Datasheet

LT3686A
14
3686afa
necessary. This can be provided with a low performance
electrolytic capacitor. Step-down regulators draw current
from the input supply in pulses with very fast rise and fall
times. The input capacitor is required to reduce the result-
ing voltage ripple at the LT3686A and to force this very
high frequency 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 LT3686A and the catch
diode (see the PCB Layout section). A second precaution
regarding the ceramic input capacitor concerns the maxi-
mum input voltage rating of the LT3686A. A ceramic input
capacitor combined with trace or cable inductance forms
a high quality (underdamped) tank circuit. If the LT3686A
circuit is plugged into a live supply, the input voltage can
ring to twice its nominal value, possibly exceeding the
LT3686A’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 LT3686A 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
LT3686A’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good value is:
C
OUT
=
145
V
OUT
• f
where C
OUT
is in μF and frequency is in MHz. Use an X5R or
X7R type and keep in mind that a ceramic capacitor biased
with V
OUT
will have less than its nominal capacitance. This
choice will provide low output ripple and good transient
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:
C
OUT
=
83
V
OUT
• f
where C
OUT
is in μF and frequency is in MHz. However,
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.
ApplicAtions inForMAtion
Table 3.
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic [phone] www.panasonic.com Ceramic
Polymer
Tantalum
EEF Series
Kemet [phone] www.kemet.com Ceramic
Tantalum
T494, T495
Sanyo [phone] www.sanyovideo.com Ceramic
Polymer
Tantalum
POSCAP
Murata [phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic
Tantalum
TPS Series
Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic