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LMZ14203H: Output Short-Circuit Protection

Yongmin Song
Prodigy 65 points
Part Number: LMZ14203H
Other Parts Discussed in Thread: LMZ14203,

Even if a short occurs in the LMZ14203HTZ output power, the Protection function does not operate.

Even if you forcibly short pins 1 and 2 of J19, the protection function does not work.

FB voltage is 0.8V before Short and after Short, the following waveform is measured.

I'm asking for help with what's the matter.

<Time base scale 1us>

<Time base scale 10ms>

<Schematic>

  • 0
    TI__Mastermind 27030 points

    Hi:

    Do you mean in short condition, why the output still have noise that come from internal switching , do I understand right? 

    The LMZ14203 over current protection in datasheet described as below , so the FB will be lower than 0.8V  . and output will be low but the it still have switching. 

    "Current limit detection is carried out during the OFF-time by monitoring the current in the synchronous MOSFET. Referring to the Functional Block Diagram, when the top MOSFET is turned off, the inductor current flows through the load, the PGND pin and the internal synchronous MOSFET. If this current exceeds 4.2 A (typical) the current limit comparator disables the start of the next ON-time period. The next switching cycle will occur only if the FB input is less than 0.8 V and the inductor current has decreased below 4.2 A. Inductor current is monitored during the period of time the synchronous MOSFET is conducting. So long as inductor current exceeds 4.2 A, further ON-time intervals for the top MOSFET will not occur. Switching frequency is lower during current limit due to the longer OFF-time."

    Thanks

  • 0 in reply to Daniel Li14
    Prodigy 65 points

    I forcibly made a short, but the voltage of FB does not rise above 0.92V. So the protection function doesn't work.
    When I checked the waveform with an oscilloscope, the following waveform was measured.
    I'm not sure what's wrong with my circuit.

  • 0 in reply to Yongmin Song
    TI__Mastermind 27030 points

    HI

    I don't understand your quesiton, if below waveform is FB, you mentioned is comeback to 0.8V after short, i can't read out FB comeback to 0.8V

    second, want is below waveform, Vout? 

    3.when you short 1and 2 , there is 150ohm at output side, which means it only go 12/150 which is 0.8A, it is 3A devcie, should not come to OCP when you draw 0.8A through it.

  • 0 in reply to Daniel Li14
    Prodigy 65 points

    Sorry. I think there was a misunderstanding because of the 2 oscilloscope waveforms.

    The datasheet says that the LMZ14203H has a function called "Output Short-Circuit Protection".

    In the data sheet, " If FB rises above 0.92 V the ON-time is immediately terminated.", "It can occur if the input voltage is increased very suddenly or if the output load is decreased very suddenly." is explaining.

    To make the output load drop very suddenly, I forcibly shorted pins 1 and 2 of J19.

    But, it seems that the FB voltage should rise above the internal reference voltage of 0.92V, but it does not, and it vibrates like a yellow waveform among the oscilloscope waveforms.

  • +1 in reply to Yongmin Song
    TI__Mastermind 27030 points

    HI

    "If FB rises above 0.92 V the ON-time is immediately terminated" is Output overvoltage protection NOT short circuit protection.  shorted PINS1 and PINS2 can't make the output drop suddenly because you only adding 0.8A load.  it won't been easily trigged, you can add a voltage at FB directly and quickly over 0.92V(keep it in safe voltage range), the high side fet will shutdown.  

    Thanks