This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM74700-Q1: Cannot work under typical application

Part Number: LM74700-Q1

Hello Team,

We use "LM74700-Q1" for reverse current blocking during input short. We connect a BUCK converter behind the MOSFET "RQ5E070BN". Resistive load is connected behind the buck circuit,and the maximum power is 30W, so I think it is suitable for this application.

But we test the "LM74700-Q1" and find it does not work at 15V-input. The "VCAP to ANODE" voltage is close to zero, obviously the charge pump is not working properly. And we have confirm the "EN" is correctly connected to “ANODE”, and they have 15V voltage.

We have debugged and determined that it is not a hardware connection problem, and tried to replace the chip, the problem still cannot be solved. The 22uF capacitors “Cin12 & Cin13” have absorbed most of the switching current ripple, so this will not affect "LM74700-Q1". 

Is the chip not suitable for such application, or is there a problem with our circuit parameter design? This problem is very urgent, we hope to get your quick answer. Thank you!

Best regards,

Huichuang

  • Hello Huichuang,

    LM74700-Q1 primarily designed to block reverse current quickly during input shorts.

    LM74700-Q1 works effectively to prevent reverse current during input short and the performance of LM74700-Q1 during input short is shown in First page of the datasheet "https://www.ti.com/lit/gpn/lm74700-q1" where Gate of the MOSFET is quickly turned off within 0.5us to prevent reverse current from discharging the capacitance on the output side (cathode pin). And it recovers quickly when input short is removed.

    Could you please share a test waveform showing IN, OUT, GATE and input current as well.
    This will help to identify the issue faster.

    IF VCAP-ANODE is 0V, then GATE-SOURCE voltage will also be 0V and MOSFET will be OFF. As the MOSFET is OFF, body diode of the MOSFET would prevent the reverse current.

    Can you share a waveform of the circuit operation during normal and input short condition as well.

    Regards,

    Kari.

  • Hello Kari,

    Maybe I did not explain the problem clearly. We did not test the input short condition. I mean that in our application, the MOSFET "RQ5E070BN" is not turned on normally when the circuit is working. And the current all flows through the body diode, causing a voltage drop of about 0.7V.

    The following is the detailed test process and key waveforms: When we enable the buck half-bridge, the (Vcap-Vanode) comes to zero, which may mean the charge pump does not work.

  • Hello Huichuang,

    Okay, issue is not with the input short circuit, but rather with VCAP turning off.

    VCAP-ANODE can discharge to 0V quickly only if GATE is turning ON/OFF. As requested earlier, can you share a waveform showing gate voltage along with ANODE, CATHODE, GATE and Current.

    When the BUCK is turned on, due to input ripple of the buck converter, cathode voltage is noise which can cause LM74700-Q1 gate to turn ON/OFF resulting in collapse of VCAP-ANODE.

    Could you help with the waveform as requested earlier?

    Regards,

    Kari.

  • Hello Kari,

    Thank you for reminding us of the influence of voltage noise at cathode. We found that this problem comes from the irregular PCB layout design. Thanks for your answer!

    Regards,

    Huichuang