A customer is inquiring about the VrefP and VREFN networks is because he is seeing a failure at -40C on my units related to this part and he has a hard time figuring out the root cause. He changed the capacitor values in the decoupling network to match the datasheet and replaced the tantalum capacitors with ceramics to reduce the ESR. He is still seeing the failure.
During his test sequence the unit is powered on and off several times. When everything is working, VREFP comes up at 4V. When he has a failure, VREFP only comes up to 2V, and I draw 50mA less than usual on the 3.3V supply. When this happens the part does not function and I fail my test.
For the following Plots:
CH1: 5V supply
CH2: 3.3V supply
CH3: VPref (ADC)
CH4: VNref (ADC)
This is a plot of everything working normally (CH 3 is 2V/Div)
See attached image1
This is what happens during a failure
See Attached image 2
One of his coworker saw this failure on his program a few years ago. He had to cycle power 3 times before he saw failures. He determined the failure was due to residual voltage on the 3.3V supply during power-on. He added a bleed resistor and the problem went away. He is seeing this failure sometimes as soon as he powers on the first time (see second plot), so there is no residual voltage. He is working on getting date codes off of failed units.
I would appreciate any suggestions you can give me.
Regards,
Naser
