LTC3522 datasheet(13/20 Pages) LINER

13 / 20 page

LTC3522

3522f

APPLICATIONS INFORMATION

The basic LTC3522 application circuit is shown as the

typical application on the front page of this data sheet.

The external component selection is determined by the

desired output voltages, output currents and ripple voltage

requirements of each particular application. However, basic

guidelines and considerations for the design process are

provided in this section.

Buck Inductor Selection

The choice of buck inductor value inﬂuences both the ef-

ﬁciency and the magnitude of the output voltage ripple.

Larger inductance values will reduce inductor current ripple

and will therefore lead to lower output voltage ripple. For

a ﬁxed DC resistance, a larger value inductor will yield

higher efﬁciency by lowering the peak current to be closer

to the average. However, a larger value inductor within the

same family will generally have a greater series resistance,

thereby offsetting this efﬁciency advantage.

Given a desired peak to peak current ripple,

ΔIL,therequired

inductance can be calculated via the following expression,

where f represents the switching frequency in MHz:

μH

OUT

⎛

⎝⎜

⎞

⎠⎟ ()

–

A reasonable choice for ripple current is

ΔIL = 80mA which

represents 40% of the maximum 200mA load current. The

DC current rating of the inductor should be at least equal

to the maximum load current plus half the ripple current

in order to prevent core saturation and loss of efﬁciency

during operation. To optimize efﬁciency the inductor should

have a low series resistance.

In particularly space restricted applications it may be

advantageous to use a much smaller value inductor at

the expense of larger ripple current. In such cases, the

converter will operate in discontinuous conduction for a

wider range of output loads and efﬁciency will be reduced.

In addition, there is a minimum inductor value required

to maintain stability of the current loop (given the ﬁxed

internal slope compensation). Speciﬁcally, if the buck

converter is going to be utilized at duty cycles over 40%,

the inductance value must be at least LMIN as given by

the following equation:

LMIN = 2.5 • VOUT (μH)

Table 1 depicts the minimum required inductance for

several common output voltages.

Table 1. Buck Minimum Inductance

OUTPUT VOLTAGE

MINIMUM INDUCTANCE

0.6V

1.5μH

0.8V

2.0μH

1.2V

3.0μH

2.0V

5.0μH

2.7V

6.8μH

3.3V

8.3μH

Buck Output Capacitor Selection

A low ESR output capacitor should be utilized at the buck

output in order to minimize voltage ripple. Multi-layer

ceramic capacitors are an excellent choice as they have

low ESR and are available in small footprints. In addition

to controlling the ripple magnitude, the value of the output

capacitor also sets the loop crossover frequency and there-

fore can impact loop stability. There is both a minimum and

maximum capacitance value required to ensure stability of

the loop. If the output capacitance is too small, the loop

cross-over frequency will increase to the point where

switching delay and the high frequency parasitic poles

of the error ampliﬁer will degrade the phase margin. In

addition, the wider bandwidth produced by a small output

capacitor will make the loop more susceptible to switch-

ing noise. At the other extreme, if the output capacitor is

too large, the cross-over frequency can decrease too far

below the compensation zero and also lead to degraded

phase margin. Table 2 provides a guideline for the range

of allowable values of low ESR output capacitors. Larger

value output capacitors can be accommodated provided

they have sufﬁcient ESR to stabilize the loop or by increas-

ing the value of the feedforward capacitor in parallel with

the upper resistor divider resistor.

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