Datasheet

LT3681
12
3681f
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 0.05Ω 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.
Catch Diode
The integral power Schottky 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.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3681 due to their piezoelectric nature.
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
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 fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3681. A ceramic
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-
age can ring to twice its nominal value, possibly exceeding
the LT3681’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
Frequency Compensation
The LT3681 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, 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
V
C
pin, as shown in Figure 2. Generally a capacitor (C
C
)
and a resistor (R
C
) in series to ground are used. In ad-
dition, there may be a lower value capacitor in parallel.
This capacitor (C
F
) is not part of the loop compensation
but is used to fi lter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
VENDOR PHONE URL PART SERIES COMMENTS
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