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The information presented in this cross reference is based on TOSHIBA's selection criteria and should be treated as a suggestion only. Please carefully review the latest versions of all relevant information on the TOSHIBA products, including without limitation data sheets and validate all operating parameters of the TOSHIBA products to ensure that the suggested TOSHIBA products are truly compatible with your design and application.
Please note that this cross reference is based on TOSHIBA's estimate of compatibility with other manufacturers' products, based on other manufacturers' published data, at the time the data was collected.
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什么是输入容限功能?
具有输入容限功能的CMOS逻辑IC允许在电源激活或在0V时向输入端施加高达最大工作电压的电压。输入容限功能允许电平从较高电压转换为较低电压。
电平转换示例
从5V向下转换至3V:采用74VHC或74LCX系列。
从3V向下转换至到1.2V:采用74VCX系列。
以下是典型CMOS逻辑IC的等效电路。
在输入侧插入二极管以进行ESD保护,而输出侧的二极管是寄生二极管。
如果施加的电压高于VCC或在IC断开时施加电压,则输入端和电源之间的二极管可能会导通。在这种情况下,产生的大电流可能会损坏IC。通过使用具有输入容错功能的IC,即输入端和电源之间无二极管的IC,可以防止器件损坏
很容易区分一个IC是否具有输入容错功能。
让我们看一下TC74HC和74VHC系列IC的数据表。
请参阅工作范围表中显示的输入电压。
TC74HC系列的数据表显示VIN=0至VCC。这意味着输入引脚的最大容限为VCC。因此,TC74HC系列没有输入容错功能。
相比之下,74VHC系列的数据表显示VIN=0至5.5V,这意味着无论VCC如何,均可将高达5.5V的电压施加到输入引脚。因此,74VHC系列具有输入容错功能。