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LMZ10501 Soft Start Rate

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Philip Meyer
Posted by
on Nov 30 2011 17:39 PM
Expert3390 points

Team,

One customer wants to be able to adjust the soft start time on this device to 200us.  There is a statement in the datasheet of the relation between the soft start rate and Vcon:

 

Are there any workarounds to change the rate?

Thanks,

Phil

 

soft start LMZ10501
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  • Denislav Petkov
    Posted by
    on Nov 30 2011 20:25 PM
    Expert4870 points

    Hello Phil, 

    The constraint is that FB has to be above 375mV at the end of the ~75us so that the output short circuit protection does not engage during startup. Higher Vcon capacitor can be used but that constraint must be satisfied. It may be possible to achieve 200us of soft start time but it depends on the output voltage setting, loading and output capacitor. I can try this fairly quickly and let you know tomorrow.

    What I need is the output voltage setting, the input voltage, and the output capacitor they are considering. I will assume 1A load (worst case) unless you say otherwise.

    Regards, 
    Denislav 

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  • Philip Meyer
    Posted by
    on Dec 01 2011 08:53 AM
    Expert3390 points

    Hi Denislav,

     

    Thanks for looking into this!  The input voltage is 5V and output is 1.2V.  Please select the output cap that will get us closest to the 200us.

     

    Thanks,

     

    Phil

     

    From: Denislav Petkov [mailto:noreply@e2e.ti.com]
    Sent: Wednesday, November 30, 2011 8:26 PM
    To: int_nonisolated_dcdc_forum@e2e.ti.com
    Subject: Re: [INT - Non-Isolated DC/DC Forum] LMZ10501 Soft Start Rate

     

    A Message from the TI E2E™ Community

    Texas Instruments

     

    Hello Phil, 

    The constraint is that FB has to be above 375mV at the end of the ~75us so that the output short circuit protection does not engage during startup. Higher Vcon capacitor can be used but that constraint must be satisfied. It may be possible to achieve 200us of soft start time but it depends on the output voltage setting, loading and output capacitor. I can try this fairly quickly and let you know tomorrow.

    What I need is the output voltage setting, the input voltage, and the output capacitor they are considering. I will assume 1A load (worst case) unless you say otherwise.

    Regards, 
    Denislav 



    View this message online or reply to this message

    E2E http://www.ti.com/e2e-community

     

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  • Denislav Petkov
    Posted by
    on Dec 01 2011 10:45 AM
    Verified Answer
    Verified by
    Expert4870 points

    Hello Phil,

     

    Here are a few scope shots. The component values are noted in the images. The Rt and Rb values also play a role in the timing. 

     

     

     

     

     

    Regards, 

    Denislav

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  • Denislav Petkov
    Posted by
    on Dec 02 2011 14:33 PM
    Expert4870 points

    I am attaching an excel calculator I put together to predict the softstart time. It is a bit simplified, but it is fairly close to reality. 

    Regards, 

    Denislav

    3122.softstart_estimate.xlsx

     

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  • Bishnu Ban
    Posted by
    on Apr 04 2013 03:06 AM
    Intellectual270 points

    Hi. Denislav!

    I could not download (open) the excel file (attached one).

    can you pls. forward me (bishnu.ban@tttech.com)!

    regards

    Bishnu

    here is the message why I could not open it: 403 - Forbidden: Access is denied. You do not have permission to view this directory or page using the credentials that you supplied
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  • Denislav Petkov
    Posted by
    on Apr 04 2013 14:01 PM
    Expert4870 points

    Hello Bishnu, 

    Here is the file again. See if you can download it and if not, I will send it via email. 

    7103.softstart_estimate.xlsx 

    Regards, 
    Denislav

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