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SN74CBT3257C: Voltage drops.

Guru 10385 points
Part Number: SN74CBT3257C
Other Parts Discussed in Thread: SN74CBT3257

Hi team.

When my customer uses SN74CBT3257 as follows, the voltage drops by 0.8V.
VCC : 5V, A : Output , B1 : 5V(Input) , B2 : GND(Input)

Could you tell me why the voltage drop?
It seems like a voltage drop by the diode.
However, from logic diagram in the datasheet , there seems to be no diode.

Sincerely.
Kengo.

  • Hi Kengo,

    The SN74CBTXXXX family of switches are single NMOS FET switches.  In order for the FET to have a low impedance path from drain to source (A to B) the Vgs voltage must be greater than the FETs threshold voltage.  With this type of switch you will only be able to pass a signal about 1 V < than the Vcc voltage. 

    If you want to pass a voltage up to the value of Vcc you will need a different family of switch that has two FETs an NMOS and PMOS in parallel or a switch with a charge pump.

    You can find more details on common switch structures here.

    What do you think about trying TMUX1574? It is has a transmission gate structure and supports rail to rail voltage in addition to other features like the 1.8-V compatible control inputs, Fail-safe logic, Powered-off protection.

    Thanks

    Saminah