Recently we have several electronics that don’t pass the production tests due to the fact that the +3.3V made by the LM2655MTC-ADJ cannot start. The schematic below has been used for years on almost all our designs:
We have investigated and found that the +3.3V needs about 1.6ms to rise to 3.3V on a “good” electronic and about 5ms on a “bad” electronic, as shown hereunder on the two oscillo plots. We have to say that in both cases the +6V is not yet stable when the LM2655 starts. But the question behind it is why then, it needs more time to rise the +3.3V on some electronics ?
In the “good” plot hereunder, we have a “soft start” capacitor = 3.3nF which is what we have always put until now. But due to the problem of long starting, we had to change the “soft start” capacitor to 33nF for the “bad” plot hereunder.
Z1: +6V input of LM2655-MTC-ADJ
Z2: +3.3V
Z3: Uss Soft start capacitor
Z4: LM2655 output
Measurement is done at about Tamb = 22°C
The questions are :
1) Could you explain the difference of timing on both electronics ? Is is the current limitation at startup that gives this strange shape on the rising of 3.3V ?
2) Is it a good solution in our case to increase the soft start capacitor to 33nF or even a bit more to have margin ? (let say 47nF)
Thank you for your help.
Hi my customer wrote the following:
