SN65HVD75: Damaging during board power ON

Supriya,

How does the damage show up in the device's behavior?  For example, do you see something like a low-impedance path from the A/B lines to ground?  Does the transceiver consume high ICC (or no ICC)?  Do transmit or receive functions work individually?

At any time before the damage is detected, can you think of any situations that would expose the transceiver to conditions outside of its "absolute maximum" ratings?  Some common situations may be overshoots on VCC ramp-up or coupling of stresses (ESD, short-circuits, etc.) to connector ports affecting A/B.

I would be happy to review your schematic if it could be shared.

Regards,
Max

Hello Max,

A. Impedance Test

I have check A/B line to GND impedance. 

For OK PCB it is around 200 K to 210 K

For Most of faulty PCB it same around 200K to 210K but on few PCB it shows Very high impedance (open).

 B. ICC is same on board. around 1mA 

C. Individual transmitter and received testing not done yet.

another observation is, SN65HVD75DR Vcc voltage is 3.3V and RS-485 device voltage is 5V means A/B line with respect GND is 2.5V. Is it create any issue.

Find the above Schematic.  

RS485A and B directly connected to RJ45 connector. 

Thank you,

Supriya

Hi Supriya,

The schematic didn't come through; could you please re-post it?

Do you mean that you apply 5 V to the VCC pin of the SN65HVD75 device?  This is not recommended.  It was designed for VCC = 3.3 V.  We have other transceivers designed to support 5 V - see, for example, THVD1550.  

Please try to test a failing PCB a little more to better understand the type of failure.  A high impedance on A or B is not a very typical failure mode for a transceiver, so it would be good to rule out simple explanations like a soldering/connectivity issue before investigating less-common issues (e.g., component damage, bond wire failure, etc.).

Have you tried swapping the transceivers on a working and non-working board to see whether the failures follow the transceiver (rather than the PCB)?

Regards,
Max

Hello,

1. Find the attached schematic

2. No we are giving 3.3v supply to IC. This IC is communicating with VFD which RS485 working voltage is 5V. Is it create any effect on IC 

3. We have check all other things like soldering, continuity but all things are ok

4. We have swap PCB but failure IC fails on other PCBs, If we change IC of failure board then it is working fine. 

Thank you,

Supriya

Hi Supriya,

Thanks for the clarifications.  The A/B pins are designed to work over a wide voltage range, so interfacing to a 5-V transceiver would not be a problem.  There are no issues with the schematic portion you shared.

It's good you confirmed that the failing behavior is tied to the IC itself.  It sounds like the impedance/ICC readings are mostly normal, though, so it doesn't seem like there is a catastrophic issue with these failing ICs.  In case it is something more subtle, I would like to see if you would be able to measure the signal waveforms at the various pins (DE, RO, DE, A, and B) on a failing device so that we can get a better understanding of the failure and where to look next.

Regards,
Max