LM3503 datasheet(14/20 Pages) NSC

14 / 20 page

Application Information

WHITE LED CURRENT SETTING

The white LED current is controlled by a DC voltage at the

Cntrl pin.

The relationship between the Cntrl pin voltage and Fb pin

voltage can be computed with the following:

Cntrl: Cntrl Pin Voltage. Voltage Range: 0.2V

≤ V

Cntrl

≤

3.5V.

Fb:

Feedback Pin Voltage.

LED CURRENT

The LED current is set using the following equation:

To determine the maximum output current capability of the

device, it is best to estimate using equations on page 16 and

the minimum peak current limit of the device (see electrical

table). Note the current capability will be higher with less

LEDs in the application.

Aside from varying the DC voltage at the Cntrl pin, white LED

dimming can be accomplished through the RC filtering of a

PWM signal. The PWM signal frequency should be at least a

decade greater than the RC filter bandwidth. Figure 3 is how

the LM3503 should be wired for PWM filtered white LED

dimming functionality. When using PWM dimming, it is rec-

ommended to add 1-2ms delay between the Cntrl signal and

the main Enable sginal (En1) to allow time for the output to

discharge. This will prevent potential flickering especially if

the Sub display is compose of 2 LEDs or less.

The equations below are guidelines for choosing the correct

RC filter values in relation to the PWM signal frequency.

Equation #1:

Equation #2:

RC:

RC Filter Bandwidth Cutoff Frequency.

PWM: PWM Signal Frequency.

Chosen Filter Resistor.

Chosen Filter Capacitor.

For example, using the above equations to determine the

proper RC values. Assume the following condition:V

IN= 3.6V,

C=0.01µF and F

PWM = 500Hz, then FRC= 50Hz by relation to

equation 2. By rearranging equation 1 to solve for R; R =

318.5K ohms (standard value, R = 316K).

WHITE LED DIMMING

20128634

FIGURE 3. If V

OUT2 is not used, En2 must be grounded

LM3503 Datasheet(PDF) 14 Page - National Semiconductor (TI)

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