Datasheet

LT3688
16
3688f
APPLICATIONS INFORMATION
additional bulk capacitance may be necessary. This can be
provided with a lower performance electrolytic capacitor.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT3688 input and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 2.2F capacitor is capable of this task, but only if it is
placed close to the LT3688 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3688. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3688 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3688’s
voltage rating. See Linear Technology’s Application Note
88 for details.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated by the
LT3688 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3688’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
OUT
=
50
V
OUT
•f
SW
where f
SW
is in MHz and C
OUT
is the recommended output
capacitance in F. Use X5R or X7R types, which will
provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor, but a phase lead capacitor across the feedback
resistor R1 may be required to get the full benefi t (see the
Compensation section).
High performance 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 0.1
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 3 lists
several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR PART SERIES COMMENTS
Panasonic Ceramic
Polymer
Tantalum
EEEF Series
Kemet Ceramic
Tantalum T494, T495
Sanyo Ceramic
Polymer
Tantalum
POSCAP
Murata Ceramic
AVX Ceramic
Tantalum TPS Series
Taiyo Yuden Ceramic
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 limit. 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 4 lists several Schottky diodes and their
manufacturers.