I have had two buck converters (from different vendors) fail in very similar ways.
The first was blamed on electrical over-stress (i.e. too high of a voltage was applied to a logic-level pin) – the second is as-yet-undetermined but suspect the same.
Both controllers have weak internal current sources to pull-up their enable lines, so they turn on if EN is unconnected. The EN threshold is 1 V.
Both controllers failed and don't turn on. EN is ~0.64 V – suspicious Si diode level…
When EN is externally pulled up, OUT is approximately 1/2 the expected value, i.e. a 5 V rail regulates to 2.4 V, and a 12 V rail regulates to 5.8 V.
Obviously something in the controller is damaged. The question is what could cause this failure, and what subcircuit of the controller would cause it to continue to regulate, but at 1/2 its setpoint?
Best Answer
It's distinctly possible that the enable pin is actually controlling the setpoint of the converter. Pull it down, and the setpoint becomes zero. Pull it up to 1V, and the setpoint becomes whatever the internal reference is. It's imaginable that the damage is to the internal amplifier or reference or pullup rail, in which case the setpoint will also be different.
Obviously this is just speculation. But I'd be curious to know what happens if you force the enable pin to 1V with an external source.