LT3514
17
3514fa
For more information www.linear.com/LT3514
APPLICATIONS INFORMATION
group of two V
IN
pins. The ceramic bypass capacitors
should be located as close to the V
IN
pins as possible.
See the sample layout shown in the PCB Layout section.
All four V
IN
pins should be tied together on the board and
bypassing with a low performance electrolytic capacitor
is recommended especially if the input power source has
high impedance, or there is significant inductance due to
long wires or cables.
Step-down regulators draw current from the input sup
-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3514 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3514 and the catch diode; see
the PCB Layout section. A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3514. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3514 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3514’s
voltage rating. This situation can be easily avoided by add
-
ing an electrolytic capacitor in parallel with the ceramic
input capacitors. See Application Note 88.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor
, it filters the square wave generated by
the L
T3514 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
L
T3514’
s control loop.
Ceramic capacitors have very low equivalent series re
-
sistance (ESR) and provide the best ripple performance.
A good value is:
C
OUT
= 33/(V
OUT
• f
SW
) for Channels 1, 4
C
OUT
= 132/(V
OUT
• f
SW
) for Channel 3
where C
OUT
is in µF and f
SW
is in MHz. Use X5R or X7R
types 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, if the compensation network is also
adjusted to maintain the loop bandwidth.
A lower value of output capacitor can be used, but tran
-
sient performance will suffer. Also, a lower value output
capacitor may result in increased sensitivity to noise which
can be alleviated by adding a 100pF phase lead capacitor
from FB to V
OUT
.
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.
Table 3. Capacitor Vendors
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series
Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP
Murata (404) 436-1300 www.murata.com Ceramic
AVX www.avxcorp.com Ceramic, Tantalum TPS Series
Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic