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LM5122-Q1: SET MAX DUTY

Raymond He
Expert 1990 points
Part Number: LM5122-Q1
Other Parts Discussed in Thread: LM5122

dear 

      If you want to limit the maximum duty cycle LM5122, how to set？thanks in advance

  • 0
    TI__Genius 15485 points

    Hello Raymond,
    Thanks for reaching out to us via e2e.

    The LM 5122 has a minimum OFF time for the low side FET of a few hundred nanoseconds (dependent on the VCC voltage).
    But there is no option to manually limit the duty cycle any further.
    The actual duty cycle during operation is determined by the peak current control loop and the voltage control loop.

    Best regards,
    Harry

  • 0
    Expert 1990 points

    thanks harry

    oher question:

    1) LM5122 has over-current protection function, when need larger power output, expand pulse width, input current increases, to a certain extent MOS transistor off, pulse width Max

    2) but the state at this time is the current is off, power amplifier terminal or supply voltage, whether there is MOS switch function to supply voltage off, in order to achieve full protection power amplifier circuit design?

    3) at present, the LM5122 directly activates the protection mechanism when it outputs a certain current value. Is there a circuit that can be designed as a buffer, where the power remains constant and the current increases until the current increases close to the current protection threshold, while the supply voltage drops from 25V to 20v, instead of the direct supply voltage jumping from 25V to 12V? For example, the ISL81805 of Renesas has a step-by-step output function?

    any advise?thanks in advance

  • 0 in reply to Raymond He
    TI__Genius 15485 points

    Hello Ahmad,
    Thanks for reaching out to us via e2e.

    I am very sorry, but it is quite difficult for me to understand your actual questions.

    So, I will try to explain the current limit features a bit.

    The Cycle-by-cycle Current limit (regulation loop) is not the same as an over-current protection.

    During regular operation:
    Dependent on the load current and input voltage there is a certain demand for energy that needs to be stored in the inductor.
    The peak current limit will either be reached earlier or later. When it is reached, the Low side FET turns OFF. This defines the Duty cycle.

    But this does NOT activate any protection mechanism.

    There are two steps that can happen, when the current limit is exceeded for a longer period of time:

    a)
    7.3.10 Cycle-by-Cycle Current Limit
    ...
    For the case where the inductor current may overshoot, such as inductor saturation, the current limit comparator skips pulses until the current has decayed below the current limit threshold.

    b)
    7.4.3 Hiccup-Mode Overload Protection
    If cycle-by-cycle current limit is reached during any cycle, a 30-μA current is sourced into the RES capacitor for the remainder of the clock cycle. If the RES capacitor voltage exceeds the 1.2-V restart threshold, a hiccup mode overload protection sequence is initiated ...

    Dependent on the size of the RES capacitor it will require many cycles of overcurrent until the protection mechanism gets triggered.

    So, your statement that "the LM5122 directly activates the protection mechanism when it outputs a certain current value" is not true.
    There is some configurable period of "tolerance for overcurrent" until the mechanism turns off the converter.

    Best regards,
    Harry