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TPS54623 noisy pulse width on the supply

Other Parts Discussed in Thread: TPS54623, TPS54622

We have a design with a number of TPS54623 buck converters, all running from a rail of 12.6 V. We see a noisy pulse width on the supply with the lowest output voltage, 1.2V. A demo board with the same circuit and output voltage produces the same problem. The situation is made worse by higher input or lower output voltages; probably, more specifically, the ratio between the two. Matters are also made worse by decreasing the output current or decreasing the output inductor value. If conditions are pushed far enough, I see the pulse width break into alternating wide and narrow pulses. I believe that what I'm seeing is referred to as sub-harmonic oscillation, found on current-mode devices. Is there an easy fix for this condition? There does seem to be a weak interaction with the compensation network, which may only be indirect. It would seem that the 47 pF that comes with the demo board actually worsens the situation.

  • What happens if you increase the inductor value and/or increase the output capacitance?  Are you using a low ESR inductor and output capacitors?

    I notice the TPS54622 data sheet has small signal circuit models for loop response, current mode control, etc.  You could evaluate loop response and check gain and phase margin with a circuit simulator.  Then, optimize the compensation network as you simulate gain and phase margin.  

    Also, you could evaluate loop response in the lab if you have the equipment.  I've used something called a "Ridley box" for this in the past.    


    Eric Hooker

    RF/analog Consultant

  • Can you post your schematic and some waveforms that demonstrate the behavior?

  • I would refer you to the schematic for the demo board (image of schematic attached) with the following changes:
    Lower voltage to 1.2V, 10kΩ for R7.
    Frequency to 380kHz by R3 = 127kΩ.
    Compensation, R4 = 4.99kΩ.
    Change inductor L1 to 4.7μH.

    With a 1.2A load, the pulse width is "stable" at 10V input, although there is noise visible.  Switching to 12.6 V input, the pulse width breaks into the split pulse width cycle, wide-narrow-wide-narrow.  This is shown in the attached images.