lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 15

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
Table 2. Inductor Vendors
VENDOR
Murata
TDk
Toko
Coilcraft
Sumida
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors,
and choose one to meet cost or space goals. Then use
these equations to check that the LT3695 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continu-
ous. Discontinuous operation occurs when I
than ΔI
Input Capacitor
Bypass the input of the LT3695 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 2.2μF to 10μF ceramic capacitor is adequate to
bypass the LT3695 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
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 LT3695 and to force this very high frequency
L
/2.
URL
www.murata.com
www.componenttdk.com
www.toko.com
www.coilcraft.com
www.sumida.com
PART SERIES
CDRH6D38
SLF10145
MSS7341
MSS1038
SLF7045
CDRH74
LQH55D
D62CB
D63CB
D73C
D75F
CR54
CR75
OUT
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
TYPE
Open
Open
Open
Open
is less
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 LT3695 (see the PCB Layout section
for more information). A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3695. A ceramic input capacitor
combined with trace or cable inductance forms a high-Q
(underdamped) tank circuit. If the LT3695 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT3695’s voltage
rating. For details see Application Note 88.
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
LT3695 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
LT3695’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
where f
output 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 capacitor if the compensation network is also
adjusted to maintain the loop bandwidth. A lower value
of output capacitor can be used to save space and cost
but transient performance will suffer. See the Frequency
Compensation section to choose an appropriate compen-
sation network.
When choosing a capacitor, look carefully through the
data sheet to fi nd out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolytic capacitors can be used for the output capacitor.
C
OUT
SW
=
V
is in MHz, and C
50
OUT
f
SW
OUT
is the recommended
LT3695
15
3695f