LM3488MM datasheet(19/24 Pages) NSC

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Typical Applications (Continued)

where D

MAX is the maximum duty cycle.

The turn-on and turn-off transitions of a MOSFET require

times of tens of nano-seconds. C

RSS and Qg are needed to

estimate the large instantaneous power loss that occurs

during these transitions.

The amount of gate current required to turn the MOSFET on

can be calculated using the formula:

G =Qg.FS

The required gate drive power to turn the MOSFET on is

equal to the switching frequency times the energy required

to deliver the charge to bring the gate charge voltage to V

(see electrical characteristics and typical performance char-

acteristics for the drive voltage specification).

Drive =FS.Qg.VDR

INPUT CAPACITOR SELECTION

Due to the presence of an inductor at the input of a boost

converter, the input current waveform is continuous and

triangular, as shown in Figure 11. The inductor ensures that

the input capacitor sees fairly low ripple currents. However,

as the input capacitor gets smaller, the input ripple goes up.

The rms current in the input capacitor is given by:

The input capacitor should be capable of handling the rms

current. Although the input capacitor is not as critical in a

boost application, low values can cause impedance interac-

tions. Therefore a good quality capacitor should be chosen

in the range of 100µF to 200µF. If a value lower than 100µF

is used, then problems with impedance interactions or

switching noise can affect the LM3478. To improve perfor-

mance, especially with V

IN below 8 volts, it is recommended

to use a 20

Ω resistor at the input to provide a RC filter. The

resistor is placed in series with the V

IN pin with only a bypass

capacitor attached to the V

IN pin directly (see Figure 13). A

0.1µF or 1µF ceramic capacitor is necessary in this configu-

ration. The bulk input capacitor and inductor will connect on

the other side of the resistor with the input power supply.

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:

Where

and D, the duty cycle is equal to (V

OUT −VIN)/VOUT.

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 Induc-

tance Converter) applications. An example of SEPIC using

LM3488 is shown in Figure 14. As shown in Figure 14, 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 inher-

ent 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.

10138893

FIGURE 13. Reducing IC Input Noise

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