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LM74900-Q1: OCP vs Short Circuit

PY (Pengyu) Song
Expert 5860 points
Part Number: LM74900-Q1

Hi team,

What is difference between overcurrent protection and short circuit protection within LM74900-Q1, in terms of - 

  • Behavior? 
  • Response time?
  • Threshold? 
  • Can use short circuit protection with higher threshold set to protect against overcurrent (as short circuit protection has faster response time)? Any potential drawback? 
  • 0
    TI__Guru** 105361 points

    Hi Pengyu,

    Thanks for reaching out!

    Equation 3 in the data sheet can be used to set the overcurrent protection threshold. The response time is a function of capacitor on the timer pin.

    Riscp resistor can be selected using Equation 6 to set short circuit protection threshold independently. It is also a type of overload protection but with faster response time (typ 5us). 

    You can only use short circuit protection and disable overcurrent protection by shorting ILIM pin to GND.

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    TI__Expert 5860 points

    Hi Rakesh,

    Thank you for your feedback but I'm still not clear on - 

    1. Fundamentally, why this device has both overcurrent protection as well as short circuit protection?
    2. In other words, to answer Q1, what are the key differences between overcurrent protection and short circuit protection? For example - 
      1. Short circuit protection uses comparator which is faster but less accurate?
      2. Overcurrent protection uses current sense amp which is slower but more accurate? 
    3. Use case - 
      1. Short circuit protection to set at higher current limit than overcurrent protection? 
      2. If application need faster response, can customer use short circuit protection as overcurrent protection? Would the potential drawback include lower accuracy?
      3. And any other drawback like fault response? 
  • +1 in reply to PY (Pengyu) Song
    TI__Guru 57737 points
    PY (Pengyu) Song said:
    has both overcurrent protection as well as short circuit protection?

    Hi Pengyu,

    As you mentioned there are two levels of protection - one is over current protection and the other is short circuit protection. Generally, the short circuit threshold is set higher than over current threshold.

    When the load current goes above over current threshold (programmable using RILIM and RSET resistors) , the LM74900-Q1 controller allows the over current to flow for Timer duration (programmable using TMR pin) before turning OFF the HGATE and cutting off the power path. As you can see in diagram below,

    • Instance 1: When the load current is above over current threshold for less than timer duration, the controller allowed the current to flow and did not turn OFF the HGATE
    • Instance 2: When the load current is above over current threshold for more than timer duration, the controller turns OFF the HGATE and blocks the power path.

    When the load current is above short circuit threshold (programmable using RISCP) the controller turns OFF the HGATE immediately with in 5us. This protection is not dependent on the timer duration.