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TPS56121 Resistor value for over current protection

K. Sasaki14
Guru 10200 points
Other Parts Discussed in Thread: TPS56121, TPS56221

Please tell me the resistor value range for OCP.

It is described that Programmable OCP level at ILIM is from 6 mV to 50 mV. With a scale factor of 2, the actual OC trip point across the low-side FET is in the range of 12 mV to 100 mV in datasheet.

I think this means the resistor value range should be set from 600[ohm] to 5k[ohm].

Is my thinking right?

  • 0
    TI__Genius 17240 points

    Hi Kohei,

    You are right about the resistor value range.

    Regards,

    Na

  • 0 in reply to Na Kong
    Guru 10200 points

    I tested over current operation with TPS56121EVM.

    I have a question about the OCP setting level.

    My test result is the following,

    ROCSET       OCP setting value       OCP detection value

    620[ohm]               7.97[A]                         6.28[A]

    750[ohm]               9.34[A]                         6.28[A]

    820[ohm]               10.07[A]                       10.6[A]

    910[ohm]               11.02[A]                       10.39[A]

    1000[ohm]             11.97[A]                       10.27[A]

    1100[ohm]             13.02[A]                       13.75[A]

    1200[ohm]             14.07[A]                       13.89[A]

    1300[ohm]             15.13[A]                       16.28[A]

     

    OCP detection level didn't change linearity. I found the step-by-step change.

    And, the error between the setting value and detection value was big when ROCSET is 620[ohm] or 750[ohm].

     

    Doesn't the OCP detection level change linearity?

    When ROCSET value is low, does actual OCP detection level become lower than setting value?

     

  • 0 in reply to K. Sasaki14
    TI__Genius 17240 points

    Hi Sasaki-san,

    The ILIM pin is a multiplexed pin for OCP setting and low-side gate drive.

    To save to OCP setting on ILIM pin, the voltage on ILIM pin is recorded internally in a digital format when the part is initially powered on. Later on, the ILIM is used for low-side gate drive. Due to the resolution of the analog-to-digital conversion for saving the OCP setting information, when changing the ROCSET from 620ohm to 750ohm, the OCP setting saved in the digital format does not change, hence the OCP detection level does not change. That's the reason you see step-by-step change by changing the ROCSET.

    Since the TPS56121 is a device for 15-A full load current, it is not intentionlly designed for OCP level below 10A. So we suggest using ROCSET value above 1kohm to get relative accurate OCP tripping point.

    Regards,

    Na

  • 0 in reply to Na Kong
    Guru 10200 points

    Thank you for your answer.

    If you suggest using ROCSET value above 1kohm, could you write-down it into OCP section of datasheet at next revision?

     

  • 0 in reply to K. Sasaki14
    Guru 10200 points

    How is the TPS56221?

    Do you suggest the resistor value above 1kohm　like TPS56121?

    Or above 2kohm?

  • 0 in reply to K. Sasaki14
    TI__Genius 17240 points

    Hi Sasaki-san,

    Same as TPS56121, 1kohm above should be accurate.

    Regards,

    Na