LM2576 Waveform

[dan3460]

I'm using a LM2576 5.0 to power a project. This is the first time that I have use that chip. From this power supply I have a LM350 that gives me a 3.3V supply, on the project there are parts that need 5V and 3.3V. After building this thing on a proto board I'm only get 4.89V, the only load that I have so far is the LM350 and its ancillary circuit.
I put the scope at the output pin on the LM2576 (pin 2) and I get the form shown below. I followed the circuit described on page 19 of http://www.ti.com/lit/ds/symlink/lm2576.pdf the only difference is the diode which I used a 1N5819.
Is the waveform shown below, normal. It seems to me that there is a lot of ringing. The square wave part is running at about 54kHz, the ringing at about 1.31Mhz. The top of the square wave is at 11.75V. The Vin is 12V.

[Benta]

It's normal. It happens when it's running with no load (or almost no load).

More precisely, it happens when there is no continuous current through choke. If choke current drops to zero, it will ring at the choke's parallel resonant frequency, which is the inductance in parallel with the parasitic capacitance between the windings (plus some smaller parasitics).

Give it a load larger than half choke peak-peak ripple current, and all will be fine.

[David Hess]

I agree; that looks like the normal discontinuous conduction waveform which occurs at low output current.

There are various ways to solve that if it is a problem including adding a snubber or increasing the inductance.