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LM5030: SS Pin of LM5030

Masami Michino
Genius 5070 points
Part Number: LM5030
Other Parts Discussed in Thread: LM25037


Dear, All

Customers are considering using the LM5030.
Please tell me about Soft Start when using COMP PIN.
If external error AMP output is input from COMP PIN, does the Soft Start function of the device operate with SS PIN?
Also like the LM25037, is it a way to lower the voltage input from COMP so that it does not exceed the voltage of SS Pin?

Thanks, Masami M.

  • 0
    TI__Expert 3095 points
    Hi Masami Michino, The external error amplifier should be configured so that it can only pull down on the COMP pin. Placing a diode from the COMP pin to the amplifier output should achieve this. The internal 5kR resistor should pull up the COMP pin to ensure that the diode is always forward biased in normal operation. During SS the COMP pin can then be internally pulled down to provide a soft-start function.
    Thanks
    Joe Leisten
  • 0 in reply to Joe Leisten
    Genius 5070 points


    Hi, Joe-san,

    Thank you for answering.
    Can I understand that it is as follows?

    The internal circuit of the LM5030 pulls down the COMP PIN voltage below the voltage of SS PIN.

    Thanks, Masami M.

  • 0 in reply to Masami Michino
    TI__Expert 3095 points

    Hi Masami-san,

    My understanding is that you should do as follows:

    1) Pull the VFB pin to GND. This will force the internal error amplifier to give a high output. This is provided by an internal 5kR pull up resistor.

    2) Connect your external error amplifier to the cathode of a diode. Connect the anode of this diode to the COMP pin. This allows the external amplifier to pull down the COMP pin but not to push it up. The COMP pin will rise when the external error amplifier rises due to the action of the internal 5kR pull-up resistor.

    3) Now the internal SS circuit can pull down the COMP pin at start-up.

    You are correct the SS circuit will pull down the COMP pin at start-up. For this to work your external error amplifier must only be able to pull current out of the COMP pin. It must not be able to push current into the COMP pin or the SS circuit will not be able to work.

    I hope all is now clear.

    Thanks, Joe Leisten

  • 0 in reply to Joe Leisten
    Genius 5070 points

    Hi, Joe-san,

    Thanks for your support.

    I understand that you need to use Open Drain type Op-AMP to connect to COMP PIN.
    What the customer wants to know is how the input of the SS Pin acts on the COMP Pin.
    Therefore, I asked you to confirm whether the following understanding is correct.

    The internal circuit of the LM5030 pulls down the COMP PIN voltage below the voltage of SS PIN.

    Thanks, Masami M.

  • 0 in reply to Masami Michino
    TI__Expert 3095 points
    Hi, Masami-san,
    You are correct the SS circuit will pull down the COMP pin at start-up. It will follow the SS pin voltage up until limited by either the internal or external error amplifier limits it or max COMP voltage is reached.
    Thanks
    Joe Leisten
  • 0 in reply to Joe Leisten
    Genius 5070 points
    Hi, Joe-san,

    Thank you for your answer.
    I will respond to customers.

    Thanks, Masami M.