LT3689 LINER [Linear Technology], LT3689 Datasheet - Page 17

• Manufacturer Part Number: LT3689
• Description: 700mA Step-Down Regulator with Power-On Reset and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3689 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3689’s voltage rating. For a complete discussion,
see Linear Technology’s Application Note 88.
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
LT3689 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
LT3689’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
where f
put capacitance in μF . 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
Frequency 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 0.1Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance because the
C
OUT
SW
=
V
is in MHz, and C
OUT SW
50
f
OUT
is the recommended out-
capacitor must be large to achieve low ESR. Table 3 lists
several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
Catch Diode
The catch diode conducts current only during switch-off
time. Average forward current in normal operation can
be calculated from:
where I
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. The overvoltage protection feature in the LT3689
will keep the switch off when V
allows the use of a 40V rated Schottky even when V
ranges up to 60V. Table 4 lists several Schottky diodes
and their manufacturers.
I
D AVG
(
OUT
)
=
is the output load current. The only reason to
I
OUT
(
V
V
IN
[phone]
[phone]
[phone]
[phone]
[phone]
IN
LT3689/LT3689-5
−
PHONE
V
OUT
)
IN
> 38V (typical), which
www.taiyo-yuden.com
www.sanyovideo.com
www.panasonic.com
www.avxcorp.com
www.murata.com
www.kemet.com
URL
17
3689fa
IN