Driving Light Emitting Diodes

The LM3354 can be used to drive LED’s of nearly any color.

The 4.1V option is ideal for driving the White LED’s required

for the backlight of small color displays.

Figure 4 shows the

circuit used to power White LED’s. The LED current is set by

the resistors R

where V

The brightness of the diodes may be controlled using the

shutdown pin. A PWM signal on the shutdown pin may be

used to adjust the brightness by varying the duty cycle. A

signal between 60Hz and 200Hz may be used for best

linearity. In this case the equivalent LED current is approxi-

mately equal to the maximum current multiplied by the duty

cycle. Using frequencies above 200Hz may cause less linear

results as the charge and discharge time of the output

capacitor becomes more significant.

Layout Considerations

Due to the 1 MHz typical switching frequency of the LM3354,

careful board layout is a must. It is important to place the

capacitors as close to the IC as possible and to keep the

traces between the capacitors and the IC short and direct.

Use of a ground plane is recommended.

Figure 5 shows a

typical layout as used in the LM3354 evaluation board.

DS200188-16

FIGURE 5. Typical Layout, Top View (magnification 1.5X)

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