LT3640IUFD 查看數據表（PDF） - Linear Technology
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LT3640IUFD Datasheet PDF : 24 Pages
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LT3640
Applications Information
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:
COUT1
=
150
VOUT •
fS
where fS is in MHz, and COUT 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:
COUT2
=
100
VOUT2 •
fS
In the case where VIN2 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 VIN2 input capacitor value is usually smaller
than the high voltage output capacitor.
Low ESR ceramic capacitors for VIN2 input and high volt-
age channel output could form resonant tank and cause
jitter in certain operating area. Avoid VIN2 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:
ID(AVG) =
IOUT (VIN − VOUT )
VIN
where IOUT 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
VR
PART NUMBER
(V)
On Semiconductor
MBRM120E
20
MBRM140
40
Diodes Inc.
B120
20
B130
30
B220
20
B230
30
DFLS240L
40
International Rectifier
10BQ030
30
20BQ030
30
IAVE VF AT 1A VF AT 2A
(A)
(MV)
(MV)
1
530
595
1
1
500
1
500
2
500
2
500
2
500
1
420
470
2
470
3640f
15
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