SN65C1168E-SEP: Powered-Down Protection

Hi Manuel,

So basically the silicon itself is rated to the absolute maximum datasheet values - i.e. we guarantee these values:

The issue with these types of parts that would make them problematic for power-down protection is back-powering through ESD diodes on the device. This is prevented on the single ended inputs by not having a diode to VCC pathway - you can tell if that diode is there from the abs max table - if the max voltage is independent of VCC then VCC's state is not important w.r.t. damage causing signals. There is a diode to GND - you can tell because the minimum will be either -0.3V or -0.5V (which is GND - diode_drop). 

The drivers structure does not allow for back-powering so the max of 7V is independent of VCC and will be high-z when unpowered. 

RS-422 receivers (A/B pins) must be able to handle common mode shifts beyond supply - so their maximum rating is independent of VCC state because it is required by RS-422 standard that they must at least handle -7V to 7V input range (this device exceeds that). However the device will still load the bus - its input impedance is also generally independent of input signal and I generally wouldn't consider 17k as "high-z" - so it's still acts a load - i.e. you can't turn it off by forcing VCC=0V to reducing loading on the differential bus - the loading would remain unchanged. 

That being said - the "R" pin cannot have a voltage applied to it when VCC is 0V because there is a diode pathway and its max rating is dependent on VCC state. Generally this isn't a huge issue - as the "R" pins when they do have  pull-up - they are pulled up to devices VCC so the max condition is still respected - however you can't apply an external signal when VCC = 0V because you will back power the VCC rail and depending on signal input it could fry the "R" pin. 

So essentially :

1. All single ended inputs are always high-impedance and max rating is independent of VCC state - this is guaranteed within spec'd operational range. 

2) single ended outputs are dependent on VCC state and cannot have voltages applied when VCC = 0V - once again this is a device guarantee. 

3) differential I/O ratings are independent of VCC - but the receiver will still load the bus and realistically there should only ever be 1 driver on RS-422 bus (if you need multiple drivers on same differential pair you need to move to RS-485 which is multi-point not multi-drop like RS-422)

Please let me know if you have any other questions!

Best,

Parker Dodson