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TPS92640:About the OCP design of TPS92640

?? ??
Prodigy 10 points
Part Number: TPS92640

Tool/software:

Dear Texas Instruments person in charge.

Please provide me how to calculate OCP of TPS92640.

I want to check OCP current design value.    

However, I could not understand how to calculate OCP from the explanation in section 7.3.11 of the data sheet.

Please confirm and answer.

Best regards.

  • 0
    TI__Mastermind 23150 points

    Hi Customer,

    The peak current limit of 1.28 V (with VIN = 85 V at room temperature) is sensed across the high side FET (from SW to VIN). Therefore, if you do not want to trigger overcurrent protection, you should keep the V_DS of the high side FET always below 1.28V.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 10 points

    Dear Steven-san

    Thank you for  prompt reply.

    I couldn't understand the specific OCP value from the catalog.

    Please teach me the specific method for calculating OCP.

  • 0 in reply to ?? ??
    TI__Mastermind 23150 points

    Hi Customer,

    Steven Li said:
    Therefore, if you do not want to trigger overcurrent protection, you should keep the V_DS of the high side FET always below 1.28V.

    To be detailed, you need to keep the max V_DS of the high side FET, which can be estimated by (I_LED + 0.5 * I_L_ripple) * R_DS(ON), always below 1.28V. You should leave some margin here for the ringing that may occur during switching. The meanings of the parameters are shown below:

    • I_LED = average LED current
    • I_L_ripple = peak-to-peak inductor current ripple
    • R_DS(ON) = on resistance of the high side FET

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 10 points

    Dear Steven-san

    Thank you for  prompt reply.

    I have additional thing.

    If I want to directly calculate the OCP current value using the calculation formula you provided, is the following calculation method correct?

    I_LED OCP =(1.28/R_DS(ON))- 0.5 * I_L_ripple

    Best Regards

    Arakawa

  • 0 in reply to ?? ??
    TI__Mastermind 23150 points

    Hi Arakawa-san,

    Yes. You can estimate the max LED current in this way. You should leave some margin here for the voltage ringing that may occur during the switching of the FETs.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 10 points

    Dear Steven-san

    Thank you for  prompt reply.

    Based on the formula you provided, I calculated the OCP of the power supply under design as shown below.

    L 100 μH
    Vin 49.63 V
    Vout 47.14 V
    ton 1.956 μs
    ΔILED 0.048704
    I_L_ripple 0.048704 A peak-peak
    R_DS(ON)  0.021 Ω
    Vd-s OVP 1.28 V
    ILED 60.928 A 
    As a result, the calculated OCP value was 60A, which significantly exceeded the FET component rating.
    I think the calculation results are incorrect because the parts break before reaching the OCP current value. 
    Please let me know if there is any mistake in the calculation method.



  • 0 in reply to ?? ??
    TI__Mastermind 23150 points

    Hi Arakawa-san,

    As the FET gets hotter during switching, its R_DS(ON) will become higher. So the OCP value will be lower.

    Based on my experience for most common applications, considering the possible ringing / noise on the VIN and SW pin, it is better to keep your LED current smaller than 1/10 of the calculated OCP value.

    Best Regards,

    Steven