LT3681
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
COMMENTS
Panasonic
(714) 373-7366
www.panasonic.com
EEF Series
Kemet
Sanyo
(864) 963-6300
(408) 749-9714
www.kemet.com
T494, T495
POSCAP
www.sanyovideo.com
Murata
AVX
(408) 436-1300
www.murata.com
www.avxcorp.com
Ceramic,
Tantalum
Ceramic
TPS Series
Taiyo Yuden
(864) 963-6300
www.taiyo-yuden.com
When choosing a capacitor, look carefully through the
data sheet to find 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
electrolyticcapacitorscanbeusedfortheoutputcapacitor.
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 0.05Ω 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.
frequencies, generating audible noise. Since the LT3681
operates at a lower current limit during Burst Mode op-
eration, the noise is typically very quiet to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
A final precaution regarding ceramic capacitors concerns
themaximuminputvoltageratingoftheLT3681.Aceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3681 circuit is plugged into a live supply, the input volt-
agecanringtotwiceitsnominalvalue, possiblyexceeding
the LT3681’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
Catch Diode
Frequency Compensation
The integral power Schottky catch diode conducts current
only during switch off time. Average forward current in
normal operation can be calculated from:
The LT3681 uses current mode control to regulate the
output.Thissimplifiesloopcompensation.Inparticular,the
LT3681 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
I
= I (V – V )/V
OUT IN OUT IN
D(AVG)
where I
is the output load current.
OUT
Ceramic Capacitors
V pin, as shown in Figure 2. Generally a capacitor (C )
C
C
and a resistor (R ) in series to ground are used. In ad-
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
whenusedwiththeLT3681duetotheirpiezoelectricnature.
When in Burst Mode operation, the LT3681’s switching
frequency depends on the load current, and at very light
loads the LT3681 can excite the ceramic capacitor at audio
C
dition, there may be a lower value capacitor in parallel.
This capacitor (C ) is not part of the loop compensation
F
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
3681f
12
Linear [ Linear ]
