Above LM22670 based circuit is used for the DC DC conversion from 12 V to 5 V and is used for our GPS based application. The switching frequency is adjusted to 500KHz by using R32, L3 and C71, whose values are chosen using WebBench Software tool.
But our unit is causing interference to another communication equipment working at 75 MHz, when our unit isi powered. There is also an internal battery present supplying 3.7 volt and charged through LM22670. No interference is seen when our unit is powered using internal battery. Also there is interference upon isolating power supply to our unit and the above communication unit.
So we have infered that the interference is caused by radiated emission from the LM22670 power switching circuit.
The project is developed as a GPS Vehicle Tracking System and is used by Kerala Police for Dial100 system. The noise affected communication system is Motorola GM950 plus and the same is used by Kerala Police.
Please suggest some remedies for overcoming the interference.
Here are our findings and comments:
1) The input capacitor in the EMI filter seems to be very small. I believe you have 1uF capacitors. That ought to be atleast 20 to 30uF. You could have a 75MHz ringing there which is going through.2) You could put an RC snubber from switch to ground. This will not completely cure the problem. But it will reduce the ringing at the switch and consequently at the output. To design the snubber, you could visit this website: http://www.ti.com/ww/en/analog/power_management/snubber_circuit_design.html3) Are your 100uF caps at the output Aluminium Electrolytics or Tantalums? If they are Al-Els, then you might want to consider switching to tantalums. Additionally the 0.1uF cap is probably not doing much. What you need to do is add a few more small caps at the output whose self resonant frequency (SRF) is null at the ringing frequencies that you seeing. You could scope the output and measure the ringing frequencies and accordingly choose the capacitors. I have also attached a presentation file that you could refer to.4) We would also like to look at your layout for the LM22670. Please send us some images when you get a chance. If the layout is not optimized, there will be a lot of ringing that could be causing the problem. I am also attaching another presentation on mitigating EMI by optmizing layouts in a different reply to this post.
I hope this helps.
Here is the presentation on the layout optimization.
Thanks for the quick response Mr. Akshay.
I am using 100 uF tentalum capacitor at the output. I will definitely try to make corrections in my design as you have suggested and will get back to you soon.
Thank you once again for sending me a nice presentation.
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