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LM74502H-Q1: LM74502H-Q1 VCAP Low Voltage and MOSFET Gate Drive Issue

Malek Hattab
Prodigy 105 points
Part Number: LM74502H-Q1
Other Parts Discussed in Thread: LM74502-Q1, LM74800-Q1

Hello Support Team,

I am using the LM74502H-Q1 for over-voltage protection. The schematic is attached below:

Protection circuit.pdf 

The system is intended to operate with an input range of 5 V to 36 V, and it has been functioning normally for a long period across multiple samples.

However, after extended operation, the circuit suddenly stopped working as expected. We cannot clearly identify the root cause, but the issue may be related to powering/resetting the system.

To troubleshoot, I removed the 4.7kohm resistor and the Zener diode on the VS pin. With this modification, the IC started working again, but still not as expected.

I measured the following voltages:

Charge pump capacitor (VCAP) < 1V or lower, also VGS of first MOSFET is less than 1V

Around 2.5 V drop across VDS of the MOSFET

Could you please explain the possible reason for this failure?

Also, we would appreciate guidance on how to resolve this issue. This is urgent and critical, as we are in the final phase of testing and certification.

We faced a similar issue earlier when using a 10 kohm resistor on the VS pin as recommended in the datasheet. Reducing it to 4.7 kohm solved the problem at that time. Replacing the IC allows the system to work again, but we still need to identify the root cause and implement a permanent fix as soon as possible.

Thank you,

Malek Hattab

  • 0
    Prodigy 105 points

    Hello again Support Team,

    I forgot to mention additional points regarding the VS pin resistor. When we used 10 kohm, we experienced difficulty operating at low input voltage (~5 V). After reducing it to 4.7 kohm, the system was able to operate at low voltage, but it could not operate reliably above 18 V until we increased the soft-start time of the 3.3 V power supply directly after the protection circuit.

    Could this behavior be related to inrush current?

    I am planning to order LM74502-Q1 (non-H version) for comparison testing. I have also reserved space for an RC delay at the gate of the first MOSFET.

    In addition, I noticed that the LM74800-Q1 reference design (another IC) includes additional capacitance at SRC pin and a diode from SRC to VS, which may help stability during startup or transient conditions. Correct?



    Currently, I am using LM74502H-Q1, following a schematic very similar to the datasheet, except for the EN circuitry, which is not connected to the VS pin.

    Should the EN pin be tied to VS for always-on operation, and then enable the downstream power supply after the protection circuit?
    Is there a defined relationship between EN voltage and VS voltage that could impact startup behavior?

    Additionally, could a high impedance/resistance power path contribute to improper device operation?

    It is strange that the issue appeared after long-time operation, it was tested and passed over the full input range (5–38 V). Even when I tested the over-voltage disconnect behavior up to ~75 V, the system responded correctly, connecting  and disconnecting as expected.

    Thanks,

    Malek

  • 0 in reply to Malek Hattab
    Prodigy 105 points

    Hello TI Support Team,

    It appears that the issue occurs at high input voltage, typically above 32 V, and not on all boards. Additionally, the affected boards do not fail immediately, the failure may appear only after multiple power cycles (sometimes after tens of cycles).

    To investigate further, I selected one failing board and performed the following changes one by one, testing after each modification:

    *  Replaced the charge pump capacitor with higher one

    * Added a gate resistor

    * Removed the Zener diode on the gate

    * Connected the EN pin to VS for always-on operation

    None of these changes resolved the issue.

    However, after I soldered a diode between SRC pin and  VS pin, the circuit started working reliably. I believe this diode may be necessary for stable operation.

    I am now testing five boards continuously at 38 V, connecting for minute and disconnecting for another minute using a relay module. So far, all boards are working good.

    I will continue long-duration testing, but before I proceed with updating the manufacturing files, I would like to ask:

    Do you recommend this modification as the diode is not shown in the datasheet? Is there anything else we should verify or measure?

    Thank you,
    Malek

  • +1 in reply to Malek Hattab
    TI__Guru 56645 points

    Hi Malek,

    Are you designing the system for 24 automotive battery? What is the maximum input voltage? Asking this to so that we can calculate the minimum required resistance instead of 4.7Kohms.

    The diode from SRC to VS can be added, there isnt an issue in that. 

    Regards.

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 105 points

    Hi Shiven,

    Yes, the system will work on 24 V batteries. The design was intended to handle clamped input voltages or overvoltage conditions up to 75 V.

    I kept five boards under continuous testing for five days, and none of the boards failed. I will proceed with the diode solution, as I experienced significant issues with the VS series resistor.

    Thank you for your support.

    Best regards,
    Malek

  • 0 in reply to Malek Hattab
    TI__Guru 56645 points

    Hi Malek,

    The resistance is added to keep the zener under it safe during surge. Please add some resistance according to zener power rating.

    Regards,

    Shiven Dhir