Datasheet

LT3641
15
3641fa
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3641.
A ceramic input capacitor combined with trace or cable
inductance forms a under damped tank circuit. If the LT3641
circuit is plugged into a live supply, the input voltage can
ring to twice its nominal value, possibly exceeding the
LT3641’s voltage rating. This situation can be easily avoided
(see the Linear Technology Application Note 80).
Output Capacitors and Output Ripple
The output capacitor has two essential functions. In steady
state, it determines the output voltage ripple. In transient,
it stores energy in order to satisfy transient loads and
stabilize the control loop. Ceramic capacitors have low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
OUT1
=
150
V
OUT
•f
S
where f
S
is in MHz, and C
OUT
is the recommended output
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
A good starting value for the low voltage channel output
capacitor is:
C
OUT2
=
100
V
OUT2
•f
S
In the case where V
IN2
is connected to the high voltage
channel output, the high voltage channel output capacitor
can be used as the low voltage channel input capacitor.
The required V
IN2
input capacitor value is usually smaller
than the high voltage output capacitor.
Low ESR ceramic capacitors for V
IN2
input and high volt-
age channel output could form resonant tank and cause
jitter in certain operating area. Avoid V
IN2
input capacitor
if possible.
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. Table 3 lists several capacitor
vendors.
Table 3. Capacitor Vendors
PART SERIES VENDOR
Ceramic, Polymer, Tantalum Panasonic/Sanyo
www.panasonic.com
Ceramic, Tantalum Kemet
www.kemet.com
Ceramic Murata
www.murata.com
Ceramic, Tantalum AVX
www.avxcorp.com
Ceramic Taiyo Yuden
www.taiyo-yuden.com
Catch Diode
The high voltage channel requires an external catch diode
to conduct current 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. Use a 1A or 2A
rated Schottky diode. Peak reverse voltage is equal to
the regulator input voltage. Use a diode with a reverse
voltage rating greater than the input voltage. Table 4 lists
several Schottky diodes and their manufacturers. Diodes
Inc. PDS360 is recommended for high current, H-grade
applications. Diodes Inc. DFLS260 can be used for smaller
circuit footprint in non-H-grade applications.
APPLICATIONS INFORMATION