Datasheet

LT3680
12
Rev C
For more information www.analog.com
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 0.05
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 neces
-
sary 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. Table 3 lists several Schottky diodes and their
manufacturers.
Table 3. Diode Vendors
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 3A
(mV)
On Semiconductor
MBRA340 40 3 500
Diodes Inc.
PDS340
B340A
B340LA
40
40
40
3
3
3
500
500
450
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3680 due to their piezoelectric
nature. When in Burst Mode operation, the LT3680’s
switching frequency depends on the load current, and at
very light loads the LT3680 can excite the ceramic capaci-
tor at audio frequencies, generating audible noise. Since
the LT3680 operates at a lower current limit during Burst
Mode operation, the noise is nearly silent to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Table 2. Capacitor Vendors
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
APPLICATIONS INFORMATION
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