LTC3127EDD LINER [Linear Technology], LTC3127EDD Datasheet - Page 13

• Manufacturer Part Number: LTC3127EDD
• Description: 1A Buck-Boost DC/DC Converter with Programmable Input Current Limit
• Manufacturer: LINER [Linear Technology]

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:
where
I
I
I
T = period of the load pulse
D = load pulse’s duty cycle
V
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.
The droop due to the capacitor equivalent series resistance
(ESR) is:
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.
V
=
PULSE
STANDBY
IN(MAX)
DROOP
DROOP ESR




V




I
I
PULSE
DROOP LOAD
PULSE
= pulsed load current
= amount the output falls out of regulation in volts
_
= programmed input current limit in amps
= static load current in standby mode
−
_
−






V
V
IN
IN
=
•
•
I
IN MAX
V
I
IN MAX
V
OUT
(
OUT
(
C
)
OUT
)
•
•
h
h
−
−
I
STAND
I
STAN
D D BY
B B Y














• •
•
D T
ESR V
( )
( )
V
The total output voltage droop is given by:
High capacitance values and low ESR can lead to instability
in typical internally compensated buck-boost convert-
ers. Using proportional compensation, the LTC3127 is
stable with low ESR output capacitor values greater than
1000µF .
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 10µF 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:
Table 2. Capacitor Vendor Information
SUPPLIER
Vishay
AVX
Cooper Bussmann
CAP-XX
Panasonic
V
I
C
•
LOAD MAX



 
DROOP
OUT MIN
I
PULSE
V
D T
DROOP
(
(
•
= V
−
)
)
= =



DROOP_LOAD
=
( )
V
F
V
IN
PHONE
402-563-6866
803-448-9411
516-998-4100
843-267-0720
800-394-2112
IN
•
•
D V
I
IN MAX
V
I
IN MAX
•
OUT
(
(
OUT
+ V
)
)
•
•
DROOP_ESR
h
h
−
WEB SITE
www.vishay.com
www.avxcorp.com
www.cooperbussmann.com
www.cap-xx.com
www.panasonic.com
( )
I
STANDBY
A
LTC3127
(V)








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