lt3685emse-trpbf Linear Technology Corporation, lt3685emse-trpbf Datasheet - Page 11

• Manufacturer Part Number: lt3685emse-trpbf
• Description: 38v, 2a, 2.4mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Table 1. Inductor Vendors
Of course, such a simple design guide will not always re-
sult 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 opera-
tion, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (V
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3685 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 LT3685 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
VENDOR
Murata
TDK
Toko
Sumida
URL
www.murata.com
www.componenttdk.com
www.toko.com
www.sumida.com
PART SERIES
LQH55D
SLF7045
SLF10145
D62CB
D63CB
D75C
D75F
CR54
CDRH74
CDRH6D38
CR75
OUT
/V
IN
> 0.5), there
TYPE
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Shielded
Shielded
Open
high impedance, or there is signifi cant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a lower performance
electrolytic capacitor.
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 LT3685 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 LT3685 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 LT3685. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3685 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3685’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 2.2μF ceramic capaci-
tor can be used for local bypassing of the LT3685 input.
However, the lower input capacitance will result in in-
creased 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 LT3685 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
LT3685 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
LT3685’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
OUT
=
V
OUT SW
100
f
LT3685
11
3685f