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LM74700-Q1: MOSFET selection

Takuya Terada
Expert 4565 points
Part Number: LM74700-Q1

Hi team,

I have a question about MOSFET selection for LM74700-Q1

Customer would not like to use low RDS(on) MOSFET. (because of the cost)

Datasheet said reccomended RDS is between 22mV/Id and 100mV/Id.

ID that they will use is 2A and RDS that they want to use is 60mΩ ~ 80mΩ.

Are there any problem or bad effect of higher RDS on?(for example longer on/off transient time)

and also on datasheet, gate Vth recomended 2-V to 2.5-V maximum.

if Vth exceed this value, what is the bad effect?

best regards,

Takuya 

  • 0
    TI__Genius 16305 points

    Hello Takuya,

    There are no major issues in using slightly higher RdsON MOSFETs with LM74700-Q1 except that the MOSFET will be fully enhanced most of the time.

    Low Vth is mainly concern for low RdsON or high current application, so it is not a concern for a 60mohm MOSFET. Low Vth is mainly a concern for high current application, where there could be gate oscillation at very light loads <1mA. At very high current application, 1mA load is easily achieveable.

    The additional power dissipation is something you can take care off in your design.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    TI__Expert 4565 points

    Hi Kari,

    thank you very much for your support.

    >> the MOSFET will be fully enhanced most of the time.

    I am sorry, I am not sure what this mean, could you tell me about this more detail?

    I understand low Vth is not big concern. How about high Vth (for example 3V or 4V against to recommended 2 to2.5V)

    best regards,

    Takuya

  • 0 in reply to Takuya Terada
    TI__Genius 16305 points

    Hello Takuya,

    I will get back on this question tomorrow, hope this is fine.

    Let me know if you need an urgent response from us.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    TI__Genius 16305 points

    Hello Takuya,

    Usually RdsON of the MOSFET is specified at two levels of Vgs, 4.5V and 10V.

    A 60mOhm MOSFET at 4.5V would have lower resistance at Vgs 10V.

    For example, MOSFET AO6420 has 60mohm at 4.5V Vgs, and 50mohm at 10V Vgs and .

    LM74700-Q1 A-C regulation voltage is 20mV and based on above MOSFET RdsON characteristics, at load current 0.4A (= 20mV/50mohm), MOSFET will be in regulation with its source to drain voltage at 20mV. At 0.33A, RdsON is 60mohm and Vgs will be 4.5V and at 0.4A, RdsON is 50mohm with Vgs 10V.

    Beyond 10V Vgs, RdsON will not change much and the MOSFET will be fully enhanced. So around 2A, 60mohm MOSFET stays fully enhanced.

    Using a Vgs(th) of 3-4V is also not a concern.

    Regards,

    Kari.