Datasheet

LT3685
12
3685fb
where f
SW
is in MHz, and C
OUT
is the recommended 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 compensation 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.
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 0.05
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 2 lists several capacitor vendors.
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
I
D(AVG)
= I
OUT
(V
IN
– V
OUT
)/V
IN
where I
OUT
is the output load current. The only reason to
consider a diode with a larger current rating than necessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3685 will keep the
switch off when V
IN
> 38V which allows the use of a 40V
rated Schottky even when V
IN
ranges up to 60V. Table 3
lists several Schottky diodes and their manufacturers.
Table 3. Diode Vendors
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 1A
(mV)
V
F
AT 2A
(mV)
On Semicnductor
MBRM120E
MBRM140
20
40
1
1
530
550
595
Diodes Inc.
B220
B230
DFLS240L
20
30
40
2
2
2
500
500
500
International Rectifi er
10BQ030
20BQ030
30
30
1
2
420 470
470
Ceramic Capacitors
A precaution regarding ceramic capacitors concerns the
maximum input voltage rating of the LT3685. A ceramic
input capacitor combined with trace or cable inductance
VENDOR PHONE URL PART SERIES COMMANDS
Panasonic [phone] www.panasonic.com Ceramic,
Polymer,
Tantalum
EEF Series
Kemet [phone] www.kemet.com Ceramic,
Tantalum T494, T495
Sanyo [phone] www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata [phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum TPS Series
Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic
Table 2. Capacitor Vendors
APPLICATIONS INFORMATION