Datasheet

LT8616
16
8616fa
For more information www.linear.com/LT8616
APPLICATIONS INFORMATION
switching current into a tight local loop, minimizing EMI.
A 2.2μF capacitor is capable of this task, but only if it is
placed close to the LT8616 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT8616.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir
-
cuit. If the LT8616 circuit is plugged into a live supply, the
input
voltage can ring to twice its nominal value, possibly
exceeding the LT8616’s voltage rating. This situation is
easily avoided (see Linear Technology 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 LT8616 to produce the DC output. In this role it deter
-
mines the
output voltage ripple, thus, low impedance at
the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT8616’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 output
ripple and good transient response. Transient performance
can be improved with a higher value output capacitor and
the addition of a feed forward 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 capacitance
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 LT8616 due to their piezoelectric nature.
When in Burst Mode operation, the LT8616’s switching
frequency depends on the load current, and at very light
loads the LT8616 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT8616
operates at a lower current limit during Burst Mode op-
eration, 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.
Enable Pin
The LT8616 is in shutdown when both EN/UV pins are low
and active when either pin is high. The rising threshold of
the EN/UV comparator is 1.03V, with 50mV of hysteresis.
The EN/UV pins 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/UV programs
the LT8616 to operate 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 cur-
rent 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
Table 3. Ceramic Capacitor Manufacturers
MANUFACTURER WEB
Taiyo Yuden www.t-yuden.com
AVX www.avxcorp.com
Murata www.murata.com
TDK www.tdk.com
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