We designed LM2594HVM-ADJ/NOPB on our SS3 MLB. It has been in production in many year. It is used in a negative switcher topology to generate regulated -15V. The input voltage to the switcher can vary from 19V to 33V.
Recently, it had a failure in the field. This was confirmed by our QA lab.
In the old National Semi data sheet (Dec 1999) on page 18, it used to say that it's okay to left the ON/OFF pin open (regulator will be in ON condition). This is how we designed it on our board.
ON /OFF —Allows the switching regulator circuit to be shut
down using logic level signals thus dropping the total input
supply current to approximately 80 μA. Pulling this pin below
a threshold voltage of approximately 1.3V turns the regulator
on, and pulling this pin above 1.3V (up to a maximum of 25V)
shuts the regulator down. If this shutdown feature is not
needed, the ON /OFF pin can be wired to the ground pin or
it can be left open, in either case the regulator will be in the
ON condition.
After TI acquired National Semi, they made revision changes to the data sheet of the National Semi switchers (revised April 2013). I noticed similar changes on other switcher devices. The TI data sheet looks difference. I can't find the note about the ON/OFF pin that said it's okay to leave it open. I wonder if the change is for the data sheet only or there is actual change made to the regulator IC. Is it still okay to leave the ON/OFF pin open for normal operation? Is there any other actual changes made for LM2594HVM-ADJ/NOPB from Dec 1999 till now?
I'll email in the National Semi data sheet for your reference to the ACAT team. The TI data sheet is available thru web.
