參數(shù)資料
型號: LTC3127EDD#PBF
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: SWITCHING REGULATOR, PDSO10
封裝: 3 X 3 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10
文件頁數(shù): 5/20頁
文件大小: 366K
代理商: LTC3127EDD#PBF
LTC3127
3127f
applicaTions inForMaTion
Output and Input Capacitor Selection
When selecting output capacitors for large pulsed loads,
the magnitude and duration of the pulse current, together
with the droop voltage specification, determine the choice
of the output capacitor. Both the ESR of the capacitor and
the charge stored in the capacitor each cycle contribute
to the output voltage droop. The droop due to the charge
is approximately:
V
I
V
I
V
I
DROOP LOAD
PULSE
IN IN MAX
OUT
STAN
_
(
)
=
h
DDBY
OUT
D T
C
V
( )
where
IPULSE = pulsed load current
ISTANDBY = static load current in standby mode
IIN(MAX) = programmed input current limit in amps
T = period of the load pulse
D = load pulse’s duty cycle
VDROOP = amount the output falls out of regulation in volts
h = the efficiency of the converter at the input current
limit point
The preceding equation is a worst-case approximation
assuming all the pulsing energy comes from the output
capacitor.
Thedroopduetothecapacitorequivalentseriesresistance
(ESR) is:
V
I
V
I
V
I
DROOP ESR
PULSE
IN IN MAX
OUT
STAND
_
(
)
=
h
BBY
ESR V
( )
Low ESR and high capacitance are critical to main-
taining low output droop. Table 2 and the Typical Applica-
tions schematics show a list of several reservoir capacitor
manufacturers.
The total output voltage droop is given by:
VDROOP = VDROOP_LOAD + VDROOP_ESR (V)
HighcapacitancevaluesandlowESRcanleadtoinstability
in typical internally compensated buck-boost convert-
ers. Using proportional compensation, the LTC3127 is
stable with low ESR output capacitor values greater than
1000F.
Multilayer ceramic capacitors are an excellent choice for
input decoupling of the step-up converter as they have
extremely low ESR and are available in small footprints.
Input capacitors should be located as close as possible to
the device. While a 10F input capacitor is sufficient for
most applications, larger values may be used to improve
input decoupling without limitation. Consult the manufac-
turers directly for detailed information on their selection
of ceramic capacitors. Although ceramic capacitors are
recommended, low ESR tantalum capacitors may be used
as well.
When using a large capacitance to help with pulsed load
applications, the maximum load for a given duty cycle, and
the minimum capacitance can be calculated by:
I
V
I
D V
A
C
LOAD MAX
IN
IN MAX
OUT
OUT MIN
(
)
(
)
(
)
( )
=
h
==
I
V
I
V
I
PULSE
IN
IN MAX
OUT
STANDBY
(
) h
D T
V
F
DROOP
( )
Table 2. Capacitor Vendor Information
SUPPLIER
PHONE
WEB SITE
Vishay
402-563-6866
www.vishay.com
AVX
803-448-9411
www.avxcorp.com
Cooper Bussmann
516-998-4100
www.cooperbussmann.com
CAP-XX
843-267-0720
www.cap-xx.com
Panasonic
800-394-2112
www.panasonic.com
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