R1

LM3502

Sw

En1

En2

Fb

Cntrl

AGND

PGND

PWM Signal

GND

Unconnected

Floating

R1

LM3502

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SNVS339B – SEPTEMBER 2005 – REVISED MAY 2013

APPLICATION INFORMATION

WHITE LED CURRENT SETTING

The LED current is set using the following equation:

where

•

•

•

R1: Current Setting Resistor.

(1)

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 susceptibility to piezoelectric induced stresses be used for the particular

applications that require minimal or no audible noise interference.

Figure 30.

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