate-

rials are small and don’t radiate much energy but generally

cost more than powdered iron core inductors with similar

electrical characteristics. The choice of which style induc-

tor to use often depends more on the price versus size

requirements and any radiated field/EMI requirements

than on what the LTC3403 requires to operate. Table 1

shows some typical surface mount inductors that work

well in LTC3403 applications.

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%. Normally, this

results in a reduction of maximum inductor peak current

for duty cycles > 40%. However, the LTC3403 uses a

patent-pending scheme that counteracts this compensat-

ing ramp, which allows the maximum inductor peak

current to remain unaffected throughout all duty cycles.