Freescale MC34845六路LED背景光驱动方案

Freescale 公司的MC34845是集成了电路的六路LED背景光驱动器,工作电压从5.0V到21V,能驱动多达6串,每串16个LED.6串LED的电流误差最大为±2%. MC34845的输出电压高达60V,集成了2.0A的FET,具有OTP,OCP,UVLO故障检测和LED短路/开路保护,LED电流在3.0mA和30mA之间编程,主要用在笔记本电脑,上网本,手提DVD播放器,小屏幕电视,工厂业和医疗显示器等.本文介绍了MC34845的主要特性,方框图,典型应用电路以及评估板的主要特性,详细电路图和所用材料清单.

Low Cost 6 Channel LED Backlight Driver with Integrated Power Supply

The 34845 series represents high efficiency LED drivers for use in backlighting LCD displays from 10” to 17”+. Operating from supplies of 5.0 V to 21 V, the 34845 series is capable of driving up to 16 LEDs in series in 6 separate strings. The LED current tolerance in the 6 strings is within ±2% maximum and is set using a resistor to GND.

PWM dimming is performed by applying a PWM input signal to the PWM pin which modulates the LED channels directly. An Enable Pin (EN) provides for low power standby. Alternatively, a single wire scheme selects power down when PWM is connected to the Wake Pin and held low.

The integrated boost converter uses dynamic headroom control to automatically set the output voltage. There are two device versions for boost frequency, the 34845 is 600 kHz and the 34845A is 1.2 MHz. External compensation allows the use of different inductor/ capacitor combinations.

The 34845 includes fault protection modes for LED short and open, over temperature, over current and over voltage errors. It features an internally fixed OVP value of 60 V (typical) which protects the device in the event of a failure in the externally programmed OVP. The OVP level can be set by using an external resistor divider.

MC34845主要特性:

Input voltage of 5.0 to 21 V

Boost output voltage up to 60 V

2.0 A integrated boost FET

Fixed boost frequency - 600 kHz or 1.2 MHz (34845A version)

OTP, OCP, UVLO fault detection

LED short/open protection

Programmable LED current between 3.0 mA and 30 mA

24-Ld 4x4x0.65 mm WQFN Package

MC34845典型应用:

PC Notebooks

Netbooks

Picture Frames

Portable DVD Players

Small Screen Televisions

Industrial Displays

Medical Displays



图1.MC34845内部方框图



图2.MC34845简化应用方框图



图3.MC34845单线控制典型应用电路: fS = 600 KHz(VIN = 9.0V, ILED/路= 20mA/路, 12 LEDs/路, OVP = 45V, VPWM = 3.3V)



图4.MC34845单线控制典型应用电路: fS = 1.2MHz(VIN = 9.0V, ILED = 20mA/路, 12 LED/路, OVP = 45V, VPWM = 3.3V)



图5.KIT34845EPEVME 评估板外形图

This EVB shows the functionality of MC34845 set it up under specific operation parameters.

MC34845 which is a high efficiency, LED driver for use in backlighting LCD displays from 10” to 17”+ can operate in this demo board from a supply of 5V, the device is capable of driving up to 96 LEDs in 6 parallel strings. The current through these 6 channels is matched to within ±2%.

For evaluation purposes this Demo Board includes a simple MCU that allows control of the light dimming of the LEDs as well as the PWM frequency from 200Hz to 26.5kHz.

EVB 工作参数:

Input Voltage (Vin) = 9V +/- 10%

Expected Output Voltage (Vout) = around 30V

LED Load = 10 LEDs x 6 Channels

Duty Cycle = All range (minimum pulse of 400ns)

Peak Current on all channels = all range up to 20mA

OVP = 35V (For 10Leds)

Boost Frequency = 600KHz

EVB 主要特性:

MCU that allows dimming control at different duty cycles and PWM frequency (200Hz to 26.5kHz)

Four Layer Board

Low Noise Design

Top Layer Placement

Connector for the 6 LED channels

Terminal Blocks for Input and output Voltage

Jumper signal configurations and resistors array for signal adjusting

Test points measurements



图6.EVB 电路图(1) - MC34845 部分



图7.EVB 电路图(2) - MC34845 部分



图8.EVB 电路图(3) - MC34845 MCU部分

EVB 材料清单(BOM)



In some PWM dimming frequencies, it could be perceivable some audible noise from the input and output capacitors of the boost converter. These micro-mechanical vibrations are an effect of the subharmonics produced from the boost switching frequency.

In order to minimize this effect plus slightly improve the efficiency of the system, it is recommended to replace the following components: