I am using the TPS54320 for several supplies. Some show a non-monotonic startup, and I'm not certain what is different about the affected supplies.
Some details on one of the supplies:
Vin=12V, Vout = 1.9V
Fsw = 750kHz
Tss = 2ms
Lout=6.8uH, Cout=66uF, Imax=1.25A
Rcomp=1.87k, Ccomp=22nF
Measured bode plot looks good (Fc = 40kHz, PM = 70 Deg), as does the transient response. All indications are that the supply is well compensated.
Can anyone provide some troubleshooting suggestions here? The startup waveform is non-monotonic with any load, from about 0V to 800mV. The non-monotonicity looks like a damped sine wave riding on top of the desired ramp -- the frequency of oscillation is 16kHz.
I have probed the soft-start pin, enable pin, frequency set pin, and Vin and none show any oscillation.