The above equation to solve for R

R

(3)

The LM3520 is optimized for 20V at 30 mA over the input

voltage range, for higher output current up to 50mA is

achieveable with a minimum input of 3.6V. If lower V

desired, the output current capability will be higher.

Using V

If V

figure 3 configuration (voltage mode). The LEDs can be

arranged in current mode configuration to control load cur-

rent.

MAIN & SUB ENABLE

The LM3520 has two independent enable pins to control the

main and sub displays. A high on the Main Enable signal will

enable the main display. While a high on the Sub Enable pin

will enable the sub display. The PFM_EN pin must tied to

SUB_EN for enabling the Sub display. Both Main & Sub

enable pins should not be ON at the same time during

normal operation. If for any reason, the main and Sub enable

are high, the main display will enable by default and the sub

display will disable by default. The following truth table sum-

marize the logic state.

TABLE 1.

Main_EN

Sub_EN

Main

Sub

0

0

OFF

OFF

0

1

OFF

ON

1

0

ON

OFF

1

1

ON

OFF

INDUCTOR SELECTION

The inductor used with LM3520 must have a saturation

current greater than the device switch peak current limit.

Choosing inductors with low DCR decreases power losses

and increases efficiency. A 10 µH inductor is optimal for the

applications. If a smaller inductor is used, the larger the

inductor ripple current. Care must be taken to select the

inductor such that the peak current rating of the inductor

accounts for maximum load current for the operating condi-

tion. It is best to select an inductor with a peak current rating

of the maximum switch peak current of the device. The

following equation is useful for determining the inductor

value for a given application condition. Where I

maximum output load current, V

_MIN

= minimum input voltage, V

age, I

quency.

TABLE 2.

Suppliers

Website

Coilcraft

www.coilcraft.com

Cooper Bussmann

www.cooperET.com

Murata

www.murata.com

DIODE SELECTION

To maintain high efficiency, the average current rating of the

schottky diode should be larger than the peak inductor cur-

rent. Schottky diodes with a low forward drop and fast

switching speeds are ideal for increasing efficiency in por-

table applications. Choose a reverse breakdown of the

schottky diode larger than the output voltage. Some recom-

mended diodes are MBR0530T1 from ON semiconductor

and CMMSHI-40 from Central Semiconductor.

CAPACITOR SELECTION

Choose low ESR capacitors for the output to minimize output

voltage ripple. Ceramic capacitors such as X5R and X7R are

recommended for use as input and output filiters. These

capacitors provide an ideal balance between small size,

cost, reliability and performance. Do not use Y5V ceramic

capacitors as they have poor dielectric performance over

temperature and poor voltage characteristic for a given

value. For most applications,a1µF ceramic output capaci-

20128805

FIGURE 4.

20128801

FIGURE 5.

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