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LM74900-Q1: Maximum current value for current limiting protection and overcurrent protection

Colin Chen
Intellectual 575 points
Part Number: LM74900-Q1
Other Parts Discussed in Thread: LM74912-Q1, LM74912

Tool/software:

Dear Experts,

We are currently evaluating LM74900-Q1 and LM74912-Q1, with a maximum continuous current of 250A and a short-circuit protection current of 300A; I have some questions while reviewing the data manual and hope to receive answers;
1. What is the maximum current limiting value for LM74900-Q1 and can it meet our requirement of limiting current to 250A?Does RIlim have a range of values? What does 12 in the formula refer to？What is the basis for deriving this formula

2.What is the maximum value of short-circuit protection current and what is the maximum threshold for short-circuit protection? Does RISCP have a range of values? I saw that the maximum input voltage for ISCP to SC- is 0.3V. Does this mean that the maximum short-circuit protection current for a 1m Ω Rsense is only up to 300A? Vsns-scp up to 0.3V?

3. Can LM74900-Q1/LM74912 drive external 300A MOS?

Best Regards

  • 0
    TI__Mastermind 33195 points

    Hi Chen,

    1.12 comes from the internal spec of 1.2V

    Due to the amplifier at CS+ and CS- pins, the differential voltage across CS+ and CS- will be 0V. 

    So, RSENSE x ILOAD = RSET x ISET

    ISET is spilt into IILIM and IMON in the ration 1:9

    So, IILIM = (1/10) x ISET

    Voltage on ILIM pin is compared to VSNS_TH and when the voltage is greater than VSNS_TH rising threshold (1.22V) the TMR starts charging.

    VILIM = IILIM x RLIM 

    VILIM = (1/10) x ISET x RLIM 

    VILIM = (1/10) x (RSENSE x ILOAD / RSET) x RLIM 

    When Voltage on ILIM pin reaches 1.22V, the OCP threshold (ILIM) is hit, this means the ILOAD = ILIM when VILIM = 1.22V

    1.22V = (1/10) x (RSENSE x ILIM / RSET) x RLIM 

    So, the over current threshold, ILIM will be

    ILIM = 12.2V x RSET / (RSENSE x RLIM )

    This is how the formula is derived.

    2. Indeed, the spec from ISCP to CS- 0.3V, for high current application you can choose to go lower on RSENSE, something like 330uΩ.

    3. Yes, the controller can drive 300A MOSFETs.

    Do you require features of an ideal diode or just want the circuit breaker features?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 575 points

    Hi Shiven,

    Thank you for your answer. We plan to use an ideal diode controller of LM749xx-Q1 series for the high-voltage distribution unit of automobiles, which requires high current and protection against overvoltage, overcurrent, etc.Is there no problem with this application?

  • 0 in reply to Colin Chen
    TI__Mastermind 33195 points

    Hi Chen,

    As far as the abs-max ratings are not violated, you can use this device.

    Regards,

    Shiven Dhir