LM3488 unable to start below 32 degrees F

Hi Darrell, 

We will review the schematic and provide feedback. The controller should work down to -40C.

Here are a few questions to get started:

Would it be possible to provide the board layout as well?

Has anything changed in the design?

Was the design originally tested at low temperature?

Regards, 

Denislav

Hello,

could you also provide me waveforms of:

1. switching point (drain of the MOSFET to GND)

2. Vout

3. Vin-pin

4. FB

all the waveforms with and without load, at cold temperature and ambient temperature.

Thank you,

Giuseppe

First let me tell you what I mean when I say it does not start, this is not totally accurate.  The unit starts but puts out very low voltage at load (6.5 - 9 V).   Removing 60% of the load and the unit starts at 9.5 and then works its way up to 11.9 when at zero degrees F or for that matter even at 32 degrees.

And this has been changed from the original schematic.  Reffering to same, R2 is now .027, R6 is now 20k (tried 36.5 as per webench), 20k gets me about 1.3us switching time on the drain of the FET and did help the problem.  When cold started the drain seems to go discontinuous.  Only periods of 1.3us with dead spots.   L2 is 33uh, 100ohms was added in series with pin1 and the source of the FETand C4 was doubled in value to 4.7uf MLCC.  Most of these changes where recommended by WEBench.

I suspect the internal voltage referance is drifting.

I also suspect possible ground noise and I am testing for this now. Except I can not seem to post more that one file at a time, I have sent you more replys with the layout.

V-in does not seem to make any differance 9 to 15 nominal and testing is done at 10v. I will try and get some waveforms captured.  I am posting the layout now.

Thanks

cold at normal load

Room temp normal run

Warming up

bottom layout

Top layout

Thank you for any help.

I have posted in the blog the doc's you asked for.  One interesting note is I have added some noise decoupling (3300pf) to the FB pin and this helped, but did not cure the problem.

Please read my other post.

Ti can contact me directly at dpruehsner@vergentproducts.com           (new company name)

Hello Darell,

we are planning to reproduce your schematic in the lab and test it out over temp.

Is there any change from the original schematic you sent here?

Regards,

Giuseppe

• You may not have seen this post.   All of the changes listed did help the startup, but nothing cures it.  I can send a sample of mine if you like.  Waveforms and the layout have also been posted.

  Normal load 1.25 amps

  input range is 10 to 15 volts

  designed for 2 amps max output.

  Darrell Pruehsner

   

  First let me tell you what I mean when I say it does not start, this is not totally accurate. The unit starts but puts out very low voltage at load (6.5 - 9 V) even with a nominal 12 volt input. Removing 60% of the load and the unit starts with the output at 9.5 and then works its way up to 11.9 when at zero degrees F or for that matter even at 32 degrees.

  This has been changed from the original schematic. Referring to same, R2 is now .027, R6 is now 20k (tried 36.5 as per webench), 20k gets me about 1.3us switching time on the drain of the FET and did help the problem. When cold started the drain seems to go discontinuous. Only periods of 1.3us with dead spots. L2 is 33uh, 100ohms was added in series between pin1 and the source of the FET, C4 was doubled in value to 4.7uf MLCC. Most of these changes where recommended by WEBench.

  I did suspect noise on the FB pin so I added a 3300pf to ground, this also seemed to help a bit.

  I suspect , or it appears that the internal voltage reference is drifting.

  I also suspect possible ground noise and I am testing for this now. Except I cannot seem to post more than one file at a time, I have sent you more reply’s with the layout.

  V-in does not seem to make any difference 9 to 15 nominal and testing is done at 10v. I will try and get some waveforms captured. I am posting the layout now.

   

Any word on how the testing is going?   Is there anything else you need from me?

Darrell