I've never had issues with TI Webench online tool before but lately it's been either misleading, or maybe I'm reading it wrong. Seems like the more time I spend trying to design a power supply the more it sucks..
Goal is a 48V->3.3V supply at 300mA, nothing hard. Attached circuit. According to the TI tool I remember the output voltage ripple being <10mV. But, what seems is the controller works in bursts: bursts a couple pulses, inductor goes to 0 (DCM), then rinse and repeat. While the voltage ripple during the pulses is well under 10mV (I mean it operates at 2.1MHz), the second-order ripple due to this bursty behavior at around 100KHz is something like 200mV, completely unusable for my application.
I don't see how the above could be an issue with ECO mode since this persists even at 600mA full output current. Only thing I can think of is there is a minimum ON time on the switch which happens to generate a voltage above 3.3V, enforcing burst mode?
I even double checked the output ripple voltage and other parameters using the datasheet provided equations.. So what gives? The layout is plenty tight, almost no visible ringing during the switch transitions (except low energy ringing whenever inductor current reaches 0, as expected).