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Original question:

LM5069: No Latch-Off after high overcurrent load

LM5069: Delayed current limiting? Datasheet Figure 39.

Nathan Langford
Prodigy 70 points
Part Number: LM5069
Other Parts Discussed in Thread: TPS1685

Tool/software:

Hello,

In another post (Link), someone pointed out that in Figure 39 of the datasheet, it appears the current is allowed to go up to 19A, despite a 12.5A limit, and then after 3ms the current is limited to 3A. Would someone be able to explain what is happening here?

  • 0
    TI__Guru** 105361 points

    Hi Nathan,

    Thanks for reaching out!

    When overload event happens, the device starts responding and discharges the GATE while simultaneously starts TIMER counting. Eventually, the device foldback the load current to a value corresponding to power limit (PLIM/Vds) and then turns-off when TIMER expires.

    Hope this is clear. 

    Please look at TPS1685 eFuse which is an alternative space efficient solution for LM5069

    BR,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 70 points

    Hi Rakesh,

    Thank you for the quick response. Does this mean that there will be a period of time where the current is not regulated, until the LM5069 is able to discharge the gate sufficiently? I would assume this unregulated time is determined by the MOSFET's total gate charge?

    We are considering the LM5069 for an application that needs to have the overcurrent fault level set to 150A with a fault timer of 100us and we were planning on using an Infineon IPT008N06NM5LF MOSFET, which has a much higher gate charge than the MOSFET used in the datasheet example application. It sounds like the LM5069 in our application would never be able to foldback the load current or MOSFET power before the fault timer expires. Am I understanding this correctly? Is there any information on how much current from the gate the LM5069 will sink when attempting to limit load current or FET power?

    I'm not sure the TPS1685 would be a good fit for our application given the high current levels. Do you have any suggestions for other parts that might fit our requirements, if the LM5069 won't work?

    Best regards,

    Nathan

  • 0 in reply to Nathan Langford
    TI__Guru** 105361 points

    Hi Nathan,

    Can you please share more details about the application and end product. Why the current level is so high ?

    Nathan Langford said:
    Thank you for the quick response. Does this mean that there will be a period of time where the current is not regulated, until the LM5069 is able to discharge the gate sufficiently? I would assume this unregulated time is determined by the MOSFET's total gate charge?

    Rakesh-> Correct. Till the gate is discharged, the FET allows the overload current provided current is less than the circuit breaker (CB) threshold. 

    Nathan Langford said:
    We are considering the LM5069 for an application that needs to have the overcurrent fault level set to 150A with a fault timer of 100us and we were planning on using an Infineon IPT008N06NM5LF MOSFET, which has a much higher gate charge than the MOSFET used in the datasheet example application. It sounds like the LM5069 in our application would never be able to foldback the load current or MOSFET power before the fault timer expires. Am I understanding this correctly? Is there any information on how much current from the gate the LM5069 will sink when attempting to limit load current or FET power?

    Rakesh-> LM5069 takes at least 1~2ms to foldback. So, in your case of 100us fault timer, the device won't act unless the current cross circuit breaker threshold.

    The sink will be in the order of few tens of uA but it is not a characterized parameter.

    Nathan Langford said:
    I'm not sure the TPS1685 would be a good fit for our application given the high current levels. Do you have any suggestions for other parts that might fit our requirements, if the LM5069 won't work?

    Rakesh-> multiple TPS1685 devices can be used in parallel to achieve 150A support. 

    LM5069 is also a good option only thing is we need to burn more power in shunt resistor at such high currents. Please fill design cal https://www.ti.com/tool/download/LM5069QUICK-CALC and share us for review.

    BR,

    Rakesh