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Soft-start time of LM317A

Takahiro Nishizawa
Genius 5575 points
Other Parts Discussed in Thread: LM317A

We have question about LM317AT/NOPB.

Please tell us the time from voltage input to voltage output(Soft-start time).

Vin=24V

Vout=3.3V

Iout :min :16.5mA

       type：18.6mA
      max ：19.8mA 

best regards,

Takahiro Nishizawa

  • 0
    TI__Guru 55940 points

    Hi Takahiro,

    Please find the startup for the LM317A bellow taken from the EVM User's Guide.  While this is for a 5 V output, the startup time will be similar for the 3.3 V output due to the fact that the output is a gained version of the internal reference.  It is the internal reference ramp that defines the output ramp.

    If you choose to use a feedforward capacitor or have enough output capacitance to trigger current limit, your output ramp will be slower.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Genius 5575 points
    Thank you for your answer!

    In EVM User's guide p.6 Figure 3-1 ,
    It seems soft-start time is about 1mS.
    Is this understanding correct?

    And do you have test data about soft-start time?

    Best regards,


    Takahiro Nishizawa
  • 0 in reply to Takahiro Nishizawa
    TI__Guru 55940 points
    Hi Takahiro,

    Please note that the timescale is 20 ms/div. As such The startup time from Vin rising to Vout reaching its final value is 20 ms.

    We do not have any test data on the startup time for this linear regulator. The startup time was not a key specification at the time of release of this linear regulator.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Genius 5575 points
    Thank you for your answer!

    User's guide p.6 Figure 3-1,
    Current is 500mA.
    But the current is 18.6mA in our conditions.

    Iout :min :16.5mA
    typ:18.6mA
    max :19.8mA

    We think that soft-start time will be shorter if current is so small.
    Does soft-start time change by the output current?

    We want to know the soft-start time in our condition.
    Do you have any information?

    Best regards,


    Takahiro Nishizawa
  • 0 in reply to Takahiro Nishizawa
    Genius 5575 points
    Thank you for your support!

    We have a new question about LM317A.

    Does soft-start time change by the package difference(TO-220 or SOT-223 or TO or TO-252)?


    Please also check the following questions.

    User's guide p.6 Figure 3-1,
    Current is 500mA.
    But the current is 18.6mA in our conditions.

    Iout :min :16.5mA
    typ:18.6mA
    max :19.8mA

    We think that soft-start time will be shorter if current is so small.
    Does soft-start time change by the output current?

    We want to know the soft-start time in our condition.
    Do you have any information?


    Best regards,


    Takahiro Nishizawa
  • 0 in reply to Takahiro Nishizawa
    TI__Guru 55940 points
    Hi Takahiro,

    Please note that LM317A does not have an adjustable soft-start function. As such the output voltage will track the internal reference voltage as shown in the User's Guide figure.

    The startup curve for Vadj will be the same for your customer's conditions. As such, the startup time for Vout will be the same. Only the end regulation voltage will change.

    Even though your application has a lower current than the User's Guide figure, you will still have the same startup time the LM317A current limit will not be reached for either application. Unless LM317A enters current limit or a feedforward capacitor is used across the top resistor in the feedback network, you will achieve the same startup time of 20 ms.

    Very Respectfully,
    Ryan