LTC3521



3521f

applicaTions inForMaTion

The basic LTC3521 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 volt-

age requirements of each particular application. 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 lead to lower output voltage ripple. For a fixed

DC resistance, a larger value inductor will yield higher

efficiency by lowering the peak current closer to the av-

erage. However, a larger inductor within the same family

will generally have a greater series resistance, thereby

offsetting this efficiency advantage.

Givenadesiredpeak-to-peakcurrentripple,

inductance can be calculated via the following expression,

where f represents the switching frequency in MHz:

L

f I

V

V

V

µH

L

OUT

OUT

IN

=







1

1

Δ

–

A reasonable choice for ripple current is

which represents 40% of the maximum 600mA 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 following equation:

Table 1 depicts the recommended inductance for several

common output voltages.

Table 1. Buck Recommended Inductance

OUTPUT VOLTAGE

MINIMUM

INDUCTANCE

MAXIMUM

INDUCTANCE

0.6V

1.5μH

2.2μH

1.2V

2.2μH

4.7μH

1.8V

3.3μH

6.8μH

2.5V

4.7μH

8.2μH

Buck Output Capacitor Selection

A low ESR output capacitor should be utilized at the buck

output in order to minimize voltage ripple. Multilayer ce-

ramic 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 can,

therefore, impact loop stability. There is both a minimum

andmaximumcapacitancevaluerequiredtoensurestabil-

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

loop crossover frequency will increase to the point where

the 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 crossover frequency can decrease too

far below the compensation zero and lead to a 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.

Table 2. Buck Output Capacitor Range

0.6V

15μF

300μF

0.8V

15μF

230μF

1.2V

10μF

150μF

1.8V

10μF

90μF

2.7V

10μF

70μF

3.3V

6.8μF

50μF