• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 16 subscribers
  • Views 160 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.

SN74CB3T1G125: Issue when driving A side high while B side has a 10K pull down.

Magid Fazel
Prodigy 60 points
Part Number: SN74CB3T1G125
Other Parts Discussed in Thread: SN74CBTLV1G125

Hi,

We use CB3T1G125 throughout our design as a leakage prevention device instead of level shifting. Basically, we expect this part to be closed when there is power (3.3V) and open when there is no power. We tie the Enable pin to GND - so should be enable always. The 2 sides of the FET are on 2 different power domains but both 3.3V. Each side can be independently on/off.

We are assuming that once the power is on to this device, it is just a 5 OHM series R. I.e. I should be able to drive high and low on one side and see high and low on the other side without any clamping as both sides are 3.3V.

We have seen the following behavior:

With 10K P/D on the B side, when the A side is driven high (3.3V), The B side only goes up to 2.1V.  When we remove the 10K P/D it works fine.

Any idea why?

Thanks

Magid

  • 0
    TI__Genius 14410 points

    Hello Magid,

    1. So the reason why you are seeing this behavior is because the pull down resistor of 10K is sinking a large amount of (3.3V/10K= 330ua) current and affecting the Ron of this device. 

    That is also why when you take away the pulldown you see  your "intended" signal.  A weaker pull down resistor that will sink less current could be used if you want  a pulldown resistor in your setup but still have your intended signal pass through the mux.

    2.Based on the fact that this device is a level translating mux , I believe you should use the   SN74CBTLV1G125 for this application!( It is a p2p replacement for the cb3t device)

    Here is a comparison between these devices.

    Please let me know if you need any other assistance!

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 60 points

    Hi Kameron

    Excellent response time. 

    Just to make sure I understand. If I go to CBTLV device, it will be FET only and no level shifting. Hence, I can still keep the 10K P/D with the new device?

    Correct?

    Thanks

    Magid

  • +1 in reply to Magid Fazel
    TI__Genius 14410 points

    Hello and thank you Magid,

    Yes if you use the cbtlv device you can keep the 10 K pulldown resistor. This is because this device is intended to pass signals source to drain( or drain to source) 

    Basically the ron of the mux will not be affected by the pulldown.

    Regards,

    Kameron

  • +1 in reply to Kameron Hill
    Prodigy 60 points

    Thanks a lot