LTC3521 Datasheet, PDF (13/20 Pages) Linear Technology – Wide VIN, 1A Buck-Boost DC/DC and Dual 600mA Buck DC/DC Converters

LTC3521
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.
Given a desired peak-to-peak current ripple, ΔI_L, the required
inductance can be calculated via the following expression,
where f represents the switching frequency in MHz:
( ) ^L
=
1
fΔI_L
V_OUT

^1–
V_OUT 
V_{IN }
µH
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 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 L_MIN, as given by
the following equation:
L_MIN = 2.5 • V_OUT (µH)
Table 1 depicts the recommended inductance for several
common output voltages.
Table 1. Buck Recommended Inductance
OUTPUT VOLTAGE
MINIMUM
INDUCTANCE
0.6V
1.5μH
1.2V
2.2μH
1.8V
3.3μH
2.5V
4.7μH
MAXIMUM
INDUCTANCE
2.2μH
4.7μH
6.8μH
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
and maximum capacitance value required to ensure stabil-
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
V_OUT
0.6V
C_MIN
15μF
0.8V
15μF
1.2V
10μF
1.8V
10μF
2.7V
10μF
3.3V
6.8μF
C_MAX
300μF
230μF
150μF
90μF
70μF
50μF
3521f
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