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SN74CB3Q3345: What does "rail-to-rail" mean?

Part Number: SN74CB3Q3345

HI,

SN74CB3Q3345 datasheet contains content below: "Rail-to-Rail Switching on Data I/O Ports".

Does it mean, the output Voltage Level is the same as the input, regadless of the Vcc supply?

For example, the Vcc supply is 3.3V, input signal Voltage Level is 1.8V, then the output Voltage Level

is also 1.8V? Or, the output level follows the Vcc, is 3.3V?

  • l x,

    Rail-to-Rail is a common term to say that a device will support a signal anywhere between the power supply "rails".  In your example if you supply the SN74CB3Q3345 with Vcc = 3.3V,  and provide input signal Voltage Level of 1.8V, then the output Voltage Level will also be 1.8V ?  This type of a device is a passive FET switch and what ever voltage is placed on the input will pass to the output.  

    The reason the Rail-to-Rail feature is mentioned on switches is because there are some switches that are only NMOS FET switches that will severely attenuate a signal as it approaches the supply rail and clamp it to a voltage level of about Vcc - 1V.  For example if you supply the SN74CBTXXXX with Vcc = 5V,  and provide input signal Voltage Level of 5V, then the output Voltage Level will be about 4.0V.

    In fact the SN74CB3QXXXX devices are more than Rail-to-Rail switches because they are NMOS FET Switches with their gate voltages biased with a charge pump.  This means they can support voltages beyond the supply rail.  For example if you supply the SN74CB3QXXXX with Vcc = 3.3V,  and provide input signal Voltage Level of 5V, then the output Voltage Level will be about 5V.

    Thank you,

    Adam