Datasheet

LT8614
17
Rev. E
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APPLICATIONS INFORMATION
A third, larger ceramic capacitor of 2.2µF or larger should
be placed close to V
IN1
or V
IN2
. See layout section for
more detail. X7R or X5R capacitors are recommended for
best performance across temperature and input voltage
variations.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir
-
cuit. If the LT8614 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8614’s voltage rating. This situation is
easily avoided (see Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT8614 to produce the DC output. In this role it
determines the output ripple, thus low impedance at the
switching frequency is important. The second function is
to store energy in order to satisfy transient loads and sta
-
bilize the LT8614’s control loop. Ceramic capacitors have
very low equivalent series resistance (ESR) and provide
the best ripple performance. For good starting values, see
the Typical Applications section.
Use X5R or X7R types. This choice will provide low out-
put ripple and good transient response. Transient perfor-
mance can be improved with a higher value output capaci-
tor and the addition of a feedforward capacitor placed
between V
OUT
and FB. Increasing the output capacitance
will also decrease the output voltage ripple. A lower value
of output capacitor can be used to save space and cost
but transient performance will suffer and may cause loop
instability. See the Typical Applications in this data sheet
for suggested capacitor values.
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capaci
-
tance under the relevant operating conditions of voltage
bias and temperature. A physically larger capacitor or one
with a higher voltage rating may be required.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8614 due to their piezoelectric
nature. When in Burst Mode operation, the LT8614’s
switching frequency depends on the load current, and
at very light loads the LT8614 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8614 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet to a
casual ear. If this is unacceptable, use a high performance
tantalum or electrolytic capacitor at the output. Low noise
ceramic capacitors are also available.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8614. As
previously mentioned, a ceramic input capacitor com-
bined with trace or cable inductance forms a high qual-
ity (underdamped) tank circuit. If the LT8614 circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8614’s
rating. This situation is easily avoided (see Application
Note 88).
Enable Pin
The LT8614 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV of hysteresis. The EN pin
can be tied to V
IN
if the shutdown feature is not used, or
tied to a logic level if shutdown control is required.
Adding a resistor divider from V
IN
to EN programs the
LT8614 to regulate the output only when V
IN
is above a
desired voltage (see the Block Diagram). Typically, this
threshold, V
IN(EN)
, is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch low under low source voltage conditions. The V
IN(EN)
threshold prevents the regulator from operating at source
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