LTC3633A/LTC3633A-1
14
3633a1fb
For more information www.linear.com/LTC3633A
APPLICATIONS INFORMATION
Several capacitors may also be paralleled to meet size or
height requirements in the design. For low input voltage
applications, sufficient bulk input capacitance is needed
to minimize transient effects during output load changes.
Even though the LTC3633A design includes an overvoltage
protection circuit, care must always be taken to ensure
input voltage transients do not pose an overvoltage hazard
to the part.
The selection of C
OUT
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 output ripple, ∆V
OUT
, is
approximated by:
∆V
OUT
< ∆I
L
ESR +
1
8 • f • C
OUT
When using low-ESR ceramic capacitors, it is more useful
to choose the output capacitor value to fulfill a charge stor-
age requirement. During a load step, the output capacitor
must instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is
dependent on the compensation and the
output capacitor size. Typically, 3 to 4 cycles are required
to respond to a load step, but only in the first cycle does
the output drop linearly. The output droop, V
DROOP
, is
usually about 3 times the linear drop of the first cycle.
Thus, a good place to start is with the output capacitor
size of approximately:
C
OUT
≈
OUT
f • V
DROOP
Though this equation provides a good approximation, more
capacitance may be required depending on the duty cycle
and load step requirements. The actual V
DROOP
should be
verified by applying a load step to the output.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are available
in small case sizes. Their high ripple current, high voltage
rating and low ESR make them ideal for switching regula
-
tor applications. However, due to the self-resonant and
high-Q
characteristics of
some types of ceramic capaci-
tors, care must be taken when these capacitors are used
a
t
the input. 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
V
IN
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
IN
large enough to damage the part. For
a more detailed discussion, refer to Application Note 88.
When choosing the input and output ceramic capacitors,
choose the X5R and X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac
-
teristics of all the ceramics for a given value and size.
INT
V
CC
Regulator Bypass Capacitor
An internal low dropout (LDO) regulator draws power
from the V
IN1
input and produces the 3.3V supply that
powers the internal bias circuitry and drives the gate of
the internal MOSFET switches. The INTV
CC
pin connects
to the output of this regulator and must have a minimum
of 1µF ceramic decoupling capacitance to ground. The
decoupling capacitor should have low impedance electrical
connections to the INTV
CC
and PGND pins to provide the
transient currents required by the LTC3633A. This supply
is intended only to supply additional DC load currents as
desired and not intended to regulate large transient or AC
behavior, as this may impact LTC3633A operation.
Boost Capacitor
The LTC3633A uses a “bootstrap” circuit to create a voltage
rail above the applied input voltage V
IN
. Specifically, a boost
capacitor, C
BOOST
, is charged to a voltage approximately
equal to INTV
CC
each time the bottom power MOSFET is
turned on. The charge on this capacitor is then used to
supply the required transient current during the remainder
of the switching cycle. When the top MOSFET is turned on,
the BOOST pin voltage will be equal to approximately V
IN
+ 3.3V. For most applications, a 0.1µF ceramic capacitor
closely connected between the BOOST and SW pins will
provide adequate performance.
