SN65HVD3080E, SN65HVD485E with photocoupler

Hello Asaka san,

I see no issue using the optocoupler with the listed transceivers.
Usually you have an external pull-up resistor to VCC and the transistor integrated in the coupler switching to ground. Do you worry about the internal resistance of the coupler or the transceiver input pins?

When proposing our digital isolators you can promote the reduced number of parts needed and the saved board space. For the SN65HVD485E for example you could propose the ISO7341C, which is a 3 channel forward plus one channel backwards device and so integrates all needed isolation channels. So you have to compare this with 4 of the above mentioned photocouplers.
Do you know which requirements for the isolation voltage your customer has?

Another option could be a complete integration of the isolation into the transceiver devices, which would even save more boardspace and reduce the number of parts to one. You can check the ISO3082 for half-duplex and the ISO3080 for full-duplex applications.

I am glad to help you if you have any other questions.

Kind regards,
Chris

Asaka san,

you are always welcome to ask!

As long as the voltage at RE, DE and D is less than 800 mV for a low logic input state and above 2 V for a high logic input state as specified in the datasheet, this works fine.

With the internal resistances given in the equivalent input diagrams on page 10, I have calculated a low level input voltage of about 250 mV for the setup you have shown above. So it should work without a problem.

Best regards,

Chris

Hello Asaka san,

you are right. There will be a variation of the internal resistance. The better parameter to look at is the input current for the D, DE, RE pins.

It is specified as maximum 100 µA in the datasheet. This is based on simulations for VCC at 5V. So you can conclude to a minimum internal resistance to VCC of 50 kOhms. With an external 10 kOhm resistor you can calculate the voltage divider and get an absolute maximum logic low input voltage of about 833 mV.  
This is above this threshold.

So to be absolutely safe you should use an external resistor with a value smaller or equal to 9 kOhms (also take the tolerance of the external resistor into account, I calculated with +/- 5%).

The 100 µA input current seems to be specified quite conservative though, since it is a common practice to use 10 kOhm external pull-up or pull-downs.
Bench tests have shown much lower maximum currents, which safely allow the use of 10 kOhms for an external pull-down.

Thanks for your good input.

Best regards,
Chris