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Original question:

TPS7A19: How soft start works?

TPS7A19: How soft start works? II

Kazuhiko Hirano
Genius 11613 points
Part Number: TPS7A19

Jason,

 

My customer still has some more questions.

 

Q1: Looking at Figure 14 in the datasheet, Vout ramp is not controlled after the point at RED circle.

Under what condition, Vout ramp control has been released?

Q2: After this Vout ramp control release point, my customer worries about undesired inrush current possibility.

Do you have any comments on this concern?

 

Best regards,

 

K.Hirano

  • 0
    TI__Mastermind 23361 points

    Hi Hirano-san, 

    Please allow some time for me to get in touch with our design engineers on the answers. I will get back to you by 4/10. 

    Regards, 
    Jason

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points

    Hi Hirano-san, 

    The soft-start of the TPS7A19 is designed in a way that the soft-start becomes activated in the beginning and soft-start is released when Vout reaches a certain % of the final value after that since there is no soft-start then output changes slope and ramps up quickly to the final value. Usually, this kind of design is to speed up the startup. The threshold value at which soft start stops varies from LDO to LDO, on this LDO it seems to be 70% of the final value looking at the waveform below

    Yes, the fast ramp after the soft-start period also creates a fast output voltage rise, but since the voltage on the output is already built up, the effective capacitance is much smaller comparing to the effective capacitance at ground level with the degrading of the capacitance. The in-rush will be much smaller when the output voltage is already near its target.

    Does this answer your question? 

    Regards, 

    Jason Song

  • 0 in reply to Jason Song
    TI__Genius 11613 points

    Jason,

     

    Thank you for your response, however my customer still has a question.

     

    Last Oct, my customer evaluated TPS7A19 with conditions, Vin = 24V, Vout=7V. I out = several tens of mA.

    Then, the threshold value is <50% of target Vout value, different from 70%.

    Is this devise to device variation?

    Or the threshold value depends on Vin, Vout and other conditions?

    If it depends on Vin, Vout and other conditions, any ways to estimate roughly how much it would be?

    Best regards,

     

    K.Hirano

  • 0 in reply to Kazuhiko Hirano
    TI__Mastermind 23361 points

    Hi Hirano-san, 

    Please allow some time for me to work with the design to understand a little more details on how the soft-start work for the device. I will get back to you by 4/15. 

    Regards, 
    Jason

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points

    Hi Hirano-san, 

    I had a discussion with our design team, and here is how the soft-start works in this device. There is a comparator which is comparing the internal Vref voltage to a threshold voltage to determine when the soft-start should stop. The output voltage is a gain of the Vref and the gain is set by the resistor divider. The threshold varies from LDO to LDO and it is not well controlled and this explains with different gains for different output options and some variation on the threshold of the comparator, the startup waveform will vary. But the startup will always show a well-controlled rising up to a certain point and quickly charge up to the target. 

    Does this make sense now?

    Regards, 

    Jason Song