Freescale Semiconductor S08 manual LED Driver Design Procedures, RGB LED Chip

LED Driver Design Procedures

Vin = 12V

DC-to-DC

Boost

Converter

(MC34063)

Vout = 30V

8 LEDs 8 LEDs 8 LEDs

VLED x 8

R-Channel

PWM

Driver R

G-Channel

PWM

Driver G

Driver B VREF

B-Channel

ILED = 50mA

VDROP

PWM

Rs

VRS

Figure 2-3. DC-to-DC Boost Converter and Linear LED Driver

Eight pieces of 3-in-1 RGB LED chips connected in series are used to form the multi-color light source. The LED chips are arranged in 2 x 4 format and each RGB LED string is driven by a separated constant current source. The average current through each RGB LED is controlled by an individual PWM signal generated from MCU. The final output color is determined by the mix of light emitted by RGB LEDs that are almost in linear relationship with PWM pulse width. An optical diffuser film should be placed on top of the display window for color mixing and brightness uniformity enhancement.

2.11 LED Driver Design Procedures

This section presents guidelines for selecting external components for DC-to-DC boost converter and linear drivers.

2.11.1 RGB LED Chip

The system is designed to drive eight pieces of RGB LED chips connected in a series. Assume the LED current for each color is 50mA and forward voltage is 2.3V for red LED and 3.3V for green and blue LEDs.

LED Lighting Control using the MC9S08AW60, Rev. 1

Freescale Semiconductor 19