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LP38851 / behavior of the over current protection

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: LP38851
Hello,
 
Please kindly let me know the behavior of the over curent protection of  LP38851.
 
(1)
Is LP38851 over current limit protection fold back control?
 
(2)
I tested the behavior of the over current protection by electronic load and resistor load.
Below my understanding correct ?
 
- In Electronic load
Vout=1.0V and Iout=2.4A
In this case, output voltage was decreased to 0V after turneon the electronic load.
I consider that since the electronic load pulls 2.4A , voltage was decreased to 0V.
 
-In Resistor load
Vout=1.0V, RL=0.4ohms.
In this case, output voltage was decreased from 1V(not 0V).
Iout is 2A.
I believe that this is behavior of the fold back current limit.
 
Best Regards,
Ryuji Asaka
  • 0
    TI__Mastermind 22610 points

    There is no ‘fold-back’ current limiting in LP38851.

    I presume that “current limit” is in reference to ‘ISC, Output Short-Circuit Current’.

    It is unlikely that you will be able to determine the output current by measuring the output voltage with either passive (resistor) or active load.

    You will need to use a current probe at the output pin, before Cout and any passive load. Active loads have voltage and bandwidth limitations. The Output Short-Circuit Current condition will not be sustained for any extended period of time as Pdiss vs Theta(j-a) will eventually push the junction temperature to the thermal shutdown threshold.

  • 0 in reply to Donald Jones
    Guru 20090 points

    Hello Donald san,

    Thank you for the reply.
    Please see the attached picture.
    This is output voltage waveform when 2.4A cc mode by electronic load.
    I understand that this ON/OFF operation is result of thermal shut down.
     
     
    If the Electronic Load increases constant current to 3A, what happens ?
    Iout will be restricted Isc and then Vout will be decreased ?
     
    Best Regards,
    Ryuji Asaka
     
     
     
     
  • 0 in reply to Ryuji
    TI__Mastermind 22610 points
    "... If the Electronic Load increases constant current to 3A, what happens ?..."
     
    Depends on how the electronic load behaves when it is programmed to sink more current than is available.
     
    ' ... Iout will be restricted Isc and then Vout will be decreased ?..."
     
    Yes. The output current reaches the limit and by definition (E = I x R) the output voltage must fall as the load demand increases (i.e. the Rload value decreases).