LTC3872-1_15 Datasheet, PDF (12/20 Pages) Linear Technology – No RSENSE Current Mode Boost DC/DC Controller

LTC3872-1
Applications Information
where:
I_IN(PEAK)=
₁₊
χ
2
•
I_O(MAX)
1– D_MAX
For the bulk C component, which also contributes 1% to
the total ripple:
C_OUT
≥
I_O(MAX)
0.01• V_O •
f
For many designs it is possible to choose a single capacitor
type that satisfies both the ESR and bulk C requirements
for the design. In certain demanding applications, however,
the ripple voltage can be improved significantly 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 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 7. The RMS
output capacitor ripple current is:
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.
L
D
V_IN
SW
V_OUT
C_OUT R_L
6a. Circuit Diagram
I_L
I_IN
6b. Inductor and Input Currents
I_SW
t_ON
6c. Switch Current
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.
I_D
t_OFF
I_O
6d. Diode and Output Currents
V_OUT
(AC)
V_COUT
V_ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
6e. Output Voltage Ripple Waveform
Figure 6. Switching Waveforms for a Boost Converter
38721f
12
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