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LM2655: DO NOT START IN PRODUCTION

Part Number: LM2655

 

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.

 

lm2655.docxlm2655.docx

Hi my customer wrote the following:

  • The problem with your start up is entirely due to the start up of your 6 V input voltage. The SS cap charges up immediately to 0.6 V, then the SS current increases to 10 uA and SD/SS ramps up to 1.3 V. At this pont the current limit is zero and there in no switching. Also VIN is still below the UVLO of 3.8 V. Eventually the input ramps up above the UVLO as switching starts. Now the variable time you see is dependent on how long it takes VIn to rise above the level needed for regulated 3.3 V output. If it were me, I would hold the SD/SS pin low until the input voltage is fully regulated or extend the SS time so that the SS time completes after the input is fully regulated.
  • hi John,

    what can we do to solve the issue?

    regards,
    Kamal
  • 3 possibilities:
    1, Best is to control the ramp up of your 6 V input voltage so it reaches regulation much quicker.
    2. You can assert SD/SS low until the input voltage is regulated.
    3. Extend the SS time so tha it last longer than the time required for the input to reach regulation. It looks like the time for the input voltage to stabilize varies a lot between your two cases.

    I think these are your only options.