LM3502 datasheet(13/18 Pages) NSC

13 / 18 page

Application Information

WHITE LED CURRENT SETTING

The LED current is set using the following equation:

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LED: White LED Current.

Fb:

Feedback Pin Voltage. V

Fb = 0.25V, Typical.

R1:

Currrent Setting Resistor.

WHITE LED DIMMING

For dimming the white LED string with a pulse-width-

modulated (PWM) signal on the Cntrl pin, care must taken to

balance the tradeoffs between audible noise and white LED

brightness control. For best PWM duty cycle vs. white LED

current linearity, the PWM frequency should be between

200Hz and 500Hz. Other PWM frequencies can be used, but

the linearity over input voltage and duty cycle variation will

not be as good as what the 200Hz to 500Hz PWM frequency

spectrum provides. To minimize audible noise interference, it

is recommended that a output capacitor with minimal sus-

ceptibility to piezoelectric induced stresses be used for the

particular applications that require minimal or no audible

noise interference.

If V

OUT2 is not used , En2 must be grounded

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FIGURE 5.

LM3502 Datasheet(PDF) 13 Page - National Semiconductor (TI)