Linear Technology LT3480 Manual page 11

APPLICATIONS INFORMATION
Table 1. Inductor Vendors
VENDOR URL
Murata www.murata.com
TDK www.componenttdk.com
Toko www.toko.com
Sumida www.sumida.com
Of course, such a simple design guide will not always result
in the optimum inductor for your application. A larger value
inductor provides a slightly higher maximum load current
and will reduce the output voltage ripple. If your load is lower
than 2A, then you can decrease the value of the inductor
and operate with higher ripple current. This allows you to
use a physically smaller inductor, or one with a lower DCR
resulting in higher effi ciency. There are several graphs in
the Typical Performance Characteristics section of this data
sheet that show the maximum load current as a function of
input voltage and inductor value for several popular output
voltages. Low inductance may result in discontinuous mode
operation, which is okay but further reduces maximum
load current. For details of maximum output current and
discontinuous mode operation, see Linear Technology Ap-
plication Note 44. Finally, for duty cycles greater than 50%
(V /V > 0.5), there is a minimum inductance required
OUT IN
to avoid subharmonic oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3480 circuit with a ceramic capaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3480 and will easily handle the ripple current.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is signifi cant inductance due to
long wires or cables, additional bulk capacitance may be
PART SERIES TYPE
LQH55D Open
SLF7045 Shielded
SLF10145 Shielded
D62CB Shielded
D63CB Shielded
D75C Shielded
D75F Open
CR54 Open
CDRH74 Shielded
CDRH6D38 Shielded
CR75 Open
necessary. This can be provided with a lower performance
electrolytic capacitor.
Step-down regulators draw current from the input supply in
pulses with very fast rise and fall times. The input capaci-
tor is required to reduce the resulting voltage ripple at the
LT3480 and to force this very high frequency switching
current into a tight local loop, minimizing EMI. A 4.7μF
capacitor is capable of this task, but only if it is placed close
to the LT3480 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
LT3480. A ceramic input capacitor combined with trace or
cable inductance forms a high quality (under damped) tank
circuit. If the LT3480 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT3480's voltage rating. This situation is easily
avoided (see the Hot Plugging Safety section).
For space sensitive applications, a 2.2μF ceramic capacitor can
be used for local bypassing of the LT3480 input. However, the
lower input capacitance will result in increased input current
ripple and input voltage ripple, and may couple noise into
other circuitry. Also, the increased voltage ripple will raise
the minimum operating voltage of the LT3480 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated by the
LT3480 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3480's control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
100
=
C
OUT
V f
OUT SW
where f is in MHz, and C
SW
capacitance in μF . 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
LT3480
is the recommended output
OUT
11
3480fb