lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 14

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3686A
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 fi lters 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:
Table 3.
14
VENDOR
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
C
OUT
=
V
OUT
145
• f
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[phone]
[phone]
[phone]
PHONE
www.taiyo-yuden.com
www.sanyovideo.com
www.panasonic.com
www.avxcorp.com
www.murata.com
www.kemet.com
URL
where C
X7R type and keep in mind that a ceramic capacitor biased
with V
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
benefi t (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 specifi ed 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
OUT
OUT
=
will have less than its nominal capacitance. This
V
is in μF and frequency is in MHz. Use an X5R or
OUT
is in μF and frequency is in MHz. However,
83
PART SERIES
• f
Tantalum
Tantalum
Tantalum
Tantalum
Ceramic
Polymer
Ceramic
Ceramic
Polymer
Ceramic
Ceramic
Ceramic
COMMENTS
T494, T495
TPS Series
EEF Series
POSCAP
3686af