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Original question:

LM76003: Definition of EN rising and PG rising

LM76003: Softstart time definition

Andreas Nestler
Intellectual 350 points
Part Number: LM76003
Other Parts Discussed in Thread: LM76002

Related to another post here in E2E ("LM76003: Definition of EN rising and PG rising") I have some dedicated questions regarding softstart time definition:

- there is a delay in the start of Vout output relative to enable -> what is the maximum delay value we can expect (e.g. 5ms)?

Datasheet mentioned "The LM76002/LM76003 employs the internal soft-start control ramp and starts up to the regulated output voltage in 6.3 ms typically." It refers to the time  between "EN reaches 1.2V" and "PGOOD reaches 90% of the defined voltage".

- What about the definition of soft start time if determined by external capacitor? Does the calculated time include the delay between rising edge of EN and the start of Vout rampup or does it only consists of the rampup time? I would expect the last mentioned scenario.

Thanks and Regards Andreas

  • 0
    TI__Guru 64365 points

    Hello

    Regarding the delay from EN to start of switching, we have not characterized this delay.

    If I had to guess I would say that the worst case may be 2x typical. 

    However, this specification is not guaranteed in any way.

    The external SS time calculation refers to the ramp up time.

    Thanks

  • 0 in reply to Frank De Stasi
    Intellectual 350 points

    Hello Frank,

    thanks for your response. If worst case ist 2x typical what can I expect for typical? According your graphs in datasheet the delay seems to be in the range of 1.5 to 2 ms. If I look at the scope screenshot in the other E2E forum post ("LM76003: Definition of EN rising and PG rising") the delay looks to be ca. 3 ms. So for my calculations I have estimated a max. delay of 5 ms from EN to start of switching. Can you confirm my guess value?

    Furthermore I would kindly ask that someone from TI checks the figure content and the figure caption below. From my point of view this does not really match in any case. Examples: Figure 65 and figure 70 content seems to be identical, in figure 67caption missing information if this is for LM76002 or LM76003. Figure 70 caption may be checked for correct device.

    Regards Andreas

  • 0 in reply to Andreas Nestler
    TI__Guru 64365 points

    Hello

    I think that 5ms should be OK.

    I agree that those images have some issues.

    I will add that to our list of data sheet changes.

    Thanks