Electronics > Beginners
SMPS Ringing Help
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mgbolts:
Hi There,

I have been building a bunch of different SMPS for a few months and are starting to get some OK results for my needs.  Still a long way to go, its certainly a whole bunch of electronics knowledge to learn for just one circuit!

My latest project is to build a ~0.75amp super small 5V reg.  I am pretty happy with the result but.....  There is minimal ripple a lot of ringing on the output.

I have tried various comp values will little effect.  Also tried more output capacitance, little improvement.  The biggest improvement came from changing the diode from a cheap Chinese ss34 to an ON-Semi number. 

Facts:

MP4459 Reg
Input Caps = 1000pF, 0.1uF and 10uF
Output Caps = 1000pF, 0.1uF and 20uF
MBR230LSFT1G 2A 30V diode
4.7uH 1.9A Inductor

The chip is running at ~1.8mhz with the ringing in the high 10mhz (+50mhz)

Whats annoying me is that the power appears clean but for the ringing....I was hoping to get it down to sub 50 peak to peak.  The power supply is 15v and output is 5V. I am feeding from a lipo to ensure clean supply.  The probe is using sprung wire for the ground ......  I am also running AC coupling and 20M BW limit.

Any ideas appreciated.

Thanks
mgb
mgbolts:
Some quick labels on the layout....
StillTrying:
"running AC coupling and 20M BW limit."

I'm surprised you're seeing 90 MHz ringing.
mgbolts:
A good point, I will rerun and take a pic....
mgbolts:
I had another look.  Definitely set to 20M BW limit.  I have the scope running at 10ns per division with 20M BW limitin the attached pic  The sinusoidal ring has a period just under 1 division which calcs to a frequency of ~80-100 mhz...

Also,  pic of the probe setup..
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