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LMZ14203: Short Circuit Problem in 3V3 Output

Olivier Eanuely
Prodigy 10 points
Part Number: LMZ14203

Tool/software:

Good Morning,

I contact you to have some information about the LMZ14203 behaviour in short-circuit.

We impleted this component to generate two output voltage level : a 3V3 and a 5V from a single supply in range [8V;30V].

Schematic 5V :

Schematic 3V3:

In our short-circuit test, we checked the current waveform on the output.

We were surprised to see different behaviour between the 5V output and the 3V3 output.

Here are the oscilloscope screen :

P5V : (Voltage in blue and current in purple)

We though that this screen is a good functionning with low output voltage and some repetitively low current spike which mean the component try to check if the Short-circuit is still present.

P3V3 (in green voltage and in purple Current):

On this test, we see large current spike with remaining voltage.

In fact, we don't see it on this screenshot but the first spike seems to have the same behaviour as with the P5V.

All the following spikes are like the figure above.

During this test, the Input voltage of the component is stable. Same configuration of test are used for both capture above, a direct short circuit.

We wanted to ask you if you have an idea why this behaviour happened ?

How is managed the shortcircuit protection ?

Thank you for your help,

With best regards,

Olivier EMANUELY

  • 0
    TI__Intellectual 825 points

    Hi Olivier,

    I will check and get back to you by tomorrow EOD (IST).

    Regards,

    Arpita

  • 0 in reply to ARPITA NATH
    TI__Intellectual 825 points

    Hi Olivier,

    Could you please share zoomed in waveforms of FB pin and Inductor current   for debugging? Please refer to datasheet section "7.3.3 Current Limit" for short circuit protection feature.

    Regards

    Arpita

  • 0
    Prodigy 10 points

    Hello Arpita

    Unfortunately, the inductor is integrated in the component so i can't measure current flowing throw the inductor.

    The Feedback voltage will only be a fraction of the output voltage (green curve on the screen above).

    Approximatively:   Vfb = 0.24*0.15 = 30mV

    With regards,

    Olivier

  • 0 in reply to Olivier Eanuely
    TI__Intellectual 825 points

    Hi Olivier,

    • Please note the switching depends on value of FB pin voltage.  Current limit detection is carried out during the OFF time by monitoring the current in the synchronous MOSFET. If this current exceeds the current limit, 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 the current limit. Can you please provide zoomed in waveforms of FB voltage for better understanding of the circuit behavior? 
    • The first schematic shown as Vout=5V, but the feedback resistors give Vout=3.3V, and same for second schematic. can you please check this?

    Regards

    Arpita

  • 0 in reply to ARPITA NATH
    Prodigy 10 points

    Hello Arpita,

    • The first schematic shown as Vout=5V, but the feedback resistors give Vout=3.3V, and same for second schematic. can you please check this?
      It is my bad, i switched the two capture:

    The 5V (4.93V) :

    The 3V3 : 

    • For the FB pin, i will take the capture.

  • 0 in reply to Olivier Eanuely
    Prodigy 10 points

    HEllo Arpita,


    Here are the capture. See that in comparison between the P5V and P3V3, I took same scale in axis. Behaviour is totally different between the two.

    Note that i measured Vin, and it is a stable 29V.

    Note also that there is electronic on the 3V3, there is a consumption of 300mA in output before the short circuit. There is nearly nothing in output of the 5V (approx 10mA).

    P3V3 behaviour :

    P3V3_short-circuit

    Zoom :

    P5V behaviour :

    Zoom:

  • 0 in reply to Olivier Eanuely
    TI__Intellectual 825 points

    Hello Olivier,

    Please provide the below information for better understanding. How are you shorting the output terminals in P5V case? Are you shorting using electronic load? The P5V case waveforms shows Vout has two level, 5V and 2V. Vout is expected to be lesser than these levels when output terminals are shorted.

    Are you shorting the output in pulsating manner like shorting the output for nearly 0.35ms and removing the short circuit for rest of cycle i.e. 1.65ms, and repeating the pattern?

    Regards

    Arpita