• State Resolved
  • Locked Locked
  • Replies 10 replies
  • Subscribers 64 subscribers
  • Views 897 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM25037: Current Limit Restart

Kuramochi Tadahiko
Guru 28715 points
Part Number: LM25037

Hi,

I received the following question from my customer.

It is mentioned that "The SS voltage increases at its normal rate, gradually increasing the duty cycle of the output drivers." on the datasheet p11.

However my customer hopes to return normal duty operation after removing current limit immediately.

Then, can the SS pin be opened?

Or is there any other way?

Best Regards,

Kuramochi

  • 0
    TI__Mastermind 40580 points
    Hello Kuramochi-san,

    I have notified the appropriate team of your question, and an answer should be posted soon.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    TI__Mastermind 25570 points

    Hello Kuramochi-san

    The restart sequence that delays the normal operation after a current limit may be defeated by tying the RES pin to GND as described in the DS extract below.

    Please let me know if you need any further information.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Guru 28715 points

    Hi colin-san,

    Thank you for your response.

    I'm understanding that current limit works after current limit detection immediately when connecting a resistor during the RES pin and GND such as attached file.

    LM25037.xlsx

    My customer hopes  "returning" normal duty operation after removing current limit immediately.

    Is my understanding wrong ?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 25570 points

    Hello Kuramochi-san

    Your understanding is correct but I'm not really sure of what you are trying to illustrate in the diagrams.

    Current limiting is done on a cycle-by-cycle basis. The controller will terminate a cycle when the CS pin exceeds the limit threshold of 255mV. This is described in more detail in section 7.3.5 of the data sheet. Connecting the resistor from RES to GND prevents hiccup mode operation and allows continued operation in current limit if the overload persists or an immediate return to normal operation when  when the current limit condition is removed - ie when the load current is reduced to less than the current limit point.

    This is just a summary of how the controller behaves when RES is connected to GND. Please have another look at the DS, sections 7.3.5 and 7.3.6 and if you have questions then please post a message here and I'll try to answer them.

    Regards
    Colin

  • 0 in reply to Colin Gillmor
    Guru 28715 points
    Colin-san,

    Sorry for my uncleanness.
    What should they do for following their request?
    >my customer hopes to return normal duty operation after removing current limit immediately.

    Best Regards,
    Kuramochi
  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 25570 points

    Hello Kuramochi-san

    If the RES pin is tied to GND the controller will continue to operate in cycle-by-cycle current limiting mode if there is an output overload. The controller will resume normal operation immediately after the load current is reduced below the current limit threshold - that is, recovery is immediate. Do note that if the controller is operating in cycle-by-cycle current limit mode the output voltage will drop. How much it drops by is a function of the overload current. When the overload condition is removed, here will be a short transient as the output voltage recovers to its original regulation setpoint.


    I hope this answers your question, if it does not then please re-post.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Guru 28715 points

    Colin-san,

    Thank you for your help.

    My way of the question was wrong.

    I'm understanding that the LM25037 cannot resume normal operation immediately after the RES voltage hits to 2V.

    Because  it takes the t2(Figure 3) to resume normal mode nevertheless the overload timer function.

    So I'm considering that the Css must be removed to correspond customer's request.

    How is my understanding...?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 25570 points
    Hello Kuramochi-san

    You are correct, if RES reaches 2V then it triggers a hiccup mode / soft-start cycle. If there is no capacitor at the SS pin then the soft-start cycle will be very short - there will always be some stray capacitance at the SS pin. If there is no capacitance at the SS pin then the system will lose the soft start feature at start-up. The best way to ensure a rapid return to normal operation after a current overload is to tie the RES pin to GND - this defeats the hiccup mode and allows the system to return to normal operation immediately the overload is removed. If you do this, you can leave the soft start capacitor at the SS pin.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Guru 28715 points
    Hi Colin-san,

    Thank you for your answer!

    When the RES pin tied to GND, the Css is not discharged if the RES pin reaches 2V.
    So the system returns to normal operation immediately the overload is removed.
    Is my understanding correct?

    Best Regards,
    Kuramochi
  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 25570 points
    Hello Kuramochi-san

    Your understanding is correct, if the RES pin is tied to ground the system returns to normal operation immediately the overload is removed - Note that if the RES pin is tied to ground then it can never reach 2V.

    Regards
    Colin