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LM5122: FET Switching

JAEHO CHOI
Prodigy 90 points
Part Number: LM5122

Tool/software:

Hello

Why is FET not switching under the conditions below?

Output Voltage : 23.5V

Load Condition : 9.5A

Output Power : 223.25W

Vin(Min) : 13V

Vin(Max) : 16.8V

Switching Freq : 600kHz

Ipeak : 37.5A (Current Sense RES : 0.002R)

For your information, it works fine in Load 9A Condition

Thank you.

  • 0
    TI__Mastermind 30030 points

    Hi Jaeho,

    Thanks for using the e2e forum.
    In theory, the LM5122 device should be capable of supporting this application.
    Assuming an inductance of ~1.5uH, there should be enough margin between the expected peak inductor current and the set current limit.

    There are some factors that affect the output power capabilities.
    Please check the following:

    - Inductor ratings (can it support this current without falling into saturation?)
    - Compensation (does the system run stable under all load conditions?)
    - Temperature (Is the board heating up too much and overtemperature protection is triggered?)

    You can also take some measurements on the board to check what is preventing operation:
    - SS and VCC voltage present? If SS goes low, the device is resetting
    - SW signal stable during lower load? If you see oscillation, it might indicate instability

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 90 points

    Hi Niklas.

    As a result of further checking, it works normally under Vin 15V conditions.

    Is there any other influence depending on the input voltage?

    Thank you.

  • 0 in reply to JAEHO CHOI
    TI__Mastermind 30030 points

    Hi Jaeho,

    When the input voltage is increased, the input voltage decreases, as the input power stays the same ( P_in = I_in x V_in).
    If the application works fine at VIN 15V, but fails at VIN 13V, the issue may be related to the input current, which affects inductor, sense resistor and duty cycle regulation, so it is difficult to narrow down the root cause based on this information.

    Best regards,
    Niklas