Datasheet

LTC3814-5
16
38145fc
For many designs it is possible to choose a single capacitor
type that satisfi es both the ESR and bulk C requirements
for the design. In certain demanding applications, however,
the ripple voltage can be improved signifi cantly by con-
necting two or more types of capacitors in parallel. For
example, using a low ESR ceramic capacitor can minimize
the ESR step, while an electrolytic capacitor can be used
to supply the required bulk C.
Once the output capacitor ESR and bulk capacitance
have been determined, the overall ripple voltage wave-
form should be verifi ed on a dedicated PC board (see PC
Board Layout Checklist section for more information on
component placement). Lab breadboards generally suffer
from excessive series inductance (due to inter-component
wiring), and these parasitics can make the switching
waveforms look signifi cantly worse than they would be
on a properly designed PC board.
The output capacitor in a boost regulator experiences high
RMS ripple currents, as shown in Figure 8d. The RMS
output capacitor ripple current is:
I
RMS(COUT)
I
O(MAX)
•
V
O
–V
IN(MIN)
V
IN(MIN)
Note that the ripple current ratings from capacitor manu-
facturers are often based on only 2000 hours of life. This
makes it advisable to further derate the capacitor or to
choose a capacitor rated at a higher temperature than
required. Several capacitors may also be placed in parallel
to meet size or height requirements in the design.
Manufacturers such as Nichicon, Nippon Chemi-con
and Sanyo should be considered for high performance
throughhole capacitors. The OS-CON (organic semicon-
ductor dielectric) capacitor available from Sanyo has the
lowest product of ESR and size of any aluminum electrolytic
at a somewhat higher price. An additional ceramic capaci-
tor in parallel with OS-CON capacitors is recommended
to reduce the effect of their lead inductance.
In surface mount applications, multiple capacitors placed
in parallel may be required to meet the ESR, RMS current
handling and load step requirements. Dry tantalum, special
polymer and aluminum electrolytic capacitors are available
in surface mount packages. Special polymer capacitors
offer very low ESR but have lower capacitance density
APPLICATIONS INFORMATION
Figure 8. Switching Waveforms for a Boost Converter
than other types. Tantalum capacitors have the highest
capacitance density but it is important to only use types
that have been surge tested for use in switching power
supplies. Several excellent surge-tested choices are the
AVX TPS and TPSV or the KEMET T510 series. Aluminum
electrolytic capacitors have signifi cantly higher ESR, but
can be used in cost-driven applications providing that
consideration is given to ripple current ratings and long
term reliability. Other capacitor types include Panasonic
SP and Sanyo POSCAPs. In applications with V
OUT
> 30V,
however, choices are limited to aluminum electrolytic and
ceramic capacitors.
V
IN
LD
SW
8a. Circuit Diagram
8b. Inductor and Input Currents
C
OUT
V
OUT
R
L
I
IN
I
L
8c. Switch Current
I
SW
t
ON
8d. Diode and Output Currents
8e. Output Voltage Ripple Waveform
I
O
I
D
V
OUT
(AC)
t
OFF
ΔV
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
ΔV
COUT
38145 F08