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Original question:

LM25069: Upper limit of gate capacitor

LM25069: On the influence of gate pin capacitance on IC

Yuto Sakai
Mastermind 6100 points
Part Number: LM25069

Hi,

I will ask you questions related to the previous question.
Due to the large capacitance of the gate pin, can it be stressful for the inside of the LM25069?

When the capacitance value of the gate pin is 1 μF, between GATE and GND pin are short fault when hot plugging is repeated.

As you commented on the previous post, I asked the customer to reduce the capacitance value(1nF).
Then it does not seem to be in a short fault.

I'd like to know if the capacitance of the gate pin is large correlated with short fault of LM25069 gate pin.
Could you give me your advice?

Best regards,
Yuto Sakai

  • 0
    TI__Guru 57737 points

    Hi Yuto Sakai,

    I have  gone through your previous discussion with Rakesh.

    Looks like your customer is placing 1uF capacitor at the Gate pin for Soft Start/Slew rate turn ON.  This will be a problem for the 'MOSFET' during continuous output short condition. Below is more detailed explanation.

    • Say, there is a continuous output short. Then, LM25069 detects the short based on the current flowing through the Sense resistor. The Gate of the Mosfet is pulled low immediately with 260mA pull down current by the controller. As you can understand, the higher the gate capacitance,  larger will be the time to discharge it with a constant current source.
    • Until the Gate is pulled low below Vth of the Mosfet, there will be current flowing through the FET and there is a voltage of approx. Vin across the FET. This will cause power dissipation in the FET.
    • Longer the time taken for the Mosfet to discharge, more chance of the FET to get damaged as it will be dissipating power until the Gate is pulled low below Vth. So, there is a problem that the FET can get damaged. The Controller - LM25069 will not get affected. 

    Now, if you place a circuit like below, the additional Gate capacitance is discharged through the PNP immediately and makes sure the gate of Mosfet is discharged quickly.

    Hope this answers your question.

  • 0 in reply to Praveen GD
    Mastermind 6100 points
    Hi Praveen-san,

    Thank you for your reply.
    I'm sorry for bothering you.

    Is it possible for the 260 mA constant current source to be somewhat damaged by longer discharge time?

    This time, the gate pin of LM25069 is damaged when 1μF capacitor is connected.
    It is not damaged when 1nF capacitor is connected.
    I think that there is an influence of the capacitance value, could you confirm it?

    Best regards,
    Yuto Sakai
  • 0 in reply to Yuto Sakai
    TI__Guru 57737 points
    Hi Yuto Sakai,

    Can you let us know how did you confirm the damage of Gate pin ?
  • 0 in reply to Praveen GD
    Mastermind 6100 points

    Hi

    Characteristics between GATE and GND pins were measured.
    As a result, I confirmed that it is in internal short condition.

    Best regards,
    Yuto Sakai

  • 0 in reply to Yuto Sakai
    TI__Guru 57737 points

    Hi Yuto Sakai,

    Increasing the gate capacitance will increase the discharge time and this will increase the stress on the controller's internal gate circuitry. Yes, it is possible that the gate circuit can get damaged if it sinks 230mA of current for increased period o time.

    We strongly recommend that you implement the below circuit at your gate.

  • 0 in reply to Praveen GD
    TI__Guru 57737 points
    Hi Yuto Sakai,

    Do you have any other questions? If not can we close this thread ?