SN751178: SN751178 Conditions for parts output cutoff

Overvoltage damage would be permanent. The problem going away after some time sounds as if this is a thermal problem.

• Is the chip hot? (You should be able to feel it, but be careful.)

A thermal problem would be caused by a very high power dissipation, i.e., high bus currents. The DC current in your circuit should be OK, but the 300 pF capacitors will create higher current peaks.

• Can you measure the current (i.e., the voltage drop over the 68 Ω resistor)? I'm interested in whether the peaking capacitors result in a noticeable increase of the average current.

330 pF appears to be very high. The LED capacitance of the HCPL-2430 is only 20 pF, and the datasheet's example circuit uses 56 pF for a 20 Mbit/s signal. If you want to experiment, replace the 330 pF with something smaller, and check if the waveform is still OK.

The SN75ALS1178 has a lower power consumption, but I do not know if this actually implies a higher output current before thermal shutdown happens; it might be worth trying. The SN75C1168 is a CMOS device with a different architecture; its absolute maximum ratings are lower, so I doubt that it is more robust.

RS485IC temperature measurement data.pdf

Clemens Radish
Mr. Parker Dotson

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Thank you for answering my question. This is very helpful.

Although I have not yet made any measurements by changing the capacitance of the capacitor, I hastily measured the waveform and temperature of 330pF.

The temperature measurements taken this time are for reference only, and are likely to be lower than the actual location where the problem is occurring. The reason is as follows.
a. The power was not turned off at the location where the problem occurred (the power was always on), and the data measured this time was taken 20 minutes after the power was turned on, so there is a possibility that the temperature is not at the upper limit.
b. The location where the problem occurred is always kept in a rack, but this time we pulled it out with an extender and measured the temperature.
c. In the location where the problem occurred, there is a possibility that there is also a board in the adjacent slot and that the problem is being affected by the heat of the adjacent board.

In the future, I would like to change to the 56pF capacitor you mentioned and measure waveforms and temperatures.

The current situation is above, and we will continue to measure.

There are indeed huge current spikes; the ideal waveform would be a square wave.

The HCPL-2430 datasheet suggests 56 pF for a 20 Mbps signal. You have less than 10 Mbps, so you would not need more than 27 pF; a more common value like 33 pF or 47 pF would also be OK. (And this is so small that no capacitor at all might work fine.)

I did not expect the 330 pF capacitor to become hot (it should not actually dissipate much power), and 46 °C is not dangerous. I was asking about the temperature of the SN751178 itself.

Hi,

So as Clemens also pointed out - there are some huge current spikes across the series resistor that is due to the 330pF of capacitance. Using a smaller capacitance with the data signal that you have something like 33pF/47pF would be a good value - however you could test without the capacitor because it may not be necessary with the current data rate. 

Also knowing the temperature of the IC should be more important the capacitor itself - this will be the case temperature however so there is going to be an increase from case temperature on the actual junction of the device (which is what thermal shutdown is preventing - an overtemp on the junction). The power usage will be greatly reduced with a smaller capacitor - and you may be able to get rid of the capacitor entirely. 

I'd suggest doing a similar test and just depopulating the capacitor during the test to see if the same temperatures are being recorded - I'd imagine it will help and you can avoid putting the device into thermal shutdown.

Is it possible to run a test without the capacitor to see if it changes the results being measured - you may still need a capacitor in actual application - but for a quick test it would help confirm that this is the main driver of the issue at hand - I expect it is, but it is probably a good idea to verify. Please let me know if this is possible.

Best,

Parker Dodson

A thermal shutdown would be active only during high temperatures. If the TXD_B outputs does not work after days, then it is damaged. (And even if it works again later, it is likely to be degraded.)

Hi,

So first off - I think you are focusing on the wrong things - remove the cap or shrink it considerably as repeated high heat/high power exposure is going cause issues eventually. If you are constantly putting device into thermal shutdown  there could be risk of reduced lifetime of parts - this is a fault protection feature and shouldn't be getting hit every time you run the system. This can be tested easily by just removing the capacitor and testing it. As of right now the issue seems to be due to the specific application design. 

Second - the cursor on the thermal image is pointing towards the hottest spot in the frame which is above the IC - not on it; it looks like a capacitor or some type of passive component is closest. The IC seems to be a different temperature as the cursor in the image is not on it

1. If the B pin doesn't come back its damaged - most likely due to design - remove the cap and retest with a fresh IC - and check if you see the same problems. You are feeding a RS-422 device into a non-RS-422 load the behavior could also be somewhat due to the other parts on the bus. At this time the best course of action is to take off the capacitor and retest - hopefully with a new device because you could have damaged this one. 

2. No thermal shutdown ends when the device cools back to operating range. If it takes days than the device is broken - please remove cap and retest. 

Best,

Parker Dodson