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LM34910C Output spikes

Thomas Hansen
Prodigy 60 points
Other Parts Discussed in Thread: LM34910C, LM34910

Hello there

I have an issue with my 5 V generated from a LM34910C.

I see short spikes on the output, with a period of 5 nsec ringing out in a couple of periods.

The time between spikes depend on the load.

I have used components specified by WEBENCH.

There is a scope picture below:

What can I do to remove these spikes?

Best regards

   Thomas Hansen

  • 0
    TI__Guru**** 191445 points

    You cannot tell form your picture, but most likely they are "switching noise".  Do the spikes align in time with the rising and falling edges of the switching node (ISEN pin of LM34910C)?  If so then it is in fact switching noise.  You have to be very careful about measuring those.  It is best to use a very short ground lead or better still "tip and ring" method directly across the output capacitor to accurately observe that noise.  For countermeasures, you can add an RC snubber from ISEN to GND and include small value ceramic capacitors at the output (0.1 uF, 0.01 uF, 0.001 uF, etc).

  • 0 in reply to JohnTucker
    Prodigy 60 points

    Thank you for the info.

    I tried shortening my ground wire for the oscilloscope, which greatly reduced the amplitude of the spikes, so I think the loop was most of the problem.

    I will add footprints for a snubber on ISEN and also add more filtering on the output.

    Thank you for your help.

    Best regards

       Thomas Hansen

  • 0 in reply to JohnTucker
    Intellectual 525 points

    Sorry to post in this manner but the site simply is not intuitive as to starting a new question, so;

    LM34910   Vin seems to be acting like it has a 7v  zener internaly.  Key difference between my eariler successful design with the LM34910 is that this time the thermal pad is not grounded.  Although the datasheet said 'can be grounded'  I wounder if this is related and thermal  'must be grounded'  ?

    Checked for shorts etc. Nothing amiss.

    Please reply as soon as possible.

    Thank you.

    Jaye

  • 0 in reply to Jaye Waas
    TI__Guru**** 191445 points

    i don't normally support this device.  I'll monitor it to see if you get an answer. 

  • 0 in reply to Jaye Waas
    Intellectual 525 points

    Well, I think I solved the issue. Seems that the LM34910 does not apprecieate 200 Ohm resistive load attached to the inductive output filter.  When I increased the resistor value to >2K then the Vin was not clamped near 7v.  Along the way I noticed that the low ohm load can cause the internal Fet switch to lock into the 100% on state which in turn upset the internals, draws a lot of current through the fet switch and clamped at 7volts. But the chip did not become hot.  I increased the D.C. load to >2K and everything operates correctly.  Interesting that the total current load, in my case, suggests a load of 10 Ohms will work. Just not at start-up time.

    My application uses the LM34910 switch to drive the gate of a high power MosFet to create a 2Kw D.C. output. earliy testing shows it will make the goal.

    Thank you.

    Jaye