LT3694 Datasheet, PDF (13/28 Pages) Linear Technology – 36V, 2.6A Monolithic Buck Regulator With Dual LDO

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.
Output Capacitor Selection
The output capacitor filters the inductor current to generate
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.
Output ripple can be estimated with the following
equations:
V_RIPPLE
=
8•
ΔI_L
f • C_OUT
; Ceramic
V_RIPPLE = ΔI_L • ESR ; Electrolytic
where ΔI_L is the peak-to-peak ripple current in the inductor.
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:
I_C(RMS) =
ΔI_L
12
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
C_OUT
> 10
•L
•
⎛
⎝⎜
I_LIM
V_OUT
⎞
⎠⎟
2
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
with applied voltage and at temperature extremes. Because
loop stability and transient response depend on the value
of C_OUT, this loss may be unacceptable. Use X7R and X5R
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
sufficiently low ESR for the required output ripple. 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.
Table 3. Low ESR Surface Mount Capacitors
SERIES
TYPE
MANUFACTURER
Ceramic
Taiyo Yuden
www.t-yuden.com
TPM, TPS
T494, T495,
T510, T520,
T525, T530,
A700
POSCAP,
OS-CON
SP-CAP
Ceramic, Tantalum
Ceramic, Tantalum,
Tantalum Organic Polymer,
Aluminum Organic Polymer
Tantalum Organic Polymer,
Aluminum Organic Polymer
Ceramic,
Aluminum Organic Polymer
Ceramic
AVX
www.avx.com
Kemet
www.kemet.com
Sanyo
www.sanyo.com
Panasonic
www.panasonic.com
TDK
www.tdk.com
36941fa
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