• Resolved

SN74CB3T16210-Q1: Too low output signal level (Voltage Translation)

Part Number: SN74CB3T16210-Q1

e2e,

 

Questions

Do you see any error in reasoning in the problem description below?

Any idea why the HIGH signal on the output is only 2.0V?

 

Design parameters

I use it to switch off various logic signals from their CPU to peripherals. The following mode of operation is used:

  • VCC = 3.3V (on Pin 15)

  • Signal level on input A = 3.3V CMOS (coming from CPU)

  • Desired signal level on output B > 2.9V CMOS (going to peripheral, also 3.3V CMOS logic)

  • The related OE signal pin 47 is set to GND

 

Intended functionality

In normal operation, the switch should be conducting permanently. Only if the 3.3V supply is switched off, the signals should be disconnected from the CPU. This is done to prevent current from flowing into the switched off peripherals.

 

Problem

The input is fed with a static signal: 3.3V on pin 23 (IO 2A9). The corresponding output on pin 26 (2B9) delivers a static signal with 2.0V.

If the change the input to LOW, the output of the bus switch is also going LOW. Functionality is ok, but the HIGH level is too low (2.0V instead of 3.3V).

 

The output has a pull-down resistor of 10k on pin 26. Removing this resistor gives 3.0V signal level on pin 26 (2B9). Without the resistor, the output is only connected to the CMOS input of the peripheral.

It looks like the switch is very high impedance when signal is HIGH. According to the datasheet, the internal resistance should be <11.5 Ohms.

Thank you,

Adam

  • Hi Adam,

    Thanks for posting on E2E, I have some thoughts and additional links for you to help understand why this output voltage is not what you are expecting.

    The CB3T family of switches are level-shifiting, NMOS bus switches. These work great for down translating a 5V TTLL to a 3.3V LVTTL signal.  The following link is one of TI's App Notes that talk about the CB3T family and how the translation is achieved. 

    Here is a snapshot from the app note on page 11.

    In your situation you have Vcc at 3.3V. With the Vt of the NMOS + the Diode drop the actual output voltage is found by VO= VG-VT. 

    These devices can translate back up but require a pullup resistor as mentioned in the following E2E posts:

    If this solution doesn't fit you system needs we have other devices that achieve a similar concept of 20bit bus switch. I've linked our paramtric table that when sorted on 1:1 devices with 20 channels give 11 devices. Here is a LINK to the pre-sorted table.

    I quickly see the SN74CBTLV16210 device which is very similar to your original device but operates as a transmission gate switch. Please look into this device and see if it better suits your needs.

    Regards,

    Dakotah

  • In reply to Dakotah:

    Thanks for posting here Adam and thanks for this detailed answer Dakotah!
    Unfortunately, I cannot verify the answer. I guess because I am not the original thread creator.