lt3640 Linear Technology Corporation, lt3640 Datasheet - Page 15

• Manufacturer Part Number: lt3640
• Description: Dual Monolithic Buck Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
ring to twice its nominal value, possibly exceeding the
LT3640’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:
where f
capacitance in μF . Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
The control loop is usually easier to be stabilized by a
bigger value of output capacitor. This equation applies
for both channels.
In the case where V
channel output, the high voltage channel output capaci-
tor can be combined with the low voltage channel input
capacitor. The required V
smaller than the high voltage output capacitor. If the bigger
output capacitor can be placed close to the V
input capacitor is not necessary for the V
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.
C
OUT
S
is in MHz, and C
=
V
OUT
100
•
f
S
IN2
is connected to the high voltage
IN2
OUT
input capacitor value is usually
is the recommended output
IN2
pin.
IN2
pin, an
Table 3. Capacitor Vendors
PART SERIES
Ceramic, Polymer, Tantalum
Ceramic, Tantalum
Ceramic, Polymer, Tantalum
Ceramic
Ceramic, Tantalum
Ceramic
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:
where I
consider a diode with a larger current rating than neces-
sary for nominal operation is for the worst-case condition
of overloaded output. The diode current will then increase
to the typical peak switch current. 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.
Table 4. Diode Vendors
PART NUMBER
On Semiconductor
MBRM120E
MBRM140
Diodes Inc.
B120
B130
B220
B230
DFLS240L
International Rectifi er
10BQ030
20BQ030
I
D AVG
(
OUT
)
=
is the output load current. The only reason to
I
OUT
(
V
IN
V
IN
(V)
V
20
40
20
30
20
30
40
30
30
Panasonic
www.panasonic.com
Kemet
www.kemet.com
Sanyo
www.sanyovideo.com
Murata
www.murata.com
AVX
www.avxcorp.com
Taiyo Yuden
www.taiyo-yuden.com
−
R
V
OUT
)
I
(A)
AVE
1
1
1
1
2
2
2
1
2
VENDOR
V
F
(MV)
530
500
500
420
AT 1A
LT3640
V
15
F
(MV)
595
500
500
500
470
470
AT 2A
3640p