Hello,
I would like to ask if I only use 5 input pins of the IC, is the one not used should be pull-low to GND?
Does this pin can be floating?(Because the IC already has pull-low resistance inside, for example.)
Or if there is any other method of the connection?
Thanks for your replying!
LEE
Hi Lee,
Thank you for posting on E2E!
Unused channel inputs of a digital isolator should be tied to their device's default state, which depends on whether your device has an "-F" suffix, like ISO7763F, or not. In the case of ISO7763_, the unused input should be connected to Vcc while the output can be left floating as described in this FAQ post:
Thank you for your time,
Manuel Chavez
Hi Lee,
Once powered on, our digital isolators will always have an output state of either low or high depending on the channel input (if there is one) and the Vcc states as shown in Section 8.4 of the ISO7763 datasheet and in the truth table below:
Therefore, yes, if the input pin of a digital isolator is connected to Vcc, the corresponding output will normally remain high. The difference between "-F" and non-F devices is behavior of the output by default. The default state is relevant in situations where the input side loses power on Vcc or the channel input is floating.
ISO7763 will have an output = high by default while ISO7763F will have an output = low. These output states are not relevant during normal data transmissions, but connecting the input side of an unused channel to the default state will help keep that channel in a known state and less susceptible to noise.
Thank you,
Manuel Chavez
Hello Manuel,
Another question from datasheet.
What does "Safety Limiting Current" mean in the diagram below?
I mean it's the current from where?
Thank you very much!
LEE
Hi Lee,
You're welcome! We are happy to help.
The internal resistor connecting an ISO device to its default state (shown in Section 8.4.1 of the ISO7763 datasheet and below), is a high-impedance, low-current connection. Strong connections to the default state, like through an external 4.7kOhm resistor, are preferred for general robustness and in case transients of a magnitude that can couple to the device are nearby:
This is also useful to keep unused channels in a known state, and choosing the default state slightly reduces power consumption and electromagnetic emissions - "slightly" because these values are already very low.
Exceeding safety limiting values either on the device input, output, Vcc, or power dissipation affects the isolation barrier directly, but this is not a typical problem; if sudden current or power surge issues are foreseen in your application, including external components like current-limiting resistors, PTC fuses, etc. will be helpful in preserving the isolation barrier even during such conditions.
Respectfully,
Manuel Chavez
