LT3694/LT3694-1
APPLICATIONS INFORMATION
quality (under damped) tank circuit. If the LT3694 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3694’s
maximum input voltage rating. See Linear Technology
Application Note 88 for more details.
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3694 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
withappliedvoltageandattemperatureextremes.Because
loop stability and transient response depend on the value
Output Capacitor Selection
Theoutputcapacitorfilterstheinductorcurrenttogenerate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilize the LT3694’s
control loop. Because the LT3694 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
of C , this loss may be unacceptable. Use X7R and X5R
OUT
types instead.
Electrolytic capacitors are also an option. The ESRs of
most aluminum electrolytic capacitors are too large to
deliver low output ripple. Surge rated tantalum capacitors
or low ESR, organic, electrolytic capacitors intended for
power supply use are suitable. Choose a capacitor with a
sufficientlylowESRfortherequiredoutputripple.Because
the volume of the capacitor determines its ESR, both the
size and the value will be larger than a ceramic capacitor
that would give similar ripple performance. One benefit
is that the larger capacitance may give better transient
response for large changes in load current. Table 3 lists
several capacitor vendors.
Output ripple can be estimated with the following
equations:
∆IL
VRIPPLE
=
; Ceramic
8 • f •COUT
VRIPPLE = ∆IL • ESR ; Electrolytic
Table 3. Low ESR Surface Mount Capacitors
whereΔI isthepeak-to-peakripplecurrentintheinductor.
L
SERIES
TYPE
MANUFACTURER
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
Ceramic
Taiyo Yuden
www.t-yuden.com
TPM, TPS
Ceramic, Tantalum
AVX
www.avx.com
∆IL
12
IC(RMS)
=
T494, T495,
T510, T520,
T525, T530,
A700
Ceramic, Tantalum,
Tantalum Organic Polymer,
Aluminum Organic Polymer
Kemet
www.kemet.com
Another constraint on the output capacitor is that it must
havegreaterenergystoragethantheinductor;ifthestored
energyintheinductortransferstotheoutput, theresulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
POSCAP,
OS-CON
Tantalum Organic Polymer,
Aluminum Organic Polymer
Sanyo
www.sanyo.com
SP-CAP
Ceramic,
Aluminum Organic Polymer
Panasonic
www.panasonic.com
Ceramic
TDK
www.tdk.com
2
ILIM
COUT > 10 •L •
V
OUT
36941fb
13
Linear [ Linear ]
