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| NS LM34917A降压开关电源方案 | |||||
作者：NS 文章来源：NS 点击数： 更新时间：2008-5-7  |
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National Semiconductor Introduces Industry’s Smallest Step-Down Switching Regulators for Handheld Devices and Automotive Power Adapters The LM34917A Step-Down Switching Regulator features all the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying at least 1.25A to the load. To reduce excessive switch current due to the possibility of a saturating inductor the valley current limit threshold changes with input and output voltages, and the on-time is reduced when current limit is detected. This buck regulator contains an N-Channel Buck Switch, and is available in the 12 pin micro SMD package. The feedback regulation scheme requires no loop compensation, results in fast load transient response, and simplifies circuit implementation. The operating frequency remains constant with line and load variations due to the inverse relationship between the input voltage and the ontime. The valley current limit results in a smooth transition from constant voltage to constant current mode when current limit is detected, reducing the frequency and output voltage, without the use of foldback. Additional features include: VCC under-voltage lock-out, input over-voltage shutdown, thermal shutdown, gate drive under-voltage lock-out, and maximum duty cycle limit. LM34917A主要特性: ■ Functional Input Voltage Range: 8V to 33V ■ Micro SMD package ■ Input Over-Voltage Shutdown at ≊35V ■ Transient Capability to 50V ■ Integrated N-Channel buck switch ■ Valley current limit varies with VIN and VOUT to reduce excessive inductor current ■ On-time is reduced when in current limit ■ Integrated start-up regulator ■ No loop compensation required ■ Ultra-Fast transient response ■ Maximum switching frequency: 2 MHz ■ Operating frequency remains nearly constant with load current and input voltage variations ■ Programmable soft-start ■ Precision internal reference ■ Adjustable output voltage ■ Thermal shutdown 典型应用: ■ High Efficiency Point-Of-Load (POL) Regulator ■ Non-Isolated Buck Regulator ■ Secondary High Voltage Post Regulator  图1. 采用LM34917A的基本降压稳压器电路  图2.LM34917A典型应用和方框图  图3.LM34917A应用案例电路 The LM34917AEVAL evaluation board provides the design engineer with a fully functional buck regulator, employing the constant on-time (COT) operating principle. This evaluation board provides a 5V output over an input range of 8V to 33V. The circuit delivers load currents to 1A, with current limit set at ≊1.3A. The board’s specification are: Input Voltage: 8V to 33V Output Voltage: 5V Maximum load current: 1.0A Minimum load current: 0A Current Limit: ≊1.3A Measured Efficiency: 91.6% (VIN = 8V, IOUT = 400 mA) Nominal Switching Frequency: 1.5 MHz Size: 2.6 in. x 1.6 in. x 0.5 in  图4.采用R5,C9和C10的最小输出波纹电路图  图5.采用Cff 和R4的中等波纹输出电路图  图6.最低成本的电路图 材料清单(BOM): 
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