LT4321 Mos Selection

Hi,

According to LT4321 Mos Selection,

Vgs_th_min should not lower than 2V.

Is that any risk on it?

Will it impact the force current ?

Because we found lots of Mos Vgs_th_min<2V except Nxperia

And as Rds_on section said,

VF is between 20mV and 70mV which means Rds_on is around 66mΩ (Class 4 PD)

Will it violate the detection/Classification if we choose Rds_on out of range ?

Thanks a lot.

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  • +1
    •  Analog Employees 
    on Feb 21, 2020 5:06 PM

    Hi SnowHung,

    Alexxx is correct, the VGS(th)min ≥ 2V assures fast turn-off if the power source fails or is shorted, and minimizes reverse current transients. There are several MOSFETs available from Nexperia, Vishay, Infineon, and STMicro that have VGS(th)min ≥ 2V, VDS_max =100V, and similar RDSON values. The VGS_max should be rated for the 13V gate drive, and +/- 20V VGS_max is common for standard level MOSFETs.

    LT4321's undervoltage lockout keeps the part off during detection and it is normally enabled after classification has completed. This prevents LT4321's quiescent current from affecting the Detection or Class Signature. Detection and Classification both work with a very short cable, or a 100-meter cable (12.5 Ohms for two-pair, 6.25 Ohms for four-pair), so 66mΩ RDSON is not a problem.

    Choosing a RDSON that is too small means that you bought a bigger MOSFET than you needed, as the LT4321 will not fully turn on a 1mΩ when the load current is less than several amps. Choosing an RDSON that is too big means your design will be less efficient than it could have been with a lower RDSON MOSFET.

    Best Regards,

    Eric

Reply
  • +1
    •  Analog Employees 
    on Feb 21, 2020 5:06 PM

    Hi SnowHung,

    Alexxx is correct, the VGS(th)min ≥ 2V assures fast turn-off if the power source fails or is shorted, and minimizes reverse current transients. There are several MOSFETs available from Nexperia, Vishay, Infineon, and STMicro that have VGS(th)min ≥ 2V, VDS_max =100V, and similar RDSON values. The VGS_max should be rated for the 13V gate drive, and +/- 20V VGS_max is common for standard level MOSFETs.

    LT4321's undervoltage lockout keeps the part off during detection and it is normally enabled after classification has completed. This prevents LT4321's quiescent current from affecting the Detection or Class Signature. Detection and Classification both work with a very short cable, or a 100-meter cable (12.5 Ohms for two-pair, 6.25 Ohms for four-pair), so 66mΩ RDSON is not a problem.

    Choosing a RDSON that is too small means that you bought a bigger MOSFET than you needed, as the LT4321 will not fully turn on a 1mΩ when the load current is less than several amps. Choosing an RDSON that is too big means your design will be less efficient than it could have been with a lower RDSON MOSFET.

    Best Regards,

    Eric

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