MAX9611 and the Magic Smoke

[spec]

Spec,

Sent you a PM.

I just replied

spec

 

[Mikebits]

How were your psu remote sense terminals connected?

 

[Mikebits]

The melted pins almost sounds like the chip latched up. Really has me puzzled. I wonder if the action of pulling the cable caused something like switch bounce. But would switch bounce unleash huge current spikes? To melt the pins implies large currents for appreciable amount of time which leads back to a latch up condition maybe a supply decoupling cap was large enough value to hold a charge. Was there a minus voltage on the board ? I'm sorta just thinking out loud

 

[spec]

The melted pins almost sounds like the chip latched up. Really has me puzzled. I wonder if the action of pulling the cable caused something like switch bounce. But would switch bounce unleash huge current spikes? To melt the pins implies large currents for appreciable amount of time which leads back to a latch up condition maybe a supply decoupling cap was large enough value to hold a charge. Was there a minus voltage on the board ? I'm sorta just thinking out loud

Hi Mike,

Yes the fault is odd,

It sound like, as you say, something catastrophic occurred: latch up, supply pins reversed, short etc.

I am considering a problem with the ground connections. Do you think that if the analog and digital grounds got separated that the MAX9611 chip would self-destruct. My feeling is that it would.

spec

 

[vne147]

How were your psu remote sense terminals connected?

I'm not using the remote sense feature. If you look at the data sheet for the AE-1500-48 in my original post, on CN2 I have pin 1 jumpered to 2 and pin 3 jumpered to 4. There is only about 6 inches of 8 AWG wire between the output of the power supply and my board, and then about another 48" inches between my board and the test load. I'm just accepting whatever line loses there might be.

The melted pins almost sounds like the chip latched up. Really has me puzzled. I wonder if the action of pulling the cable caused something like switch bounce. But would switch bounce unleash huge current spikes? To melt the pins implies large currents for appreciable amount of time which leads back to a latch up condition maybe a supply decoupling cap was large enough value to hold a charge.

Agreed, although the pins are pretty small.

Was there a minus voltage on the board ? I'm sorta just thinking out loud

Possibly. Unfortunately, I have no way of knowing at this point. I'm not sure where a reverse voltage could have come from. The load is just a big power resistior.

Hi Mike,

I am considering a problem with the ground connections. Do you think that if the analog and digital grounds got separated that the MAX9611 chip would self-destruct. My feeling is that it would.

spec

Agreed that if the grounds became disconnected, the chip could self destruct. I'm not sure how that could have happened though. The two ground planes are connected to one another on the bottom side of the PCB through 3 SMD components that are placed immediately adjacent one another. I wasn't touching the board at all when the fault occurred. I was only touching the multimeter.

Additionally, that ground plane was connected after the fault. If it wasn't, many of the components that continue to work, wouldn't. I suppose it could be a really smart intermittent fault.

 

[vne147]

I have an update. I just got a few more of the MAX9611 ICs in the mail this morning and tried to recreate the problem. I was unsuccessful.
I tried it both with and without a TVS diode installed across the power supply output rails. No smoke. Maybe it was a defective part, or a really smart intermittent problem.
I suppose I can call off the dogs at this point. What I was mainly trying to get out of this thread was a sanity check and that there wasn't anything big I was failing to realize.
Thanks everyone for taking a look at this for me.

 

[spec]

I have an update. I just got a few more of the MAX9611 ICs in the mail this morning and tried to recreate the problem. I was unsuccessful.
I tried it both with and without a TVS diode installed across the power supply output rails. No smoke. Maybe it was a defective part, or a really smart intermittent problem.
I suppose I can call off the dogs at this point. What I was mainly trying to get out of this thread was a sanity check and that there wasn't anything big I was failing to realize.
Thanks everyone for taking a look at this for me.

Well that is good news Max- glad you got the circuit working.

Can I make a suggestion which is completely emotion rather than science: change the resistor coupling the digital and analog grounds to a through hole half watt or higher component or other more substantial resistor than surface mount. I have a phobia about grounds- sorry.

spec

 

[Mikebits]

Well that is good news Max- glad you got the circuit working.

Can I make a suggestion which is completely emotion rather than science: change the resistor coupling the digital and analog grounds to a through hole half watt or higher component or other more substantial resistor than surface mount. I have a phobia about grounds- sorry.

spec

I was going to suggest making it zero ohms. Albeit the installed resistor is a small value, there will still be noise dropped across it, which may or may not be a problem in a mixed signal design.

 

[Mikebits]

Possibly. Unfortunately, I have no way of knowing at this point. I'm not sure where a reverse voltage could have come from. The load is just a big power resistior.

I think you misunderstood me, I meant are you using a bipolar design? i.e. +V/-V,.

 

[Mikebits]

What I was thinking the problem may have been was that the power supply regulation went from 2 ohm load to no load, thus the supply output momentarily increased above the 48Vdc, and perhaps exceeded the max volt in on the chip thus causing a latch up condition in the parasitic PN junctions of the device (Purely guessing mind you, but you may be able to check with a good digital scope triggered on the DMM disconnect).

While I was working at a Cell Site manufacturing company we had a fuse that would blow maybe once a month. Inevitably the codec was always fried along with the fuse. The Codec had +V and -V, and once in a purplish moon the fuse would blow (keep in mind these systems were for development so power cycling was quite frequent). Turns out that the chip was latching up once in awhile, and was related to the power supply (V+/V-) sequencing.
Lesson here is that although the problem may seem to be one of, it may show again some time later down the road, whether in the board you have now, or maybe in a later build.
Glad your up and running though.

 

[spec]

I was going to suggest making it zero ohms. Albeit the installed resistor is a small value, there will still be noise dropped across it, which may or may not be a problem in a mixed signal design.

Yes, agree zero ohms would be better.

spec

 

[vne147]

Can I make a suggestion which is completely emotion rather than science: change the resistor coupling the digital and analog grounds to a through hole half watt or higher component or other more substantial resistor than surface mount. I have a phobia about grounds- sorry.

I have wondered for a while whether or not I was doing this properly. Several years ago when I was working on my first mixed signal design, I searched for a reference or some good rules of thumb on how to separate ground planes. I remember there not being a lot of clear direction out there on this topic, or maybe I just didn't know what to look for. I can't remember how exactly I settled on the 0.1 Ω resistor, the 100 nF cap, and the 1 uF cap all in parallel. But, my goal was to minimize noise in the analog portion of the circuit, and for that purpose it seemed to work pretty well for me in the past. Admittedly though I didn't do a lot of experimentation with alternate configurations so I wouldn't be shocked in the least if this arrangement isn't ideal.

I can replace the 0.1 Ω resistor with a zero Ω one as you and Mike suggest. That's no problem. But that sort of leaves me with the question about what to do with the caps. Are they doing anything for me here? What would you think about simply removing them?

Also, what if I omitted the zero Ω jumper and just replaced it with a trace connecting the ground planes instead?

I think you misunderstood me, I meant are you using a bipolar design? i.e. +V/-V,.

Sorry, I did misunderstand you. I'm not using bipolar design. There are a couple of different logic levels on the board but nothing below 0 V, at least not by design.

What I was thinking the problem may have been was that the power supply regulation went from 2 ohm load to no load, thus the supply output momentarily increased above the 48Vdc, and perhaps exceeded the max volt in on the chip thus causing a latch up condition in the parasitic PN junctions of the device (Purely guessing mind you, but you may be able to check with a good digital scope triggered on the DMM disconnect).

I'm not sure if this is still a possibility in your mind after my clarification about there not being a negative supply rail, but this was my guess too. I hope to prevent this in the future through the addition of a TVS diode across the V+ and V- outputs. I selected the SMCJ51CA-13-F. I figure if our hunch is right, the TVS will prevent this condition in the future. If the problem was something else the TVS diode may be unnecessary, but shouldn't hurt anything.

Any thoughts on this?


Thank you, gentlemen!

 

[spec]

I have wondered for a while whether or not I was doing this properly. Several years ago when I was working on my first mixed signal design, I searched for a reference or some good rules of thumb on how to separate ground planes. I remember there not being a lot of clear direction out there on this topic, or maybe I just didn't know what to look for. I can't remember how exactly I settled on the 0.1 Ω resistor, the 100 nF cap, and the 1 uF cap all in parallel. But, my goal was to minimize noise in the analog portion of the circuit, and for that purpose it seemed to work pretty well for me in the past. Admittedly though I didn't do a lot of experimentation with alternate configurations so I wouldn't be shocked in the least if this arrangement isn't ideal.

I can replace the 0.1 Ω resistor with a zero Ω one as you and Mike suggest. That's no problem. But that sort of leaves me with the question about what to do with the caps. Are they doing anything for me here? What would you think about simply removing them?

Also, what if I omitted the zero Ω jumper and just replaced it with a trace connecting the ground planes instead?

Hi VNE,

Grounding arrangements on nearly all circuits is always a critical area, even in digital systems, where you might think that because digital is only two-state, grounding would not matter. The reason for this, as I have said many times before, is that the nice solid line you see on schematics is an illusion- there is no such thing as a solid line in the real world. If you ever want to see this illustrated, just get a good low- noise differential input scope and connect one differential input to what you think is the best ground point in your system and then with the other differential input measure other 'ground' points on the circuit.

There are two broad ways to approach grounding:

(1) Isolate
(2) Solid

With the isolate approach, which you have used, you have islands of separate grounds, so you may have an analog ground and a digital ground to make sure that the noisy digital signals do not interfere with the sensitive analog circuits. This approach was widely used up to around 1985, and you can see this illustrated with audio power amplifier design especially.

Alternatively, the solid approach does not separate the grounds. Instead, it uses as solid a ground as possible (but you still separate sensitive circuits from digital circuits by physical layout). For example, you can specify a thick copper ground plane. In one case I stuck a sheet of copper on to the printed circuit board to get a solid ground. And in another case I used 5mm square copper bus bars to give a solid ground.

So where is all this leading: the problem with the isolate approach is that often it will catch you out, especially on a large system,because sooner or later the grounds have to be joined, so the modern trend is to connect all grounds together on a solid ground.

Of course, this is a broad philosophy and the are situations where you would completely isolate one ground from another, but this is where the two grounds can be separate. The classic situation is a processor controlling an industrial welder, for example. In this case you would most definitely separate the processor ground and welder ground, normally by a transformer or optocoupler. Another example could be a precision oscillator. Here you may need a separate and isolated ground for the oscillator to ensure low jitter.

So after all that, yes, I would advise joining the digital and analog grounds on your circuit by a solid conductor, but only in one place to avoid ground loops. Obviously, you can then ditch the capacitors that you mentioned.

spec

 

[KeepItSimpleStupid]

This thread got lost somewhere. Power supply interfacing gets really tricky especially if it's a grounded supply.

Guess what side gets grounded? The positive output! That in my opinion is what caused your grief.

The setpoint is referenced internally and possibly externally to the + output and the output is referenced to ground.

When I worked with analog programming of a power supply, it was better to use isolated current loop for the programming the voltage/current with a resistor across the programming voltage pin and use a pseudo differential A/D converter to measure the output. The isolation interface was often an expensive option.

 

[spec]

This thread got lost somewhere.

???

 

[KeepItSimpleStupid]

???

Not following from the beginning made the thread difficult to follow. I just added my relevant $0.02 USD to the discussion. Cooking and eating for the recent Holiday took priority.

 

[spec]

Not following from the beginning made the thread difficult to follow. I just added my relevant $0.02 USD to the discussion. Cooking and eating for the recent Holiday took priority.

Oh I see- I miss points in previous posts all the time. It is a kind of word blindness (laziness too on my behalf).

spec

 

[vne147]

Hi VNE,
So after all that, yes, I would advise joining the digital and analog grounds on your circuit by a solid conductor, but only in one place to avoid ground loops. Obviously, you can then ditch the capacitors that you mentioned.

Thanks for the advice. I removed the resistor and caps separating the different ground planes and replaced them with a solid copper trace. I also added a TVS diode across the supply rails of the Cotek power supply just in case.

One final question, at least I hope it will be the last one. It has to do with the MAX9611, specifically what to do with the SET and OUT pins. Should I leave them floating?

I am not using the features of the IC that those pins are used for and I have searched the datasheet for guidance on what to do with them in that case but I've come up empty. Any thoughts?

 

[spec]

Hi VNE,

[the question] has to do with the MAX9611, specifically what to do with the SET and OUT pins. Should I leave them floating?

I am not using the features of the IC that those pins are used for and I have searched the datasheet for guidance on what to do with them in that case but I've come up empty. Any thoughts?

Connect SET to 0V and leave OUTPUT unconnected. In general, all inputs of an integrated circuit should be set to a defined state or they may assume any voltage and cause weird effects.

spec

 

[vne147]

Connect SET to 0V and leave OUTPUT unconnected.

It is done. Thanks!