LTC3522_15 Datasheet, PDF (13/20 Pages) Linear Technology – Synchronous 400mA Buck-Boost and 200mA Buck Converters

LTC3522
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, 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 = 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 L_MIN as given by
the following equation:
L_MIN = 2.5 • V_OUT (μH)
Table 1 depicts the minimum required inductance for
several common output voltages.
Table 1. Buck Minimum Inductance
OUTPUT VOLTAGE
MINIMUM INDUCTANCE
0.6V
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.
3522fa
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