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ON Semiconductor and the ON Semiconductor logo are trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries. ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent-Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein. ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not designed, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended
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© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.0.3 • 5/14/13
AN-6022 — 使用 FSUSB30/31 以符合 USB 2.0 故障条件要求
简介
设计人员使用飞兆 FSUSB30/FSUSB31开关产品将增强的 高速 USB 功能添加到其系统中，
他们通常会问，“如何才能确保完全符合 USB 标准？” 许多设计者选择 FSUSB30/31
开关的初衷是因为其卓越的信号处理特性、低功耗及小 封装选择。 除了维持出色的信号完整性之外，
FSUSB30/31 开关必须
能够防止 Vbus 短路，从而完全符合 USB 2.0 规范。
以下内容摘自 USB 2.0 规范第 7.1.1 节：
短路耐受性
USB 收发器需要耐受 D+ 和/或 D- 到 VBUS、GND 以及其它数据线或连接器电缆屏蔽层的持续短路，时间 至少为 24 小时且不会降低性能。
建议将收发器设计为可无限期耐受此类短路故障。
在短路的情况下且发送和接收时间各占 50%（所有支持的速度）时，不得损坏器件。
发送阶段包含一个对称信号，在驱动高电平和驱动 低电平之间切换。这要求必须满足 Vbus
的最大值（5.25V）。
FSUSB30/31 的 VCC 最大值限制为 4.6V。
乍一看，人们可能会认为这种开关并不符合 USB 要求。
对于没有飞兆的断电保护电路的标准摸拟开关而言，这 个结论是正确的，但 FSUSB30/31 是首个可提供此保护 的 USB 开关。 如果配置得当， FSUSB30/31
可完全符合 USB 耐短路要求。
在两种截然不同的情况下，USB 系统会经历这种故 障状况。
第一种情况是开关断电，第二种情况是芯片带电。
图 1.有信号泄漏的典型模拟开关应用 控制
V
CC浮置或弱下拉
寄生 PMOS 源体效应二极管
D+ 或 D- =5.25V OE
正信号泄漏
S
Vsw-0.8V
AN-6022 应用指南
断电保护
一个不受保护的模拟开关要根据控制输入完全确保正确 的功能状态，则必须带电。
在开关完全带电之前，当此类开关输入上出现一个正数 据信号时，不能保证该信号可以得到正确处理。
除非模拟开关具有专门设计的电路，能够确保断电时提 供关断隔离,信号才可以泄流。无论 OE 引脚和 S 引 脚处于何种状态，信号泄漏都可以同时出现在单刀双掷 开关的两个输出引脚上。
使用一个不受保护的开关来在两个不同主机控制器之间 共享一个共用 USB 端口时，
如果正信号意外通过开关，
可能会导致未选择控制器出现错误的枚举序列。
当 VCC 引脚处于浮置或非常弱的下拉状态时，
开关输入信号
（D+、D-）可能会为开关内部电路供电，使信号 通过开关泄流。 这从图 1
中可以看出，其中开关内部的 VCC 节点显示为 Vsw- 0.8V。 内部节点带电后，开关打开且输入信号通过。
在此情况下，正电压可能出现在未选择的输出上。
USB 开关通常被置于系统外围设备中，
用于隔离内部元件与外部环境。
在 USB 短路示例中，也期望开关能够长 时间耐受这种情况（至少 24 小时）。
当将正输入电压提供给一个不受保护的开关时，可能导 致模拟开关产生不可逆性损坏。
这种损坏归因于从开关输入端口流向接地开关 VCC
引脚的过电流。开关固有的寄生 PMOS 体效应二极管将 形成此电流通路，当输入电压高于 VCC+0.5 伏时，
其行为如同一个正向偏压二极管。
该二极管导通需要一个最低的正向电压，通常假定为 0.5 伏左右。该高效二极管允许过电流通过芯片流向 VCC 引脚。输入引脚上的电压越大，则电流越大。
电压和电流呈指数性关系，可以用理想的二极管曲线来 表示。结果是，可能很快超过芯片最大额定电流。
当一个器件在过压条件下被受损后，通常会继续出现过 量泄漏，即使输入回到正常的操作状态后，它也可能不 再工作。图 2 阐明了数据输入引脚和上述 VCC
之间形成的电流通路。
图 2 过压条件下的传统模拟开关电流泄流通路
FSUSB30/31 具有专门设计的电路，可以防止关闭 电源和过压条件下的意外信号泄漏，并且保证系统可靠 性。当 VCC=0V 时，
无论 使能引脚和选择引脚处于何种状态，
开关都将隔离输入信号与输出信号来防止意外信号泄漏
。同时也将防止从信号引脚到电源引脚的电流泄漏。
当开关断电时，输入信号为高阻抗输入，且防止寄生 PMOS 体效应二极管出现正向偏压。 值得注意的是，
FSUSB30/31 在共用端口（D+、D-）提供断压保护。
共用引脚已增加保护，因为连接器共享是最常见的应用 需求。 在这种情况下，共用端口是唯一可能遭遇 Vbus 短路的端口。因此，基于飞兆半导体对 USB
应用的理解，在这两个 I/O 引脚中增加断电过压保护。图 3 显示了共用端口共享应用示例。
图 3 典型连接器共享应用 控制
VCC = 0
寄生 PMOS 源体效应二极管
D+ 或 D- = 5.25V OE
S
电流通路
AN-6022 应用指南
© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.0.3 • 5/14/13 3
带电保护
USB 2.0 规范还说明，USB 设备应该能够耐受 传输数据时出现的 Vbus 短路。
因为 USB 开关在带电状态下才可以传输数据，
所以它必须能够耐受 D+ 和 D- 引脚上高达 5.25V 的电压。 对于 D+ 和 D-，
超过 VCC 0.5V 是可以接受的，然而过量电压
没有其它电路来调整会导致可靠性故障，应当避免。
例如，如果开关带电（VCC=3.6V）且 D+/ D- 遭受 5.25V Vbus 短路，无论标准的模拟开关
还是断电保护开关都不能保证可靠运行，除非对电路进 行更改。 在此带电情况下，
SPDT 开关可确保未选择的输出
在未选择的端口上没有任何意外信号泄漏。
如图 2
所述，过流是一个现实问题，但是一个简单的原理性更 改就可以解决这一问题。在开关
VCC引脚和电源轨之间插入一个 1000hm 的串联电阻可防止
开关在带电 Vbus 短路过程中损坏。
在出现过压时，这种更改操作可以限制流回 VCC
轨道的电流，从而将电流保持在安全工作范围内。
在该应用中，开关将 5.25V 输入信号传输
到选择的输出上。 出于此原因，
USB 数据路径上的所有元件都必须 能够耐受带电 5.25V Vbus 短路。
最后，无论部件是否带电，应观察正常运行期间设计的 数据表限制。图 4
阐明了电路板的设计更改，此设计使 FSUSB30/31 完全符合 USB 2.0 规范。
图 4 通过添加 100 Ohms 电阻并与 VCC
电源串联，FSUSB30/31 在带电状态下可耐受 Vbus 短路。
结论
FSUSB30/31 关是首个已生产的完全集成断电保护的 产品，这有助于防止 USB 2.0 规范所述的 Vbus 故障状态。
只需在 VCC 引脚和 VCC 电源引脚之间增加一个 1000hm 的电阻，FSUSB30/31 即可在带电状态下耐受 Vbus 短路。
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控制 寄生 PMOS 源体效应二极管
D+ 或 D- = 5.25V OE
S
电流通路
V
CC= 3.6 V
100 Ohms
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ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein.
ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
Buyer is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not designed, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
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