LTC3872 Linear Technology, LTC3872 Datasheet - Page 11

• Manufacturer Part Number: LTC3872
• Description: No RSENSE Current Mode Boost DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
where:
For the bulk C component, which also contributes 1% to
the total ripple:
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 waveform
should be verifi ed 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
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 6. 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
I
I
C
RMS COUT
IN PEAK
OUT
(
(
≥
)
0 01
=
. •
)
I
⎛
⎜
⎝
O MAX
≈
1
(
+
I
O MAX
V
(
χ
2
O
U
)
⎞
⎟
⎠
•
•
f
)
1
I
•
O MAX
–
U
(
D
V
MAX
O
V
)
–
IN MIN
V
(
IN MIN
W
(
)
)
U
capacitor available from Sanyo has the lowest product of
ESR and size of any aluminum electrolytic, at a somewhat
higher price.
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.
Figure 5. Switching Waveforms for a Boost Converter
5e. Output Voltage Ripple Waveform
V
(AC)
I
OUT
SW
I
I
D
L
5b. Inductor and Input Currents
5d. Diode and Output Currents
V
IN
t
ON
5a. Circuit Diagram
5c. Switch Current
L
SW
ΔV
t
OFF
ESR
ΔV
D
COUT
C
OUT
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
V
OUT
I
IN
R
LTC3872
L
I
O
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