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LM5116: LM5116 DEMB Pin Question

Wei-Hao Chen (TW-FAE)
Expert 3260 points
Part Number: LM5116

Hi support team champs,

My customer is using LM5116 for the 600W PSU design for 70VIN to 14VOUT. They are having some question regarding the DEMB pin of LM5116. 

1. They are using external gate driver and SW is connected to ground. Is there a way to maintain the DEMB function while SW is grounded? There will be no zero crossing at all on SW, suppose the LM5116 will always work in full synchronous rectification mode? 

2. What happens if leaving the DEMB pin float? Any downside effect that might happen? They are having some issue during the startup when the DEMB resistor is added but removed the DEMB resistor can address the issue. I will attach the waveform tomorrow.

Thank you.

Best Regards,

Wei-Hao

  • 0
    TI__Expert 6245 points
    Hello Wei-Hao,

    The DEMB pin detects a voltage vs. ground to determine whether to turn off the LS FET or keep it on during freewheeling. You likely have a PWM node somewhere in your design even if not connected to the controller's SW pin. If this power stage is referenced to ground, you can still use this feature to get diode emulation.
    If the circuit's SW node is not referenced to ground, tie the DEMB pin to ground through a resistor large enough to generate the needed offset. The DEMB sources current to allow this.
  • 0 in reply to Robert Blattner
    TI__Expert 3260 points

    Hi Robert,

    I am confused by what do you meant by DEMB pin detects a voltage vs ground during the freewheeling stage. The internal DEMB comparator compares the SW pin voltage and DEMB pin to detect the zero-crossing as datasheet illustrated: 

    Both SW and DEMB pin are grounded now, how does diode emulation can be achieved? Internal comparator has no way to detect when the current hits zero-crossing.

    I do not understand what do you mean by using the "PWM node somewhere in your design even if not connected to the controller's SW pin"?? Could you please be more specific? 

    Lastly, what is the effect if DEMB pin is floating? I suppose the controller will always operates full synchronous mode? because the internal PNP will always be conducting and the DEMB will follow the SS voltage.

    Thank you.

    Best Regards,

    Wei-Hao

  • 0 in reply to Wei-Hao Chen (TW-FAE)
    TI__Expert 6245 points
    Hello Wei-Hao,

    If the DEMB pin is left open the part will continue to operate. It is better to tie it low through a10k resistor so that the pin is not left floating.

    Since you are building PWM converter there should be a power stage, the PWM node, somewhere in your circuit. You can send a schematic to my email, robert.blattner@ti.com if you want to pursue this but not in a public forum.
  • 0 in reply to Robert Blattner
    TI__Expert 6245 points
    Hello Wei-Hao,

    Since this thread now involves diode emulation without SW tied to the power stage's SW node, I am handing this thread over to someone with PDS experience, David.
  • 0 in reply to Robert Blattner
    TI__Expert 3260 points
    Hi Robert,

    Got it thanks.

    --

    Hi David,

    Could you please provide some idea how can achieve it?

    Thank you
  • 0 in reply to Wei-Hao Chen (TW-FAE)
    TI__Mastermind 40040 points
    Hello Wei,

    Discussion continued via email.

    David.