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SN74CB3T3245 - Low Switch Output Voltage When Using a 100K Pull-down Resistor

Other Parts Discussed in Thread: SN74CB3T3245

I'm using a SN74CB3T3245 switch as a bus isolation device, and I have it powered at 3.6V.  If I tie a 100Kohm pulldown resistor at the output of one of the switches (this particular switch is an 8-bit switch), I only see 2.4V at the output when I drive the input high (3.3V, CMOS driven).  If I remove the 100Kohm pulldown, then I see ~3.5V at the output. 

In short, the device seems to behave correctly without a pulldown resistor, but doesn't behave correctly with one.  Anybody have any ideas?

 

 

  • The CB3T device are designed to drop the input voltage to VCC. I know you can pull the output up above Vcc but I am not sure about below VCC.

      these are NMOS devices which will have outputs af about 1V below VCC so these CB3T devices have a control circuit that boosts the voltage above Vcc. I am not sure if this pulldown would effect thet or not.

    Can the CMOS driver drive the 100k resistor?

    Here is an explanation of the CBT3T devices that may help.

    CBT-C, CB3T, and CB3Q Signal-Switch Families Analog & Mixed-Signal scda008 - TI.com

  • Q: Can the CMOS driver drive the 100k resistor?

    A: Yes, it can.  My CMOS driver is an I/O port on a Microchip 8-bit microcontroller, and a 100K pull down will only require 3.3V/100K=33uA of sourcing current.  Microchip I/O ports can source ~20mA.

    Thanks for the link to the CBT3 documentation.  Can you help me understand the Ron (on resistance) and Io (output current) parameters?

    Q: The on resistance (Ron) seems to be a function of the input voltage.  So, if my Vcc=3.6V, and my Vin=3.3, then Ron should be ~10Kohm if Io=-1uA (see Figure 33 of the document)... so how is Ron actually defined?

    Q: Io (output current) is defined as positive in some tables and negative in others, what does the sign mean (sourcing = positive, sinking = negative), and how is it defined?  Is it Io=Vout/Rload?  So in my case it would be Vout/100Kohm?

    I appreciate any help.