LM3488

LM3488

LM3488-Q1

SNVS089M – JULY 2000 – REVISED MARCH 2013

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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)

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)

the diode.

22

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