LT3686 LINER [Linear Technology], LT3686 Datasheet - Page 14

• Manufacturer Part Number: LT3686
• Manufacturer: LINER [Linear Technology]

ApplicAtions inForMAtion
LT3686
a high quality (underdamped) tank circuit. If the LT3686
circuit is plugged into a live supply, the input voltage can
ring to twice its nominal value, possibly exceeding the
LT3686’s voltage rating. This situation is easily avoided;
see the Hot-Plugging Safely section.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3686 to produce the DC output. In this role it deter-
mines the output ripple so low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3686’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good value is:
where C
X7R type and keep in mind that a ceramic capacitor biased
with V
choice will provide low output ripple and good transient
Table 3
VENDOR
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden

C
OUT
OUT
OUT
=
will have less than its nominal capacitance. This
V
is in μF and frequency is in MHz. Use an X5R or
OUT
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www.taiyo-yuden.com
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response. Transient performance can be improved with
a high value capacitor, but a phase lead capacitor across
the feedback resistor, R1, may be required to get the full
benefit (see the Compensation section).
For small size, the output capacitor can be chosen ac-
cording to:
where C
using an output capacitor this small results in an increased
loop crossover frequency and increased sensitivity to
noise, requiring careful PCB design.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specified by the supplier and should be 0.1Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 3 lists
several capacitor vendors.
C
OUT
OUT
=
V
OUT
is in μF and frequency is in MHz. However,
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PART SERIES
Tantalum
Tantalum
Tantalum
Tantalum
Polymer
Polymer
Ceramic
Ceramic
Ceramic
Ceramic
Ceramic
Ceramic
COMMENTS
T494, T495
TPS Series
EEF Series
POSCAP
3686fa