MAX3232-EP: IC is failing when using RS232 by connecting it to PC

Hi,

When you connected MAX3232 to PC (or more specifically the converter), was the device powered on? How're the device and the converter grounded? Is there any potential difference? Also can you give some detail about the failing devices, like which pin failed, what's impedance of the failing pin, etc? BTW, can you also provide the voltage rating of the external cap (C82, C83, C89, C91, C93)?

Regards,

Hao

Hi Hao,

1. Yes, the device was powered on when we connected MAX3232 to PC through MOXA USB to Serial Converter.

2. in serial port side of Converter, Ground pin is connected to the Ground in the MAX3232 Transceiver. 

   USB side of converter is directly connected to PC.

3. Potential difference? I think no.

4. Pins 11, 12, 13 14 is failed. I dont know about 7, 8, 9, 10 as we are not using. How to measure impedance of the failing pin?

5. Voltage rating of the all external cap is 16V.

Please let me know for further clarification & help me out to identify and resolve an issue.

Hi,

How often does the failure happen? Is it possible to test with the MAX3232 and PC ground connected? If you do the connection first then power the device up, would it help? You can use a multimeter to measure the impedance from the pin to Vcc/ground and compare the result with that of the good units. It looks to me that the device is damaged due to overcurrent or overvoltage.

Regards,

Hao

Hi,

1. Failure happened approximately once in 4-5 days. Totally 3 ICs failed.

2. Both MX3232 & PC ground is connected

3. It is same if u do connection and then powered ON also.

4. 

5. Yes i feel the same. But no other IC and circuit in the board got damaged/failed.

I will share Impedance details later after measuring.

Hi,

Can you share the model of MOXA's converter? I will check it out.

As I re-looked at the schematic, I noticed SMBJ36CA was applied. Is there any reason? The abs max of the RX input is +/-25V and the TX +/-13.2V.

Regards,

Hao

Hello, 

1. MOXA Uport 1110 USB to Serial Converter

2.  SMBJ36CA is for Transient Protection

Hi,

Thanks for your information. In littlefuse's datasheet, SMBJ36CA starts breaking around 40V and the maximum clamp voltage is 58.1V, which is way above the MAX3232's abs max. In transient events, I'm afraid the diodes may not be able to provide any protection for the IC.

Regards,

Hao

Hao,

Is this the only reason behind the failure of the IC? 

Because When were testing our board without ATE (Automated Test Equipment), we found no issues with the IC for more than 6 months.

But we started testing with ATE, we found failure of IC 3 times in a month.

Hello Hao,

In addition to previous reply, can you please suggest me the appropriate TVS diodes for Rx & Tx lines by considering absolute maximum operating consitions of MAX3232 IC?

The diodes is the most suspicious place so far. You can consider SMBJ8.0CA for the TX and SMBJ20CA for the RX.

Regards,

Hao

Hao,

You mean to say, diodes might have not required? 

Hi,

The diodes might not be required if the device is not exposed to ESD strikes in applications.

Regards,

Hao