参数资料
型号: LT3640EUFD#PBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: 3.4 A DUAL SWITCHING CONTROLLER, 2350 kHz SWITCHING FREQ-MAX, PQCC28
封装: 5 X 4 MM, LEAD FREE, PLASTIC, MO-220, QFN-28
文件页数: 7/24页
文件大小: 506K
代理商: LT3640EUFD#PBF
LT3640
3640f
Output Capacitors and Output Ripple
Theoutputcapacitorhastwoessentialfunctions.Insteady
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 fS is in MHz, and COUT is the recommended output
capacitance in F. 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
S
2
100
=
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
electrolyticcapacitorscanbeusedfortheoutputcapacitor.
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
toconductcurrentduringswitchoff-time.Averageforward
current in normal operation can be calculated from:
I
V
D AVG
OUT
IN
OUT
IN
(
)
(
)
=
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
PART NUMBER
VR
(V)
IAVE
(A)
VF AT 1A
(MV)
VF AT 2A
(MV)
On Semiconductor
MBRM120E
MBRM140
20
40
1
530
595
Diodes Inc.
B120
B130
B220
B230
DFLS240L
20
30
20
30
40
1
2
500
International Rectifier
10BQ030
20BQ030
30
1
2
420
470
applicaTions inForMaTion
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