LTC3414 LINER [Linear Technology], LTC3414 Datasheet - Page 9

• Manufacturer Part Number: LTC3414
• Description: 4A, 4MHz, Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
C
The input capacitance, C
dal wave current at the source of the top MOSFET. To
prevent large voltage transients from occurring, a low ESR
input capacitor sized for the maximum RMS current
should be used. The maximum RMS current is given by:
This formula has a maximum at V
I
monly used for design because even significant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
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. For low input voltage applications, sufficient
bulk input capacitance is needed to minimize transient
effects during output load changes.
The selection of C
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 as described in a later
section. The output ripple, V
The output ripple is highest at maximum input voltage
since I
placed in parallel may be needed to meet the ESR and RMS
current handling requirements. Dry tantalum, special poly-
mer, 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 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
RMS
IN
I
RMS
and C
V
= I
OUT
L
OUT/2
increases with input voltage. Multiple capacitors
OUT
I
OUT MAX
. This simple worst-case condition is com-
Selection
I ESR
(
L
OUT
U
)
V
is determined by the effective series
V
OUT
IN
IN
8
U
, is needed to filter the trapezoi-
fC
1
V
OUT
V
OUT
OUT
IN
, is determined by:
W
– 1
IN
= 2V
OUT
U
, where
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capaci-
tors have excellent low ESR characteristics but can have a
high voltage coefficient and audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming 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
mistaken as loop instability. At worst, a sudden inrush of
current through the long wires can potentially cause a
voltage spike at V
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation:
The resistive divider allows pin V
the output voltage as shown in Figure 2.
IN
V
. At best, this ringing can couple to the output and be
OUT
0 8
.
Figure 2. Setting the Output Voltage
V
1
IN
large enough to damage the part.
R
R
2
1
LTC3414
SGND
V
FB
FB
to sense a fraction of
V
OUT
R2
R1
3414 F02
LTC3414
9
3414f