. If we were to connect the non-isolated side of two of those together but power only one at a time, would the “Undetermined state” of the outputs (pins 4-6) of the unpowered device conflict with the output of the powered device? In other words, will the outputs be tri-stated at the unpowered and therefore only driven by the powered or will the unpowered device allow current to flow into/out of its pins when its Vcc (Pin 1) is disconnected?
Hi Matt,
Thanks for reaching out.
To your question in the title of this E2E post, the answer is NO, the undetermined state of outputs on the isolated side doesn't conflict with the powered non-isolated side outputs. Since these two die are completely separate and are powered by different supplies, their output undetermined state is only defined by their own power supply voltage.
If we were to connect the non-isolated side of two of those together but power only one at a time, would the “Undetermined state” of the outputs (pins 4-6) of the unpowered device conflict with the output of the powered device? In other words, will the outputs be tri-stated at the unpowered and therefore only driven by the powered or will the unpowered device allow current to flow into/out of its pins when its Vcc (Pin 1) is disconnected?
Here, you are talking about connecting outputs of two different devices together, so the answer to this question is going to be different. Since the output pins have ESD bypass diode to its power supply pin (VCC or VISO), applying voltage at the output pin externally can turn on the device through the ESD bypass diodes. For example, refer to the output schematic in below image. The image is showing output on the VISO side, the schematic for VCC side output is also the same hence, consider VCC in place of VISO in the below image for our discussion.
When two outputs are connected together and one of VCC is 0V, then a HIGH voltage (3.3V or 5V) on one output can pass to VCC of unpowered device through the ESD bypass diode that is shown in below image. This will accidentally turn on the unpowered device and the outputs will conflict with each other leading to undesired behavior.
The best way to handle connecting two outputs is to use the enable pins to enable or disable the outputs where when disabled, the outputs will be tri-stated and hence the ESD bypass will also be disconnected. Unfortunately, there are no enable pins feature available for ISOW78xx, this is currently only limited to all digital isolators and ISOW7741. If ISOW7741 is acceptable to customer, then I recommend customer to use ISOW7741 which is a second generation ISOW device with significant improved performance.
Let me know if you have any further questions, thanks.
Regards,
Koteshwar Rao
