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LM25069 outside schottky diode requirement spec

K. Sasaki14
Guru 10200 points
Other Parts Discussed in Thread: LM25069, LM5067

In LM25069 datasheet, OUT pin schottky is required for inductive load.
Please let me know the requirement spec of the schottky diode. 
We would like to know about the VF, IF and VRRM value. Should we check the VF at what IF value?
If you have any equation about this, please let me know.
Our customer system voltage is 12V.

Best Regards,
Kohei Sasaki

  • 0
    TI__Genius 12380 points

    Hi Kohei,

    The schottky is needed to protect the negative transient which occurs when the circuit shuts down from a severe overcurrent event (circuit breaker). The LM25069 has a -0.3V ABS_MAX rating due to the internal ESD diode protection structure.

    There will always be some inductance, even just from trace.

    The minimum specification we look for is the peak forward surge current rating. This is usually spec'd as a 8.3ms sine wave.

    This surge rating should be at least capable of the worst case short circuit current of the design.

    For example, with a 50A current limit (1mohm Rsns), circuit breaker threshold is 90A typ, 110A max. So once current reaches 90A, the LM25069 will begin to respond within ~2us and pull down on the GATE. It then takes another 1-2us to shut down the MOSFET's Vgs. So during this time, current will continue to rise and its peak will depend on the impedance of the short circuit.

    Thus for high power (12V, 50A-120A current limit ) designs, we have seen the SK153-TP is a very strong schottky with a 275A rating and has great performance:

    For lower power designs, we do not have a recommended part, but you may look at the peak forward surge current.

    The goal is for the schottky to clamp Vout below -0.6V during the short circuit shutdown. The ABS_MAX for Vout is -0.3V, but the pin can handle up to -0.6V if the duration is short (which it will be in this event).

    Attached are some test results which discusses the negative transient effects on hot swap operation (the LM5067 uses a similar ESD protection structure). Basically, if the voltage is kept below -0.6V, there should be no issue. If it goes to -0.7V, then the device may begin to mis-behave. Beyond -0.7V, it may damage the IC.

    LM5067_Restart_Neg_Surge_customer.pdf

    Thanks,

    Alex