LTC3626
15
3626fa
For more information www.linear.com/LTC3626
applicaTions inForMaTion
from capacitor manufacturers are often based on only
2000 hours of life which makes it advisable to further de-
rate 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. For low input voltage
applications, sufficient bulk input capacitance is needed
to minimize transient effects during output load changes.
Even though the LTC3626 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.
Additional input voltage filtering to the SV
IN
pin (signal
V
IN
) is made possible by adding optional components R
IN
and C
IN2
as shown in the Functional Diagram. Generally,
the inherent supply rejection of the LTC3626 makes the
addition of these components unnecessary, however, users
with large, asynchronous noise on the input supply may
choose to populate these components. Typical values for
R
IN
and C
IN2
are 5Ω and 0.33µF respectively.
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 value, lower cost ceramic capacitors are now
available in small case sizes. Their high voltage rating
and low ESR make them ideal for switching regulator ap
-
plications. However, due to the self-resonant and high-Q
characteristics of some types of ceramic capacitors,
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
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 or X7R dielectric formulations. These
dielectrics provide the best temperature and voltage
characteristics for a given value and size.
INTV
CC
Regulator
An internal low dropout (LDO) regulator produces a
3.3V supply voltage used to power much of the internal
LTC3626 circuitry including the power MOSFET gate
drivers. The INTV
CC
pin connects to the output of this
regulator and should have a minimum 1μF of 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 LTC3626. The user may connect a maximum load
current of 5mA to this pin but must take into account the
increased power dissipation and die temperature that
