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LM74700EVM: LM74700EVM

Jose
Intellectual 315 points
Part Number: LM74700EVM
Other Parts Discussed in Thread: LM74700-Q1

Hi,

We tested the LM74700EVM in our application, with input voltage between 0V and 15V, and load current between 0A and 4A, and it performed as expected.


We are going to start designing our board, but we wanted to avoid increasing the BOM by using the DMT6007LFG, which is the Mosfet used in the LM74700EVM.

We already have another Mosfet N on our board, and it would be great to use the same part-number, but there are some differences in terms of specifications.

DMT6007LFG
Vgs(th): 0.8V to 2V
RDSon: 6.5 to 8.5 mohm @ Vgs=4.5V, Id=15A


MOSFET in BOM
Vgs(th): 1.2 to 2.8V
RDSon: 18 to 22 mohm @ Vgs=4.5V, Id=7A

With a slightly different Vgs(th), and with a much higher RDson, what will be the main changes in the circuit's performance?

regards

Jose

  • 0
    TI__Guru 52625 points

    Hi Jose,

    Usually, the controller operates in the VGS range of (Vth to full enhanced region) if recommended RDS_ON is chosen. In your case since RDS_ON is high, operation will be in full conduction mode most of the times. Although at very low current say 100mA or as far as RDS_ON is not achieved controller will be in regulated mode.

    This will not alter the functionality of your circuit.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 315 points

    Hi Shiven Dhir,

    Thank you for your quick response.

    We will use several LM74700 in the circuit, and all with the ideal diode function.

    However, we were thinking of using one of them as a Power Switch, that is, the EN pin was activated through a mechanical switch to activate the system's energy. In this case we would have a double function, that is, as a Power Switch, and also as an ideal diode.
    The Power Swith function could therefore be performed by a type N Mosfet, as we have one free in a package of 2 Mosfets.

    Is this concept functional, or is there something we are missing?

    regards

    Jose

  • 0 in reply to Jose
    Intellectual 315 points

    Hi Shiven Dhir,

    In the meantime, we found the answer in the manual, so we concluded that the LM74700 cannot be used as a Power Switch.

    When the LM74700-Q1 is in shutdown mode, forward current flow through the external MOSFET is not interrupted but is conducted through
    the MOSFET's body diode.

    We will have to maintain the current solution.

    regards,

    Jose

  • 0 in reply to Jose
    TI__Guru 52625 points

    Hi Jose,

    If you want a ideal diode and a switch behavior, you can consider our devices such as LM748XX family or LM749XX family which can drive back-to-back FETs. One FET will act as a diode and other will act as blocking FET.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 315 points

    Hi Shiven Dhir,

    Thank you for your answer.

    We are doing some tests with the LM74700EVM board.

    What is supposed to happen if after removing C3 (220uF) we apply a voltage of 15V with inverted polarity to the Vout output ?
    There will be no voltage at the Vin input in this test.

    regards 

    Jose

  • 0 in reply to Jose
    TI__Guru 52625 points

    Hi Jose,

    -15V at output should be fine. Anode will follow cathode since body diode will be conducting. May I know the use case where output goes negative?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 315 points

    Hi Shiven,


    If I understand correctly, as long as the body diode supports the current, nothing happens to the LM74700 at -15V at output.

    Is that what happens?

    We've finished the tests, and we're going ahead with the PCB design, and we have an important question.


    We have two LM74700 very close to each other on the PCB, and as we already have a Dual Mosfet in the BOM, we're going to use the same part-number for the two LM74700. In other words, two LM74700s and a single Dual Mosfet.

    It turns out that we can't follow the PCB layout suggested in the datasheet, so the question is how important is it to follow that layout?

    What are your suggestions in the situation I've described?

    regards

    Jose

  • +1 in reply to Jose
    TI__Guru 52625 points

    Hi Jose,

    LM74700 can withstand negative voltage at output as far as input is not powered. If ANODE is positive and CATHODE is negative, IC can get damaged.

    Layout in the datasheet is just a recommendation. It is not a rule to follow it. You can design your own layout as well. Although you can follow the points mentioned and implement in your layout. 

    Regards,

    Shiven Dhir