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TPS54227 ripple - switch waveform appearing on output

ctw
Prodigy 20 points
Other Parts Discussed in Thread: TPS54227

Hi all

I have an unwanted amount of the switch waveform (square wave) appearing on the output using a very standard TPS54227 design (15V - 1.9V).

The waveform looks extremely similar to figure 13 in the datasheet, p14 - www.ti.com/.../tps54227.pdf

Except that the square pulses are large enough at about 40mV to be worrisome, regardless of load.

In my experience of sync buck circuit theory and from checking simulations it is not caused by current flow in the circuit, even if a ground was bad or solder joint was dry - the filter would still lead to the textbook capacitor ripple shape.

This leads me to blame either

a) layout - i followed the layout guidelines quite closely but may have tried to miniaturize a bit too much

b) measurement technique - we are measuring using a tight loop with gnd spring - but might be a fairly cheap probe.

Thankful if you have any similar experiences/advice.

-Cal



  • 0
    TI__Guru**** 191445 points
    That square type waveform can come from two sources. You may be picking up some coupling of the switch waveform. Usually that is from grounding of the probe or too long a ground return on the probe. You need to probe directly across the output capacitor(s) using a very short (< 1cm) ground lead. Second, it could be the ESL of the capacitors is too high or you have other parasitic inductance present (GND or VOUT traces to thin).