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LM5122: Regarding Vss

Part Number: LM5122

Hi Team,

 My customer is evaluating the LM5122, and they want to know the detail of SS pin voltage(Vss). 

When the no load, Vss is around 3.8v, but when the load is 10A/phase, Vss is around 1.2v.

Is this a nomal behavior of this devise? If so, could you let me know how Vss will change?

Customer think that  Vss matintain aroud 3.8V after started.

best regards

TATSU

  • Hi Tatsu,

    Thank you for using E2E forums, regarding your query could you provide waveforms and other information? I can try to recreate this and get back to you tomorrow.

    Whether in your case this is normal behavior or not would depend on any influence on your Vout or SW node.

    I will get back to you as soon as possible.

    BR,

    Haroon

  • Hi Haroon

    Folowings are condition in customer. 

    And, where and what waveform is needed for you?

    <Conditions>

    Vin     25V~44V

    Vout    48V

    Iout     10.53A/Phase (Master, Slave interleave)

    MODE  FPWM(MODE pin connecting to VCC)

    RES   Connecting to AGND(Off the hiccup mode)

    ・Css  0.1uF

    <Question from customer>

    ・Problem is the decrase in Vout (ex;47.4V →46.4V.)  when changing from no load to 10A load.

    When Vss drops to around 1.2V, there is a problem that Vout drops(46.4V) below the target voltage(47.4V).

     When Vss falls below the error amplifier's Ref. 1.2V, Vss (<1.2V) becomes the + reference voltage of the amplifier. Customer think  that this may be causing the decrease in Vout.

    best regards

    TATSU

  • Hi Tatsu,

    Thank you for your reply, I will get back to you as soon as possible with some answers.

    BR,

    Haroon

  • Hi Tatsu,

    Thank you for your patience, regarding your query the Vss should not drop after adding a load, normally drop in SS is due to some error in the operation of the device either OVP or over current. Now you mentioned that the Vout drops by 1V, if that was not by design or within the range of the Vout that was set during the design, this could be loop stability issue and that is why your Vss is dropping as well. 

    BR,

    Haroon

  • Hi Haroon

     

     Thank you for your reply.

    My understanding is that when OVP or over current occur, Vss will decrease. Is this correct?

    If correct, the does the fact that Vss in customer board is decreasing mean that system detect the OVP or over current?

     

    And, is this problem(Vout is decreasing) generated by Vss decreasing? Or other courses ?

     

    Best regards

     

    TATSU