Datasheet

LTC3127
13
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
I
PULSE
= pulsed load current
I
STANDBY
= static load current in standby mode
I
IN(MAX)
= programmed input current limit in amps
T = period of the load pulse
D = load pulse’s duty cycle
V
DROOP
= 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.
The droop due to the capacitor equivalent series resistance
(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:
V
DROOP
= V
DROOP_LOAD
+ V
DROOP_ESR
(V)
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:
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