In recent batches, we're having problems with the EP5368QI not starting up at -30°C and below. Of a batch of 4 devices, 3 show this behavior (the fourth works normally).
I have attached (part of) the circuit diagram. The PSU-PWR-GOOD signal is an open-collector signal that goes high when the power supply is within 10% of the nominal value (so at about 3.8V). On the +3.3V output there is some additional capacitance of about 50 uF in total (not in this part of the schematic). Attached are also measurements of startup at various temperatures and time scales, where input (Ch1), output (Ch2), PSU-PWR-GOOD (Ch3) and the switching node on pin 15 (Ch4) are displayed.
- At room temperature (25°C) startup is normal
- At low temperature (-30°C) the power supply does not start and enters a hiccup mode
With smaller timebase, it can be seen that the hiccup mode consists of approximately 16 switching cycles followed by about 60 uS of non-switching.
The really weird thing is, that I can break the hiccup mode by injecting a small current into the output. If I apply 3 V through a 100 Ohm resistor (so about 30 mA), the supply starts and keeps running even if I remove the resistor. If after a minute or so I remove input power for a couple of seconds and then re-apply the power, it will also power up normally.
In addition to the above, I have verified the output capacitance over temperature. At lower temperature, the capacitance gradually drops from 75.3 uF @ 20°C to 64.1 uF @ -40°C. So it's not that there is a cracked capacitor on the board that shorts out the output at lower temperatures.
I've also replaced the 22 uF capacitor on the output by a 47 uF one. That worsens the behavior, in that it will now fail to startup at temperatures below -5°C (instead of -30°). We've been producing the boards on which these capacitors are since 2011 or something. Since somewhere around August 2018, we're seeing boards failing to startup at low temperature, but not in numbers we're seeing now.
I just sent a personal email to you to request FA (failure analysis) for this case and i hope your cooperation to fill the FA form so i can proceed with FA.
I will let you know the next step once i received the FA form from you.