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TPS51116: why does IC set the over current as half of nominal value ?

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Replies: 15

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Part Number: TPS51116

Hi, Team.

could you tell me why does IC set the over current as half of nominal value ?

> 8.3.9 Soft-Start Function and Powergood Status The soft-start function

> of the SMPS is achieved by ramping up reference voltage and two-stage

> current clamp. At the starting point, the reference voltage is set to

> 650 mV (87% of its target value) and the overcurrent threshold is set

> half of the nominal value. When UVP comparator detects VDDQ become

> greater than 80% of the target, the reference voltage is raised toward

> 750 mV using internal 4-bit DAC. This takes approximately 85 μs. The

> overcurrent threshold is released to nominal value at the end of this period.

Thanks.

Best Regards.,

  • Masaharu-san, 

    The part was released many years ago. I can not find the original design team and ask them why OC threshold is half of nominal during soft start. 

    Sorry about it.

    Thanks

    Qian

  • In reply to Qian Chen:

    Hi, Qian-san,

    I see...

    Do you think the half of nominal OC threshold is benefit for something ?

    Thanks.

    Best Regards.,

  • In reply to Masaharu Takahashi:

    Masaharu-san, 

    I guess the purpose is to limit the inductor current during soft start, hence limit the Vout slew rate to make Vout rise up smoothly. 

    Thanks

    Qian

  • In reply to Qian Chen:

    i see,

    do you see that IC works properly by seeing below pic?

    Thanks.

    Best Regards.,

  • In reply to Masaharu Takahashi:

    Any feedback?

    Thanks.

    Best Regards.,

  • In reply to Masaharu Takahashi:

    Masaharu-san,

    The Vout waveform looks fine.

    There is a spike on inductor current at the end of soft start.  I guess it is from load current. 

    Thanks

    Qian

  • In reply to Qian Chen:

    hi, 

    point is,

    the amount of current IC limits is the overcurrent threshold is set half of the nominal value.

    but it seems IC didnt shut down even the current hits the half of current.

    could you tell me why?

    Thanks.

    Best Regards.,

  • In reply to Masaharu Takahashi:

    The over current protection only does cycle by cycle current limit. It does not shut down rail directly. 

    During steady state, if there is over current situation, the Vout will drop and rail will shut down when Vout drops below UV threshold.

    Vout UV is enabled after 1007 switching cycle. Please refer to datasheet section 8.3.13 on page 24. 

    As UV is not enabled during soft start in your waveform, the rail does not shut down. 

    Thanks

    Qian 

  • In reply to Qian Chen:

    Hi,Qian-san

    could you tell me below 2 more question my customer asked.

    Q1.

    for 1007 cycle you mentioned , does "each cycle in 1007" mean 2.5us(400kz frequency)?

    Q2.let me confirm below

    will the UVLO be enabled in 1007 cycle once the amount of current hits the current threshold, correct?

     ⑤「1007サイクル後にUV機能が働きます。」と、ご回答頂きましたが、

      どの時点から1007サイクル後にUV機能が動作するのでしょうか。

    Thanks.

    Best Regards.,

  • In reply to Masaharu Takahashi:

    Masaharu-san, 

    1. Each cycle means 2.5us if Fsw is 400kHz. 

    2. The Vout UV protection will not be enabled during 1007 cycle. It is only enabled after 1007 cycles. 

    Thanks

    Qian

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