LTC1871 Linear Technology, LTC1871 Datasheet - Page 17

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
For the bulk C component, which also contributes 1% to
the total ripple:
For many designs it is possible to choose a single capaci-
tor type that satisfies both the ESR and bulk C require-
ments for the design. In certain demanding applications,
however, the ripple voltage can be improved significantly
by connecting 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 waveform
should be verified on a dedicated PC board (see Board
Layout section for more information on component place-
ment). Lab breadboards generally suffer from excessive
series inductance (due to inter-component wiring), and
these parasitics can make the switching waveforms look
significantly 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 12. The RMS
output capacitor ripple current is:
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, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest product of
ESR and size of any aluminum electrolytic, at a somewhat
higher price.
C
OUT
I
RMS COUT
0 01
. •
(
I
O MAX
(
V
)
O
U
)
•
I
f
O MAX
(
U
)
•
V
O
W
V
–
IN MIN
V
(
IN MIN
(
)
)
U
In surface mount applications, multiple capacitors may
have to be placed in parallel in order to meet the ESR or
RMS current handling requirements of the application.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount packages. In the case of
tantalum, it is critical that the capacitors have been surge
tested for use in switching power supplies. An excellent
choice is AVX TPS series of surface mount tantalum. Also,
ceramic capacitors are now available with extremely low
ESR, ESL and high ripple current ratings.
Boost Converter: Input Capacitor Selection
The input capacitor of a boost converter is less critical than
the output capacitor, due to the fact that the inductor is in
series with the input and the input current waveform is
Figure 12. Switching Waveforms for a Boost Converter
12e. Output Voltage Ripple Waveform
V
(AC)
I
OUT
SW
12b. Inductor and Input Currents
I
I
12d. Diode and Output Currents
D
L
V
IN
t
ON
12a. Circuit Diagram
12c. Switch Current
L
SW
t
V
OFF
ESR
D
V
COUT
C
OUT
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
V
OUT
I
IN
R
L
I
O
LTC1871
17