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LM5050-1-Q1: LM5050-1Q1

Part Number: LM5050-1-Q1
Other Parts Discussed in Thread: TIDA-00992

Hi Team,

I would like to protect my electronic board form reverse polarity using a reverse polarity protection circuit.

The supply is 12V or 24V. Load current is about 35A (current absorbed by DC/DC Boost converter).

For this high-current, I evaluated LM5050-1-Q1 but the selection of the N-channel Mosfet selection is very critical.

For my application I must comply with ISO7637-2 and ISO16950-2 and it's important to fix the Vdss Mosfet:80V-100V

As reported in the datashet, the requirement is:

  22mV/I_load <Rds(on)< 100mV/I_load -> 0,62mOhm <Rds(on) < 2.8mOhm

In particuar, at 22mV the N-Mosfet with Rds(on) so low, is very difficult to find and in any case, it would be very expensive.
and so I Would be inclined to use 2 N-Channel Mosfet.


Each mosfet will then be crossed by about 18A -> 1.2mOhm <Rds(on)> 5.5mOhm

It's correct to choose a Mosfet with Rds(on)>1.2mOhm?

For example, I could choose also 4.5Ohm (always less than 5.5mOhm)  or 100mV/I-load  is a parametr only as guide-line? I dont' undestand the meaning of upper end of the range...

My further doubt is: f you also found me the  Mosfet with these characteristics, IC LM5050-1Q1  has tha ability to drive 2 Mosfet?

I think that LM74700 is more suitable (gate drive is powerful). It's correct?

As one of the requirements is to pass the test ISO7637-2 and ISO16950-2 it's correct to use same circuitery of evaluation TIDA-00992 for LM74700? Naturally means TVS and capacitors...

The datashhet of LM74700 provieds 2 different solutions for 12V and 24V supply instead evalution TIDA-00992 provieds only one solution
for which I have the doubt.

Please, let me Know.

Thanks.

  • Hello Davide,

    I have looked into your requirement in detail and based your power requirements, recommend LM74700 over LM5050, due to increased gate drive current and integrated reverse polarity protection in LM74700. LM5050 needs additional diode and MOSFET in GND path to protect itself during reverse input conditions.

    Moving further into the details, Vds rating of MOSFET is important during the ISO 7637-2 pulse 1, where -150V is applied at the 12V battery input and needs to be clamped to protect the MOSFET and abs max rating across IN-OUT in LM5050 and rating across anode to cathode in LM74700. For 24V battery, -600V is applied at the input. 

    The TIDA-00992 schematic has a 1500W 28V rated SMC TVS "1.5SMC33CA-E3/57T". This clamps input to -36V during ISO pulse -150V(refer figure 29 in TIDA-00992), where output starts to drop (loading on output capacitor). OUT-IN sees 48V (=12V+36V) and MOSFET Vds rated >48V is needed for this 28V rated TVS.

    During the -600V pulse on 24V battery (refer figure 39 in TIDA-00992), input is clamped to -54V. OUT-IN sees 78V (=24V+54V) and MOSFET Vds rated >78V is needed here.

    TIDA-00992 used 100V Vds rated MOSFET and so it works for both 12V and 24V battery application. However, this is not cost optimal for high power/current application.

    With LM5050/LM74700 40V/60V rated MOSFETs are common for 36A current and would be price competitive for 12V application, but for 24V battery 80V rated MOSFET would be still required

    To use 40V rated MOSFETs, please use two unidirectional back - back TVS at the input, +TVS needs to be rated > Load dump voltage, 600W or higher and -TVS to clamp ISO pulse 1. For 12V battery, it can be a 16V rated SMBJ TVS 600W, for 24V battery, 26V rated TVS SMBJ 600W along with 80V rated MOSFET.

    Two 1.2mohm MOSFETs are good to use for your application, hope 36V is not maximum current.

    The recommended RdsON calcuation is for nominal current and not maximum current.

    100mV limit is only a guide, not a necessary.

    Regards,

    Kari.

  • Hello Kari,

    Thanks for the reply.

    Summing up:

    IC: LM74700-1 (ok for two Mosfets in parallel for high-current and reverse polarity protection).

    Mosfet Vdss: 80V. 2 Mosfet in Parallel (as LM74700 Evaluation Module datasheet)

    Protection: SMBJ TVS 600W for 12V and for 24V battery, TVS 26V SMBJ 600W for ISO7637-2 compliance

    IC LM74700: full conduction (Vanode-cathode> 50Mohm), regulated conduction (<20mOhm Vanode-cathode <50mOhm) reverse protection.

    In my application:

    Supply 12V nominal current: 36A-> Two Mosfet each with 20mV/18A=1.2mOhm<Rds(on)<50mV/18A=2.7mOhm

    Supply 24V nominal current: 18A -> Two Mosfet each with 20mV/9A=2.4mOhm<Rds(on)<50mV/9A=5.4mOhm

    Worst case: 12V -> Two Mosfet each with 1.2mOhm<Rds(on)<2.7mOhm 

    So, I woul choose 2 Mosfet each: 2.3mOhm , Vdss:80V , circuitery protection as indicated.

    According to your opinion it could be ok?

    Really thanks.

  • Hello Davide,

    Yes, sumarry is correct.

    Note that the TVS discussed in for the negative clamp, and positive clamp protection you need to place according to your application.

    80V Vds rating is needed for 24V and for 12V 60V would be good, 80 may be costly.

    Regards,

    Kari.