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| LT3744 大电流同步降压LED驱动方案 |
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| 文章来源： 更新时间：2015/5/6 9:23:00 |
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Linear公司的LT3744是固定频率大电流同步降压DC/DC控制器,LED驱动连续电流高达20A,脉冲电流40A,PWM调光3000：1,模拟调光20：1,输入电压3.3V到36V,主要用在DLP投映仪LED驱动器,智能显示器和大功率LED.本文介绍了LT3744主要特性,框图,多种应用电路,以及演示板2339A主要特性,电路图,材料清和PCB元件布局图.
The LT®3744 is a fixed frequency synchronous step-downDC/DC controller designed to drive a LED load at up to 20Acontinuous or 40A pulsed. The peak current mode controller will maintain ±3% LED current regulation over a wide outputvoltage range, from VEE to VIN. By allowing VEE to floatto negative voltages, several LEDs in series can be drivenfrom a single Li-Ion battery with a simple, single step-downoutput stage. PWM dimming is achieved with the PWM pins.The regulated LED current is set with analog voltages at theCTRL pins. Regulated voltage and overvoltage protectionare set with a voltage divider from the output to the FB pin.
The switching frequency is programmable from 100kHz to1MHz through an external resistor on the RT pin.
Additional features include an accurate external referencevoltage, a control input for thermally derating regulationcurrent, an accurate EN/UVLO pin, an open-drain outputfault flag, OVLO, frequency synchronization, and thermalshutdown.
LT3744主要特性：
Ideal for Driving Up to 40A LEDs
Up to 3000：1 PWM Dimming
20：1 Analog Dimming
±3% Current Regulation Accuracy
±3% Voltage Regulation Accuracy
Unique Inverting Buck-Boost Topology AllowsGrounded Heat Sink to Be Used for RGB LEDs
3.3V to 36V Input Voltage Range
Peak Current Mode with DC LED Current Sensing
Open and Shorted LED Protection and Fault Reporting
Floating LED Driver Allows Single Power Solutionto Drive Multicolor LEDs or Single LED with ThreeDifferent Regulated Currents
Thermally Enhanced 5mm × 6mm QFN 36-LeadPackage
LT3744应用：
LED Driver for DLP Projectors
Heads-Up Displays
High Power LED
图1.LT3744框图
图2.LT3744 DLP投映仪LED驱动器
图3.LT3744三电流级单个LED共阳极色彩混合应用LED驱动器
图4.LT3744三电流级单个LED色彩混合应用LED驱动器
图5.LT3744 98%效率升压模式 3A LED驱动器
演示板2339A
Demonstration circuit 2339A is a high current synchronousstep-down LED driver featuring the LTR3744. The uniquedrive stage used on the LT3744 allows the anodes of threeLEDs to be connected together for better heat sinking. Thisconnection will work in both the step-down configurationand the inverting buck-boost configuration. The pros and cons of each configuration can be found in the data sheet.
This demonstration circuit 2339A is for customers to testthe step-down configuration only. The inverting buck-boostconfiguration is shown in a separate demonstration circuit.
The input of the demo board is up to 36V. The componentsare optimized for the efficiency, thermal and PWM dimmingfor a 12V input. Each of the three outputs is up to5V, 20A with a 6.05V maximum output voltage limit. ThePWM1, PWM2 and PWM3 pins are set to low by default.
A DC or PWM signal is required to connect to at least oneof the PWM pins to enable the circuit. At any giving time,output current only passes through one LED determinedby settings of PWM pins.
The CTRL1, CTRL2 and CTRL3 pins can be adjusted toprovide accurate analog dimming down to 20：1 ratio.
The minimal input voltage for the LT3744 to operate is3.3V. However, to maintain the output current regulationin a step down regulator, the minimum input voltage isdetermined by the LED voltage and the maximum duty cycle.
For a 5V output, the demo board minimum input voltagewill be around 5.5V at room temperature. The load to beused with this demo board is high current LEDs or laserdiodes. Smaller LEDs may not be able to handle the highcurrent, even for a short period of time. It is necessaryto mount the LED load on a proper heat sink. A fan maybecome necessary to avoid exceeding LED’s maximumtemperate rating.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of LinearTechnology Corporation. All other trademarks are the property of their respective owners.
The typical efficiency of the demo board is 93.5% froma 12V input to 5V, 20A load. The lower the input voltage,the higher the efficiency tends to be for a given load. Atoutput power level of 100W, even one percent of efficiencyimprovement is a big advantage in minimizing temperaturerise. If an efficiency measurement is needed in an application,the output voltage must be measured at the outputcapacitors instead of the LED load. This prevents cableloss from being counted as a loss of the board.
The demo circuit DC2399A achieves fast current rise timefrom 0A to 20A in 5μs. To see the real rise time, connectingwires between the LED and the board should be as short as possible to minimize the wire inductance and resistance. Itis recommended to measure the voltage across R32 witha short 50Ω coax cable directly into a BNC connector onthe oscilloscope. The current can be calculated from themeasured voltage. Acurrent probe adds more delays to the rise time so usinga current probe is not recommended unless rise time isnot a concern.
The LT3744 data sheet gives a complete description ofthe part, operation and application information. The datasheet must be read in conjunction with this quick startguide for demo circuit 2339A.
图6.演示板2339A测量建立图
图7.演示板2339A电路
演示板2339A材料清单：
图8.演示板2339A PCB元件布局图(顶层)
图9.演示板2339A PCB元件布局图(底层) |
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