Pmod LVLSHFT B-side power

[Ken Brown]

Pmod LVLSHFT is not working for me.  A side is 1.8V.  B side is 3.3V.  It seems like Q1 is not working to connect VCCBIN to the VCCB rail. 

I've got 3.35V at VCCBIN.  I see 120 mV at the gate of the Q1 P-FET.  And I get 120 mV at VCCB. 

Why is Q1 even needed?  It looks like it only delays when VCCB turns on (by maybe 2 mSec).  Do I need that?  

>> Can I just short across the not-installed R33?  (connecting VCCBIN directly to VCCB)

These are fresh out of their Digilent ESD product bag, purchased from DIgikey about 3 weeks ago. 

Help!  And thank you. 

 - Ken

 

[Ken Brown]

Turns out I when was probing the gate pin of Q1, I was actually probing the drain and vice-versa.  The Q1 MOSFET is actually working correctly.  The turn-on delay is about 400 uSec. 

>> I'm just curious, in what situations is this delay of the VCCB turn on needed?

(in the scope shot, green is the gate of Q1 and yellow is the 3.3V VCCB at the drain.)

 

 

[JColvin]

Hi @Ken Brown,

I reached out to another engineer about this and they let me know that Q1 is set up to effectively be a load switch to help mitigate in-rush current so that when a cable is inevitably hot-plugged it doesn't brown out the other board.

Let me know if you have any questions about this.

Thanks,
JColvin