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MAX3232: TX (DOUT1) failure

JD Heyns
Prodigy 10 points
Part Number: MAX3232
Other Parts Discussed in Thread: GD65232

Tool/software:

Hi Guys.

We are experiencing intermittent failures in the field with some of our units, occurring after random periods (from days to months). After investigation, we identified the RS232 MAX3232IPWR driver as a common point of failure. Specifically, the TX line (DOUT1) is shorting to GND, which causes the entire 3.3V line to short to GND. Removing the chip from the PCB resolves the issue. 

The TX (DOUT1) is driving 2 to 3 RX receivers simultaneously which indicates that this could be the issue. But we've measured a few of these setups and does not see any overcurrent condition happening. We also have 100's of these configurations running in the field for more than a year.

Any ideas what could cause the chip to fail and how to fix the issue?

Thanks in advance.

Chip URL: 

www.ti.com/.../MAX3232IPWR

  • 0
    Guru 318700 points

    What exactly do you mean with "3.3 V line"? The bus outputs should be at ±5.4 V.

    One RS-232 receiver has an input impedance of 5 kΩ (3 kΩ worst case). The driver is not designed to drive more than one receiver load. But the most common cause of failures is overvoltage of some kind. How are the bus lines protected against ESD/transients? Are the RS-232 lines hotplugged, or can they by touched?

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Hi,

    We are facing similar issue, as the DOUT2 is shorting with GND(becomes 0E wrt GND) when the board is powered ON and a transaction appears on DIN2, we have used NUP2105L for ESD,, and the pin becomes OPEN once board is turned OFF.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Hi Clemens,
    The 3.3V VCC line that's powering the MAX3232 and a number of other IC's like the MCU, BLE etc. Yes we do have ESD and resettable fuse protection on the RS-232 lines.

    There is a possibility that some of the units may be hot-plugged. Are you asking because this would maybe cause an overvoltage/ESD condition?

    *Our idea to improve the unit to drive multiple RS-232 receivers is to run the TX line through a voltage follower op-amp circuit with a high side P-channel FET or transistor on the output so that the MAX3232 is protected and the driving side is strong enough to handle multiple receivers at the same time. Any thoughts? 

  • 0 in reply to JD Heyns
    Guru 318700 points

    Unplugging will result in arcing, and any pins that can be touched will be touched.

    A voltage follower can work. (If you already have bipolar supplies between ±5 V and ±12 V, then you do not need to generate the bus voltages with charge pumps and can use a simpler transceiver like the GD65232.)

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks Clemens.

    If we don't want to go with the charge-pump option because we don't have the ±5 V and ±12 V supplies onboard. Can you think of a better solution to drive multiple receivers? Multiplexer perhaps? Or would you recommend charge-pump/bipolar supply option?

  • 0 in reply to JD Heyns
    TI__Guru* 76991 points

    Hi JD,

    Using a multiplexer would be a good solution to prevent the driver from seeing all 3 inputs at the same time but how sure are you the device is getting damaged from seeing the parallel receivers? What I would expect if the 3 parallel receivers are causing an issue is the RS232 driver would probably just see a collapsed V+/V- rail (likely fall to +/-3.3V). 

    Are you able to monitor the RS232 output pins with an o-scope and set the trigger level to 10V and try to perform the hot plugging to see if the signal is potentially being overvoltage during the hot plugs? 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 10 points

    Hi Bobby,

    Yes we did. We do sometimes see a fast (few microseconds) 18V spike on the line. This is on a kit on a test bench. We've repeated the plugging and unplugging for probably 50 to 100 times on the same unit as a mini endurance test but cannot get the chip to fail Sweat smile. The kit does have 3 receivers connected like in the field. Are there any other measurements or ideas we can try? And if we had to design a RS-232 protection circuit on steroids....what would you include? ESD/TVS/Zener, Current limiting resistors/resettable fuses? Does TI have an application note for something like this?

    Thank you in advance! 

  • 0 in reply to JD Heyns
    TI__Guru* 76991 points

    Hi JD,

    You may have actually found the issue with the spike. 18V on the Driver is actually above the absolute maximum rating for the device. Table 6-1 in the datasheet shows that the Driver can withstand a max voltage of 13.2V. Since the time duration is pretty short, it probably doesn't immediately break the device but slowly degrades it. This likely is causing them to fail later and more sporadically. Maybe in the field, the transients are going higher than 18V or the scope you're using isn't sampling enough to see the true peak voltage.... 

    I don't necessarily think you need to go all out to design a super robust system with everything on it. Maybe a series resistor of 10 ohms and a TVS that can clamp the voltage sooner than 13V and later than 6V. I haven't seen too many people overdesign the RS232 bus but in RS485 some people go wild with pulse proof resistors, resettable electrical fuses, Varistors, MOVS, TBUs, and TVS but generally this is to design around surge events that have huge power over a longer time frame than normal EFTs or ESD events. 

    This app note below actually recommends a TVS for the MAX3232.

    https://www.ti.com/lit/ta/sszt891/sszt891.pdf?ts=1721768833025&ref_url=https%253A%252F%252Fwww.ti.com%252Fdocument-viewer%252Flit%252Fhtml%252FSSZT891

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 10 points

    Hi Bobby

    Thanks, we are implementing this on a few units and will test it in the field.

    FIY, we tried a bunch of series resistor values on the actual system and saw that the receivers all work up until +/- 2K2 was added. We are also going with a bigger wattage TVS than the app note (just in case)

    Regards

  • 0 in reply to JD Heyns
    TI__Guru* 76991 points
    JD Heyns said:
    FIY, we tried a bunch of series resistor values on the actual system and saw that the receivers all work up until +/- 2K2 was added.

    Adding larger series resistors on the line will lower the load seen by the drivers. It kind of looks like a voltage divider from the receiver side so it makes sense you're starting to see it stop working the closer it gets to the termination impedance. (~3k to 7k in a single receiver set up).

    JD Heyns said:
    We are also going with a bigger wattage TVS than the app note (just in case)

    Just be sure it clamps the voltage before the absolute max of the device.

    -Bobby