LTC3520
15
3520fa
The basic LTC3520 application circuit is shown as the
Typical Application on the front page of this datasheet.
The external component selection is determined by the
desired output voltages, output currents, and ripple volt-
age requirements of each particular application. However,
basic guidelines and considerations for the design process
are provided in this section.
Operating Frequency Selection
The operating frequency choice is a tradeoff between ef-
fi ciency and application area. Higher operating frequencies
allow the use of smaller inductors and smaller input and
output capacitors, thereby reducing application area. How-
ever, higher operating frequencies also increase switching
losses and therefore decrease effi ciency. Typical effi ciency
versus switching frequency curves for both converters are
given in the Typical Performance Characteristics section
of this datasheet.
Buck Inductor Selection
The choice of buck inductor value infl uences both the ef-
fi ciency 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
fi xed DC resistance, a larger value inductor will yield higher
effi ciency by lowering the peak current and reducing core
losses. However, a larger inductor within the same family
will generally have a greater series resistance, thereby
offsetting this effi ciency advantage.
Given a desired peak to peak current ripple, ΔI
L
, the required
inductor can be calculated via the following expression,
where f represents the switching frequency in MHz:
L
fI
V
V
V
H
L
OUT
OUT
IN
=−
⎛
⎝
⎜
⎞
⎠
⎟
1
1
Δ
μ
A reasonable choice for ripple current is ΔI
L
= 240mA 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 effi ciency
during operation. To optimize effi ciency, an inductor with
low series resistance should be utilized.
APPLICATIONS INFORMATION
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 effi ciency will be reduced.
In addition, there is a minimum inductor value required
to maintain stability of the current loop (given the fi xed
internal slope compensation). Specifi cally, if the buck
converter is going to be utilized at duty cycles over 40%,
the inductance value must be at least L
MIN
as given by
the following equation:
L
MIN
= 1.4 • V
OUT
μH
Table 1 depicts the minimum required inductance for
several common output voltages.
Table 1. Buck Minimum Inductance
OUTPUT VOLTAGE MINIMUM INDUCTANCE
0.8V 1.1μH
1.2V 1.7μH
2V 2.8μH
2.7V 3.8μH
3.3V 4.5μH
Buck Output Capacitor Selection
A low ESR output capacitor should be utilized at the buck
output in order to minimize voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
low ESR and are available in small footprints. In addi-
tion to controlling the ripple magnitude, the value of the
output capacitor also sets the loop crossover frequency
and therefore 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 crossover frequency will increase to
the point where switching delay and the high frequency
parasitic poles of the error amplifi er will degrade the
phase margin. In addition, the wider bandwidth produced
by a small output capacitor will make the loop more sus-
ceptible to switching noise. At the other extreme, if the
output capacitor is too large, the crossover 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
