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耗尽型模拟开关:无电源也能高性能连接

由  Shawn Barden 发布  - 07-03-2019 

耗尽型MOSFET开关,一度不那么受欢迎,且常被视为典型的增强型FET的同属,却在最近几年中越来越受欢迎。安森美半导体投入该技术,开发出越来越多的耗尽型模拟开关系列。这些开关越来越多地用于很好地解决工程问题。此博客将使读者更好地了解这些实用的器件的能力,并介绍方案示例。

简介

增强型FET用于当今绝大多数电子产品中,工作模式基于简单的概念。考虑到增强型NFET采用共源极结构(图1,左)-当门极与源在相同的电势,漏源极之间的沟道电阻很高,我们认为晶体管是‘关断’的。这些FET需要一个正的门极到源电压,以导通沟道,并在漏源极之间传导。当没有完全饱和时,这些FET的沟道电阻会有很大的变化。这可能导致模拟信号的问题,需要在整个信号幅度范围的低失真。此外,当采用增强型FET的模拟开关失去电源时,其状态是不确定的;它不会很好地传导,很可能也不会很好地隔离信号。

1:增强型对比耗尽型MOSFET

耗尽型FET与增强型FET互为补足:对于采用相同结构的耗尽型NFET(图1,右),沟道电阻较低,并且沟道被认为是“导通”的。因此,在默认的无电源状态下,耗尽FET是传导的,且由于它们的设计,其沟道电阻是线性的,这使得它们在整个信号幅度范围具有极低的失真。

基于耗尽型FET的模拟开关通常有控制电路,以便在器件上电时启用或禁用开关路径。该控制电路使用电荷泵产生隔离开关路径所需的电压。因此,禁用(隔离)开关路径会耗电。对于信号隔离时间相对较短的应用来说,这通常不是问题。否则,选择一个电荷泵耗电低的器件是很重要的。

虽然许多硅供应商吹嘘他们的耗尽型方案为默认的“导通”,但其器件的电阻并不是完全线性的,导致信号失真或电阻率不一致。安森美半导体开发了专有的耗尽型FET,配合专利的设计技术,以在无电源时失真较低。 

解决降噪耳机电池电量耗尽的问题

当降噪耳机首次被广泛使用时,使用它们的好处显而易见。人们终于可以忍受那些漫长而嘈杂的飞机旅行了;在背景噪音的嗡嗡声中,听着那首最受欢迎的古典音乐,从普通的体验变成了沉浸式的体验。但是,这些耳机需要电池才能消除噪音,当电池耗尽时,耳机就没用了。有些设计试图克服这一点,通常是通过机械旁路开关,但这些方案总是需要用户亲自动手。

考虑FSA553:当供电时,这负摆幅、双通道SPST耗尽型模拟开关与降噪的DSP并联,支持设计人员通过耳机中降噪的DSP传送立体声音频,同时电池进行充电。当电池电压降到对DSP太低时,对DSP和FSA553的电压供应就会禁用。在这种状态下,音频信号绕过DSP并通过FSA553路由到耳机,从而创建改进的用户体验,其中音频聆听体验在电池放电时自动继续。FSA553的0.4欧姆(典型值)低信道电阻和-104 dBV (非A加权)超低总谐波失真加噪声(THD+N)的提供低损耗和实际无失真的立体音频旁路。

采用USB Type C移动设备附件可省电

考虑通过USB Type C将移动设备连接到受电附件的应用。一旦附件通过VCONN连接并通电,附件中仍有电流流过接地的Ra电阻(图2)。对于5V的VCONN和1千欧姆的Ra,提供5mA DC电流,这无需从移动设备获取。然而,对于使用微控制器或类似器件的附件,附件器件上的单通道SPST耗尽型模拟开关如FSA515与Ra电阻器串联可提供能力以在完成USB Type C检测之后隔离Ra电阻器接地路径。

2Type-C附件在检测后有电流流过Ra电阻

通过在附件控制器上使用GPIO和一些固件编码以在成功检测后对FSA515的VDD引脚供电(图3),在连接时,所增加的电流消耗可从5mA减小到仅约30uA,节省了近25mW的功率。此外,FSA515的超小占位减少了所增加的方案面积到2,包括分立器件。

3Type-C附件在检测后隔离Ra电阻

机会

耗尽型模拟开关具有多种用途,并特别适用于在没有电源的情况下传导高保真信号。这使得它们常用作旁路开关,可在需要时用作在电源下隔离的低功率默认路径,或者作为在讲究省电应用中降低损耗的设计灵活的方案。随着设计转向更低的损耗和增加的复杂性,耗尽型模拟开关成为在低功率产品中路由高保真模拟信号的越来越有用的工具。

了解更多关于我们的耗尽型音频开关

标签:IoT, Consumer, Portable and Wireless
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