LTC3631 Datasheet, PDF (12/22 Pages) Linear Technology – High Effi ciency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
APPLICATIONS INFORMATION
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based
only on 2000 hours of life which makes it advisable to
further derate the capacitor, or choose a capacitor rated
at a higher temperature than required. Several capacitors
may also be paralleled to meet size or height requirements
in the design.
The output capacitor, C_OUT, filters the inductor’s ripple
current and stores energy to satisfy the load current
when the LTC3631 is in sleep. The output voltage ripple
during a burst cycle is dominated by the output capacitor
equivalent series resistance (ESR) and can be estimated
by the following equation:
V_OUT
160
< ΔV_OUT
≤ I_PEAK
• ESR
where the lower limit of V_OUT/160 is due to the 5mV
feedback comparator hysteresis.
The value of the output capacitor must be large enough
to accept the energy stored in the inductor without a large
change in output voltage. Setting this voltage step equal to
1% of the output voltage, the output capacitor must be:
C_OUT
>
50
•L
•

_IPEAK
 ^V_OUT
²


Typically, a capacitor that satisfies the ESR requirement is
adequate to filter the inductor ripple. To avoid overheating,
the output capacitor must also be sized to handle the ripple
current generated by the inductor. The worst-case ripple
current in the output capacitor is given by I_RMS = I_PEAK/2.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic,
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important only to use types that have been surge
tested for use in switching power supplies. Aluminum
12
electrolytic capacitors have significantly higher ESR but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have high voltage coefficient and
audible piezoelectric effects. The high quality factor (Q)
of ceramic capacitors in series with trace inductance can
also lead to significant ringing.
Using Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now be-
coming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the input,
V_IN. At best, this ringing can couple to the output and be
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V_IN large enough to damage the part.
For applications with inductive source impedance, such
as a long wire, a series RC network may be required in
parallel with C_IN to dampen the ringing of the input supply.
Figure 5 shows this circuit and the typical values required
to dampen the ringing.
L_IN
R = ^LIN
C_IN
4 • C_IN
LTC3631
V_IN
3631 F05
C_IN
Figure 5. Series RC to Reduce V_IN Ringing
Output Voltage Programming
For the adjustable version, the output voltage is set by
an external resistive divider according to the following
equation:
V_OUT
=
0.8V
•


¹⁺

R1^
R2


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