Author Topic: Why did my LT3042 die?  (Read 615 times)

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Offline giosif

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Why did my LT3042 die?
« on: April 03, 2021, 09:26:11 am »
Hi,

I bought one of those little boards found on eBay which use an LT3042 to provide low noise voltage supply.
I set the output voltage to 15V by changing Rset to 150 kOhms, fired it up and all looked good.
I then used it for about 1 hour or so, drawing around 50 mA from it.
During this time, I noticed the LT3042 IC was getting rather hot to the touch, but thought this might be normal.
After the 1 hour, though, the IC died and I am trying to understand the reason for that.
Looking at the datasheet for the IC (to the degree I can read & understand it), I don't think I've exceeded any of its operating parameters (except maybe for heat dissipation?).

Anyone has any ideas?

Of course, this being bought from eBay, that IC could be anything *but* an original LT part, so that is one possible explanation, but I'm trying to see if I might have overlooked something else.

Thanks!
 

Online 2N3055

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Re: Why did my LT3042 die?
« Reply #1 on: April 03, 2021, 09:36:24 am »
What was the input?
 

Offline giosif

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Re: Why did my LT3042 die?
« Reply #2 on: April 03, 2021, 10:41:52 am »
Sorry, forgot to mention that: input was 20V.
 

Offline PeteH

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Re: Why did my LT3042 die?
« Reply #3 on: April 03, 2021, 11:35:21 am »
What did your wire harness look like?

Running the part at 20V when the abs max is 22V with default decoupling on the eval PCB is risky.

Does your load ever change in current?

The chip mounted on a fairly large PCB should be able to handle the ~300mW of loss but would get warm depending on the layout of the board.... Could get very hot if not done well.
 

Offline Jay_Diddy_B

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Re: Why did my LT3042 die?
« Reply #4 on: April 03, 2021, 01:08:20 pm »
giosif and the group,

Are you talking about a board like this one?

[attachimg=1]


It has ceramic input capacitors. It doesn't have an electrolytic capacitor that will provide damping. Have a look at this application note. The application note explains why you need damping.

Link: https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdf

This is probably why the LT3042 failed.

Jay_Diddy_B
 
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Offline giosif

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Re: Why did my LT3042 die?
« Reply #5 on: April 03, 2021, 04:08:28 pm »
Yep, that's the board.
Ok, I will read the application note.
Thank you!
 

Offline giosif

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Re: Why did my LT3042 die?
« Reply #6 on: April 03, 2021, 04:18:06 pm »
What did your wire harness look like?
Amm... Two simple wires going from by bench power supply into the input pins of the board?
I'm not sure what you mean with the question.

Running the part at 20V when the abs max is 22V with default decoupling on the eval PCB is risky.
I am aware of the 22V max limit but, for an output of 15V, I needed to go above that.
Also, one of the typical application examples is showing 20V at the input.

Does your load ever change in current?
Yes, but I don't think it would stray way too much from the 50mA.

The chip mounted on a fairly large PCB should be able to handle the ~300mW of loss but would get warm depending on the layout of the board.... Could get very hot if not done well.
This is the small board Jay_Diddy_B mentioned above.
 

Online 2N3055

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Re: Why did my LT3042 die?
« Reply #7 on: April 03, 2021, 06:41:52 pm »
What did your wire harness look like?
Amm... Two simple wires going from by bench power supply into the input pins of the board?
I'm not sure what you mean with the question.

Running the part at 20V when the abs max is 22V with default decoupling on the eval PCB is risky.
I am aware of the 22V max limit but, for an output of 15V, I needed to go above that.
Also, one of the typical application examples is showing 20V at the input.

Does your load ever change in current?
Yes, but I don't think it would stray way too much from the 50mA.

The chip mounted on a fairly large PCB should be able to handle the ~300mW of loss but would get warm depending on the layout of the board.... Could get very hot if not done well.
This is the small board Jay_Diddy_B mentioned above.

Like always, JDB is right on the money...
If you have a meter of cable and ceramic capacitor it can ring really bad. Also, one thing people forget, most of lab PSU have inductive output impedance(hence big elcos on the output, to compensate).
Also LT3042  is a low dropout regulator that can work with 350mV drop on regulator, i.e. for 15V you need 15.35V on input. 5V is way too much. 16V would have given you healthy reserve.
 

Online coppercone2

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Re: Why did my LT3042 die?
« Reply #8 on: April 03, 2021, 06:44:18 pm »
I put a 3042 on a little home made PCB a while back and it also died, but I got mine from digikey.

These guys are sensitive, I think it was ESD, I was pretty careful soldering it but my ESD mat was not hooked up because I moved a buncha stuff and cleaned and forgot to plug it back in.

Anyway I cut the PCB in half and ran it on batteries, the 3042 can go on a power supply board if I ever finish that amplifier.

They are touchy. I recommend sticking with 3080 series unless absolutely necessary
« Last Edit: April 03, 2021, 06:48:33 pm by coppercone2 »
 

Offline giosif

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Re: Why did my LT3042 die?
« Reply #9 on: April 03, 2021, 09:22:28 pm »
Like always, JDB is right on the money...
If you have a meter of cable and ceramic capacitor it can ring really bad. Also, one thing people forget, most of lab PSU have inductive output impedance(hence big elcos on the output, to compensate).
Something I learned today.  :)

Also LT3042  is a low dropout regulator that can work with 350mV drop on regulator, i.e. for 15V you need 15.35V on input. 5V is way too much. 16V would have given you healthy reserve.
Indeed. I should have read the datasheet more carefully, but I was being impatient...
 

Offline giosif

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Re: Why did my LT3042 die?
« Reply #10 on: April 03, 2021, 09:29:03 pm »
[...]
They are touchy.
Yes, I learned this first hand.

I recommend sticking with 3080 series unless absolutely necessary
I will check the specs for the 3080, but I'm after something with very low noise.
 

Offline Jay_Diddy_B

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Re: Why did my LT3042 die?
« Reply #11 on: April 04, 2021, 05:19:28 am »

Snip ...

Also LT3042  is a low dropout regulator that can work with 350mV drop on regulator, i.e. for 15V you need 15.35V on input. 5V is way too much. 16V would have given you healthy reserve.
Indeed. I should have read the datasheet more carefully, but I was being impatient...

giosif and the group,

Be careful, although the LT3042 can operate with only 350mV of input to output differential:

[attachimg=1]


Other parameters will be compromised if operated with a low input to output voltage differential: This is PSRR:

[attachimg=2]


Output noise spectral density:

[attachimg=3]



I would choose a differential voltage 1.3V if I want the best PSRR and noise performance. Typical applications for the LT3042 are normally for lowest noise and highest possible PSRR. To allow for tolerances in a production environment you may need to allow more voltage. If the input supply is 5% and the LT3042 is 2% and the Vset resistor is 1%, a larger differential may be required. If is for home use, you can tweak the circuit.

Best Regards,
Jay_Diddy_B
 
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Offline bingo600

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Re: Why did my LT3042 die?
« Reply #12 on: April 08, 2021, 04:33:59 pm »
For an Non EE person...

The headroom (Vin) should be around 1.3v above (Vout) ?
After reading the AN , am i correct in understanding that one should leave the ceramic input cap , and add an additional 47uF Elco in parallel ?

I have a few of these boards , that i expect to use for some OCXO regulation

/Bingo


 

Offline Jay_Diddy_B

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Re: Why did my LT3042 die?
« Reply #13 on: April 09, 2021, 12:03:24 am »
Bingo600,

Choosing the headroom, is a complex question, the answer is (as always) it depends.

1) You have to consider power loss in the regulator. The power loss is:

(Vin - Vout) x Iout

The maximum dissipation depends on the PCB, but 1 to 1.5W would be a good starting point. This may limit Vin-Vout.


2) You have to decide if you *need* the highest possible noise performance and PSRR. You might be able to operate with a Vin-Vout < 1.3V

3) You may want to consider what rails you have available, You may also be able to choose the output voltage.

4) You may need to consider the tolerances of the LT3042 and the supply being used to feed it. This is to make sure that you get the performance you need under all conditions.


In short, AN88, suggests placing an electrolytic capacitor that is 3 or more time the value of the ceramic capacitor in parallel. The ESR of the electrolytic will dampen any resonances. In most cases a 47uF electrolytic will be fine. It is if you 'hot plug' or hard switch the input supply.

Jay_Diddy_B
 


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