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

Magid Fazel

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

7 months ago

0 Kameron Hill 7 months ago

TI__Genius 16430 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 Magid Fazel 7 months ago 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 Kameron Hill 7 months ago in reply to Magid Fazel

TI__Genius 16430 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 Magid Fazel 7 months ago in reply to Kameron Hill

Prodigy 60 points

Thanks a lot

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