Datasheet

LT3640
15
3640f
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
V f
OUT
OUT S
1
150
=
•
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
V f
OUT
OUT S
2
2
100
=
•
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 find 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
www.panasonic.com
Ceramic, Tantalum Kemet
www.kemet.com
Ceramic, Polymer, Tantalum Sanyo
www.sanyovideo.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
I V V
V
D AVG
OUT IN OUT
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.
Table 4. Diode Vendors
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 1A
(MV)
V
F
AT 2A
(MV)
On Semiconductor
MBRM120E
MBRM140
20
40
1
1
530 595
Diodes Inc.
B120
B130
B220
B230
DFLS240L
20
30
20
30
40
1
1
2
2
2
500
500
500
500
500
International Rectifier
10BQ030
20BQ030
30
30
1
2
420 470
470
applicaTions inForMaTion