Hi all
My customer has implemented a window comparator circuit as illustrated in the below schematic with the LM339.
Under normal operation the output of U7 is high. If a fault condition occurs the output of U7 is pulled low.
For one PCB they find the output of U7 is stuck low regardless of the input, replacing the IC fixes the problem.
Do you see anything wrong with this circuit ?
Do you see a problem with e.g. C21 10nF capacitor at the output of U7 ?
Best regards
Ueli
Hi Ueli,
in my opinion it's very bad design practise to directly discharge a cap with the output of an comparator, without any current limiting, especially if this cap is not directly referenced to the ground pin of comparator. By this an enormous discharging current spike will run across complex ground impedances resulting in a huge ground bounce, not only outside the comparator but even inside the comparator itself, directly on the die. This can cause instability and even latch-up.
See this huge 82mA current spike and the dramatical decrease of discharging current after the introduce of 1k current limiting resistor:
In any case, I would limit the discharging current by a suited resistor mounted directly at each comparator output. Also, move C21 close to the two comparators, move both comparators close to each other and use a solid ground plane.
Also, mount a current limiting resistor in the charging and discharging path of C20 and C22 as well.
Kai
