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Original question:

LM5050-1-Q1: MOSFET selection for up to 40 A current

LM74700-Q1: Reverse Polarity Protection

John Pan
Intellectual 990 points
Part Number: LM74700-Q1
Hi Kari,
      I really enjoyed reading your report, Chapter 5 Reverse Polarity Protection, 11 Ways to Protect Your Power Path, Design Tips and Tradeoffs Using It’s Power Switches. It was well written and very informative.
      I have a few questions and hope to receive your answer.
1. Could you please explain why there are the differences between LM74700-Q1 and LM5050-Q1 in the following two aspects:
1) gate threshold voltage Vth, LM74700-Q1 limits the max gate threshold voltage to 2 V while LM5050-Q1 limits to 5 V.  
2) R_DS(ON). LM5050-Q1 data sheet (page 15) suggests that R_DS(ON) be no more than 100 mV at the nominal load current while LM74700-Q1 data sheet (page 16) suggest that R_DS(ON) be no more than 50 mV at the nominal load current.
2. For Auto application with nominal input voltage of 12 V or 24 V application, which is better choice for reverse polarity protection?  LM74700-Q1 or LM5050-Q1?
3. Another common method for reverse polarity protection is using a MOSFET and a BJT. Could you please advise why this method is not listed in your report? Could you also please comment on pros and cons of using a MOSFET and a BJT compared with using an Ideal Diode for reverse polarity protection? May I say that both methods are similar except that a MOSFET and a BJT method requires a charge pump gate driver while an ideal diode using LM74700-Q1 or LM5050-Q1 has charge pump gate driver, thus no external power is needed.
4. Could you please comment on all five reverse polarity protection methods regarding meeting ISO 16750-2? May I say that Methods 1) using an ideal diode, 2) a MOSFET and a BJT, and 3) using eFuse all meet ISO 16750-2 requirements, while methods 4) using Schottky diodes and 5) discrete MOSFETs cannot meet ISO 16750-2 requirements?
Sorry for so many questions, thank you very much for your help and I look forward to hearing from you,
John
  • 0
    TI__Genius 16305 points

    Hi

    1. There is not limitation or difference in gate threshold voltage between LM5050 and LM74700.

    Having a 2V MOSFET Vth helps to turn on quicker.

    I would need some time to get into the details, allow some more time till next week.

    2. At higher load curents, LM74700 is more suitable than LM5050 due to LM74700 increase gate drive capability. So to support higher load currents, 50mV limit is achieved so that MOSFET goes into full conduction quickly so as not to waste power dissipation. 100mV is possibly higher threshold for higher load currents.

    3. We dont talk about reverse polarity protection using MOSFET with BJT, if i am missing something, please point to the exact location in the report. But however we have a circuit in Figure 5 where N-Channel MOSFET is used in the GND path for reverse polarity protection.

    4. I will get back on this item later next week, hope this is fine

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Intellectual 990 points

    Hi Kari,

          Thanks for your quick reply.

          For Items 1 & 2 regarding the difference between LM74700-Q1 and LM5050-Q1, my understanding is LM74700-Q1 requires an integrated charge pump motor drive IC while LM5050-Q1 don't. Could you please confirm.

          Item 3 regarding reverse polarity protection using a MOSFET with a BJT, please refer to Technique 4 in TI SLVA835. Could you please discuss with your colleague, Michael Bifalco in this issue?

          Thanks and I look forward to receiving your responses next week,

          John

  • 0 in reply to John Pan
    TI__Genius 16305 points

    Hi

    1. Both LM74700-Q1 and LM5050 have inbuilt charge pump and provide reverse polarity protection.and reverse current blocking. Both can drive 2V and 4V Vth MOSFETs (Vth). No issues here, but LM74700-Q1 is better suited for driving bigger MOSFET, hence higher power applications/

    2. MOSFET + BJT solution requires external Charge pump drive to drive the MOSFET ON and it provides only reverse polarity protection and does not do reverse current blocking. 

    Regarding ISO 16750-2 and ISO 7637-2, tests require both reverse polarity protection and reverse current blocking. Mainly AC superimposed test of ISO 16750-2 requires reverse current blocking to do the rectification of the input AC (2V PP). MOSFET + BJT does not do reverse current blocking and will not be able to do rectification properly, leading to power dissipation in body diode of MOSFET.

    There are many other OEM tests which require reverse current blocking and there MOSFET + BJT is not a good solution.

    Ideal diode meets and complies to the requirements of ISO 16750-2 and ISO 7637-2.

    MOSFET + BJT and discrete MOSFET solution does not have reverse current blocking and so does not meet all the ISO requirements.

    Schottky diode is good for all applicaions as it does both reverse current blocking and reverse polarity protection, but power dissipation in MOSFET body diode is major concern especially at higher power applications. Power dissipation during AC superimpose test and normal operation is a concern.

    We recommend Ideal diode over eFuses for ISO 16750-2 and other ISO tests.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Intellectual 990 points

    Hi Kari,

          Thank you very much for the detailed explanation. This is very helpful.

          Best regards,

          John