LM3520 datasheet(13/14 Pages) NSC

13 / 14 page

Application Information (Continued)

tor is sufficient for the main-display. A minimum of 4.7µF

output capacitor is recommended for V

SUB output. Larger

output capacitor can be used to reduce ripple voltage. The

LM3520 has a maximum OVP of 24.2V, a 25V minimum

rated capacitor voltage is recommended for the application

to ensure proper biasing.

Local bypassing for the input is needed on LM3520. Multi-

layer ceramic capacitors with low ESR are a good choice for

this as well. A 10 µF capacitor is sufficient for most applica-

tions. Using larger capacitance decreases input voltage

ripple on the input. Extra attention is required if smaller case

size capacitor is used in the application. Smaller case size

capacitor typically has less capacitance for a given bias

voltage as compared to a larger case size capacitor with the

same bias voltage. Please contact the capacitor manufac-

turer for detail information regarding capacitance verses

case size. Table 3 lists several capacitor suppliers.

TABLE 3.

Suppliers

Website

TDK

www.tdk.com

Suppliers

Website

AVX

www.avxcorp.com

Murata

www.murata.com

LAYOUT CONSIDERATIONS

As for any high frequency switcher, it is important to place

the external components as close as possible to the IC to

maximize device performance. Below are some layout rec-

ommendations: 1) Place input filter and output filter capaci-

tors close to the IC to minimize copper trace resistance

which will directly effect the overall ripple voltage. 2) Place

the feedback network resistors in the Main and Sub display

close to the IC. 3) Route noise sensitive trace away from

noisy power components. 4) Connect the ground pins and

filter capacitors together via a ground plane to prevent

switching current circulating through the ground plane. Simi-

larly the ground connection for the feedback network should

tie directly to GND plane. If no ground plan is available, the

ground connections should tie directly to the device GND

pin. Additional layout consideration regarding the LLP pack-

age can be found in Application AN1187

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