Adding Three Power MOSFETs Lets a Single Step-Down Converter Drive RGB LEDs in Projectors

Description

Introduction

This application note presents an RGB LED driver reference design for a low-power projector. This high-current LED driver is based on a single MAX16821 device that drives 10A to a set of RGB LEDs in step-down mode with less than 1µs turn-on/turn-off times. Only one color LED is driven at a time, with RGB colors sharing the PWM period in required proportions.

LED Driver Specifications:

• Input supply voltage: 10V to 15V
• LED drive current: 10A
• LED forward voltage: 4.5V to 6V
• LED current rise/fall times: < 1µs
• LED current ripple: 10% peak to peak, max

Inputs:

• V_IN (J4): supply voltage input
• PWMR, PWMB, PWMG (J8 pins 1, 3, and 4): RGB PWM input signals. Amplitude should be 3.3V to 5V. As output can rise/fall within 1µs, any PWM period higher than 2µs can be used. Only one of the above three signals should be high at a time.
• PWMN (J8 pin 4): digital NOR of PWMR, PWMG, and PWMB. PWMN will be high only when all three PWM signals are low.
• ON/OFF (J1): leave open or drive +5V to enable the driver. Connect to GND for disabling the board.

Outputs:

• LEDR, LEDG, LEDB (J5, J6, and J7): 10A RGB LED outputs. Connect LED+ to pins 3, 4, and 5; connect LED- to pins 6, 7, and 8.
• OUTV (J2): provides a signal proportional to the LED current. The voltage at OUTV will be 135 times the voltage across R12||R16.
• V_{IN_OUT} (J3): input supply-voltage output for connecting to other boards. Pins 1 and 2 are V_IN+; pins 3 and 4 are GND.