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TPS7A91: About soft start time

Part Number: TPS7A91

Hi

My Customer has question.

The soft start time is a period of 10% to 90% of the output voltage.
Is this correct?

When the customer checked with TPS7A91, 0 to 100% was closer to the design value.

which one is correct?

■SPEC

SS/NR=1uF

SS_CNTL=Vin

Best Regard

T Kishi

  • Hi T Kishi,

    The soft-start equation for TPS7A91 is to calculate the entire (0-100%) rise time of the output.  The equation does not take into effect variations such as capacitor tolerance; however, you can see that your results are similar to the results shown in Figure 35 of the datasheet:

    Very Respectfully,

    Ryan

  • Hi Ryan

    Thank you for reply.

    I understand that the soft start time of TPS7A91 is specified as 0-100%.

    As one additional question, is the definition of 0-100% for other Power IC's soft start time too?
    Or is it TPS 7A91 only?

    Regard

    T Kishi

  • Hi T Kishi,

    You will always want to check the datasheet for a specific device as some devices are specified differently. Some datasheets will specify the startup time under certain conditions in the Electrical Characteristics table. When this occurs the specification is usually an abbreviated range such as 0 to 95%. In the Electrical Characteristics table, the conditions under which the LDO is tested will be specified. In the applications section when a softstart equation for the LDO is given, it will generally be for 0 to 100%. If it is for another range it should be specified.

    Very Respectfully,
    Ryan
  • Hi Ryan

    Thank you for answer.
    In the case of LDO, I understood that when there is no test condition, it is 0-100% regulation.
    Is this also true for DCDC?

    Regard
    T Kishi
  • Hi Kishi-san,

    You have a very broad question. For DC/DCs, the same answer applies.

    You should consult the D/S of the specific device and see what it says. For devices with an external SS cap to set the SS time, it is usually 0% to 100%.