• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 16 subscribers
  • Views 75 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74CBT16811C: On resistance is measuring high?

Steven White
Prodigy 20 points
Part Number: SN74CBT16811C
Other Parts Discussed in Thread: SN74LV541A, SN74LV4066A, TPS22948, UCC27519

Tool/software:

I am trying to use the bus switch to optionally to pull up a bunch of nets to 5 V. The schematic is as shown below. I want to be able to use both the 16811C and 16211C, so I implemented the bias side (B) as the source and the non-biased side (A) as the sink. I doubled up channels 1 and 2 just to reduce the on-state resistance further. With a 1.6 kΩ pull down to GND on the A-side I am seeing the on-state resistance between B and A being about 1.2k (I measure 4.99 V @ B, 3.66 V @ A). With 40 µA of pull-down I'm seeing an on-state resistance of 29.3k between A and B. 

I've confirmed when I apply 3V3 to !OE that the pull up through U900 disappears, so this suggests the chip is "ON" but no current is getting through. I measured the same behavior on every channel and two cards, so the result is consistent. What am I doing wrong, and why isn't the switch turning fully on?

  • 0
    Guru 265990 points

    CBT switches are designed for TTL signals, which do not actually go near 5 V. The switches are implemented with plain N-channel MOSFETs with the gate at 5 V, so with the signal at 5 V, the transistor is off.

    There are proper analog switches using tranmission gates like the SN74LV4066A, but with at most four channels. if you just want to connect to 5 V, you could just as well use buffers with three-state outputs, e.g., SN74LV541A.

    But when you connect all signals to the same voltage, you do not actually need multiple channels. A gate driver like the UCC27519 is a strong buffer, or a load switch like the TPS22948 is a very-low-resistance switch.