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Original question:

BQ77915: Design Review

BQ77915: Current Sense Resistor Selection

Samruddhi Patil
Prodigy 210 points
Part Number: BQ77915

Dear Sir,

I'm trying to select the Current Sense Resistor Rsense for my design.

E.g. Battery is 12V24Ah. Continuous Discharge Current is 20A & Charge Current 12A. Now I want to set the Over Current Discharge protection at 25A & Over Current Charging Protection at 15A.

bq7791506 OCD 1 Threshold is 50mV, OCD 2 Threshold is 100mV, OCC Threshold is 60mV & SCD Threshold is 300mV. 

  1. For discharge protection which OCD threshold to consider to calculate the Rsense? OCD 1 or OCD 2?

    OCD 1 = 50mV then, Rsense = 50/25 = 2mohm,

    OCD 2 = 100mV then, Rsense = 100/25 = 4mohm,

    OCC = 60mV then, Rsense = 60/15 = 4mohm

    SCD = 300mv then, ISCD = 300/4 = 75A & 300/2 = 150A.

  2. Which Rsense value to select so that both charging & discharging protections will be achieved?
  3. Can you please explain one more calculation using any random current value?

Regards,

Samruddhi

  • 0
    TI__Mastermind 42060 points

    Hello Samruddhi,

    See Section 10.2.2.1 Design Example of the datasheet for considerations when selecting your sense resistor.

    Chose a sense resistor based on the detection of your largest discharge current, and short-circuit current. The protections also have tolerances that must be taken into account, Section 10.2.2.1 briefly describes this and these can be found in Section 8.5 Electrical Characteristics — PROTECTION ACCURACIES. 

    In your scenario, you'd probably want to set it at 2-mOhms. OCD1 triggers after 700-ms, and OCD 2 for this case would trigger at 50-A after 350-ms. The OCC however in this case would be 30-A. But you would not want to set the sense resistor to 4-mOhm, as this would trigger OCD1 always under your normal discharge current: 50/4 = 12.5-A. ISCD as you know would be 150-A.

    Now, if we are taking into account the tolerances, assuming a 20% OCD tolerance and that we use the 2-mOhm resistor with a 25-A nominal OCD threshold. It is possible that 20-A may trigger the protection, and it is possible that even 30-A might not trigger it. To assure it would triggered/not triggered, the current would have to have ~19-A (to assure it will not be triggered) and ~31-A (to assure it will be triggered).

    Keep in mind this when selecting your sense resistor.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 210 points

    Dear Sir,

    Thank you for your valuable reply.

    But my confusion is  exactly the same that OCC threshold will always be greater than OCD if I design it according to OCD1 threshold.

    OCC threshold should be less than OCD threshold.

    Yes, if I design it as per OCD2 then OCD1 protection will keep happening.

    Please guide.

    Regards,

    Samruddhi

  • +1 in reply to Samruddhi Patil
    TI__Mastermind 42060 points

    Hello Samruddhi, 

    With these particular OCD/OCC ratios, this will always be the case.

    I apologize. There is nothing that could be done, as the OCD/OCC threshold are all factory programmed. You will have to choose a part that has the OCC/OCD ratios you need, the BQ7791501 seems to fit the ratios better for your current protection needs. Do keep in mind the protection tolerances.

    Best Regards,

    Luis Hernandez Salomon