LM3670 Stability

Charles Gervasi

Other Parts Discussed in Thread: LM3670

I'm seeing instability in my circuit based on LM3670 1.8V. Sometimes it comes up and it's oscillation decays. (yellow is input)

Other times it goes into oscillation:

Here is what my schematic looks like:

Here is my layout. The white square is the outline of the inductor on the other side of the board. There is a solid ground plane between the inductor (L5) and the parts show.

What could be causing this instability?

over 10 years ago

0 Charles Gervasi over 10 years ago

Expert 2270 points

I pasted the wrong plot for the one showing 1.8V oscillation. Here it is.

0 Charles Gervasi over 10 years ago in reply to Charles Gervasi

Expert 2270 points

I changed my inductor from SRR1210-330M to CDRH4D28NP -6R8NC. The problem appears to go away.

Does anyone have a guess as to why?

0 David Baba over 10 years ago

TI__Mastermind 35920 points

Hello,

Are you sure that the part has not entered into PFM mode? Has this been ruled out?

What is your load current at start up?

0 Charles Gervasi over 10 years ago in reply to David Baba

Expert 2270 points

David Baba said:

Are you sure that the part has not entered into PFM mode? Has this been ruled out?

Maybe I'll put the old inductor on to recreate the problem and then see if increasing the load makes the apparent oscillation go away. Thanks! CJ

0 Charles Gervasi over 10 years ago in reply to David Baba

Expert 2270 points

David Baba said:

Are you sure that the part has not entered into PFM mode? Has this been ruled out?

What is your load current at start up?

I tried putting 100-ohm and 10-ohm loads on the 1.8V output. In both cases it oscillates like this.

Is this known behavior with large-value inductors? Is there anything I can do to be sure I'm not near the edge of stability with my lower-value inductor?

0 David Baba over 10 years ago in reply to Charles Gervasi

TI__Mastermind 35920 points

Yes, you can try a lower value inductor it will have some effect. even better you can even try a R in series with Cout - try a 0.1ohm...

0 Charles Gervasi over 10 years ago in reply to Charles Gervasi

Expert 2270 points

Should the series resistor make it more or less stable?

It's working great with the 6.8uH. I just want to make sure it's not on the edge.

0 David Baba over 10 years ago in reply to Charles Gervasi

TI__Mastermind 35920 points

more stable if you fear you do not have enough phase margin...

0 Charles Gervasi over 10 years ago in reply to David Baba

Expert 2270 points

David Baba said:
more stable if you fear you do not have enough phase margin...

It did not oscillate when I added 1 ohm in series and used the 33uH. (I didn't have 100 mohms.)

Do you believe the solution is low-value inductor? Should we add a series resistance? Should we use tantalum cap instead of a ceramic for its higher ESR?

Thanks!

0 David Baba over 10 years ago in reply to Charles Gervasi

TI__Mastermind 35920 points

The smaller inductor may help. A higher ESR Cout will certainly help, a Tantulum may work depending on the ESR; I think you will need at least 100mR of ESR.

0 David Baba over 10 years ago in reply to David Baba

TI__Mastermind 35920 points

PS the Smaller inductor works because the LM3670 (voltage Mode Control) is internally compensated and the Zeros (2 of them) are placed in the vicininty of expected double Pole formed by the L and C.

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LM3670 Stability