LTC3643
14
3643fb
For more information www.linear.com/LTC3643
Input Capacitor (C
IN
) Selection
If the LTC3643 is only used in the boost direction, then
the input filter capacitor is only required to reduce peak
currents drawn from the input source and reduce input
switching noise. A low ESR bypass capacitor with a value
of at least 4.7µF should be located as close to the V
IN
pin
as possible.
However, in applications where buck mode
is engaged,
more bypass capacitance is required. The selection of
C
IN
is determined by the effective series resistance (ESR)
that is required to minimize voltage ripple and load step
transients as well as the amount of bulk capacitance that
is necessary to ensure that the control loop is stable. Loop
stability can be checked by viewing the load transient
response. The Input ripple, ΔV
IN
, is determined by:
ΔV
IN
< ΔI
L
1
8 • f •C
+ESR
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The output ripple is highest at maximum input voltage
since ΔI
L
increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirements. Dry tanta-
lum, special polymer, aluminum electrolytic, and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors are very low ESR but have
lower capacitance density than other types. Tantalum
capacitors
have the highest capacitance density but it is
important to only use types that have been surge tested
for use in switching power supplies. Aluminum 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
and small footprints.
Using Ceramic V
IN
and CAP 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
APPLICATIONS INFORMATION
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the V
CAP
input. 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
CAP
large enough to damage the part.
When choosing the input and output ceramic capacitors,
choose
the X5R and X7R dielectric formulations. These
dielectrics have the best temperature and voltage char-
acteristics of all the ceramics for a given value and size.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. In both the buck mode and boost mode cases,
during a load step, the V
IN
and CAP capacitor respectively
must instantaneously supply the current to support the
load until the feedback loop raises the switch current
enough to support the load.
Typically in buck mode, ~5 cycles are required to respond
to a load step but only in the first cycle does the V
IN
voltage
drop linearly. The V
IN
voltage droop, V
DROOP
, is usually
about 3 times the linear drop of the first
cycle. Thus, a good
place to start with the V
IN
capacitor value is approximately:
C
IN
= 3
IN
f
• V
The boost mode response is typically much slower than
that of the buck and has a dependency on the duty cycle
of the application. Typically, the loop response will be at
least 3 times slower than that of the buck. Thus, more
ceramic capacitance at CAP may be required. However,
in most applications, the LTC3643 will be used to charge
a bulk capacitor in which case placing the
22µF ceramic
capacitor in parallel to the bulk capacitor just to filter out
the square wave current is sufficient.
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency (1MHz) determine the
ripple current:
ΔI
L
=
V
IN
10
6
• L
1–
V
IN
V
CAP(MAX)
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