Datasheet

V
IN
V
IN
R
IN
C
IN
C
BYPASS
LM3488
LM3488
LM3488-Q1
SNVS089M –JULY 2000–REVISED MARCH 2013 www.ti.com
Figure 35. Reducing IC Input Noise
OUTPUT CAPACITOR SELECTION
The output capacitor in a boost converter provides all the output current when the inductor is charging. As a
result it sees very large ripple currents. The output capacitor should be capable of handling the maximum rms
current. The rms current in the output capacitor is:
(31)
Where
(32)
and D, the duty cycle is equal to (V
OUT
− V
IN
)/V
OUT
.
The ESR and ESL of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL
at the output for high efficiency and low ripple voltage. Surface Mount tantalums, surface mount polymer
electrolytic and polymer tantalum, Sanyo- OSCON, or multi-layer ceramic capacitors are recommended at the
output.
DESIGNING SEPIC USING LM3488
Since the LM3488 controls a low-side N-Channel MOSFET, it can also be used in SEPIC (Single Ended Primary
Inductance Converter) applications. An example of SEPIC using LM3488 is shown in Figure 36. As shown in
Figure 36, the output voltage can be higher or lower than the input voltage. The SEPIC uses two inductors to
step-up or step-down the input voltage. The inductors L1 and L2 can be two discrete inductors or two windings of
a coupled transformer since equal voltages are applied across the inductor throughout the switching cycle. Using
two discrete inductors allows use of catalog magnetics, as opposed to a custom transformer. The input ripple can
be reduced along with size by using the coupled windings of transformer for L1 and L2.
Due to the presence of the inductor L1 at the input, the SEPIC inherits all the benefits of a boost converter. One
main advantage of SEPIC over boost converter is the inherent input to output isolation. The capacitor CS isolates
the input from the output and provides protection against shorted or malfunctioning load. Hence, the A SEPIC is
useful for replacing boost circuits when true shutdown is required. This means that the output voltage falls to 0V
when the switch is turned off. In a boost converter, the output can only fall to the input voltage minus a diode
drop.
The duty cycle of a SEPIC is given by:
(33)
In the above equation, V
Q
is the on-state voltage of the MOSFET, Q, and V
DIODE
is the forward voltage drop of
the diode.
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