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LM5116 emulated diode

simone frasanni
Intellectual 285 points
Other Parts Discussed in Thread: LM5116

Hi everyone,

can someone explain me the differences between Full synchronous and Emulated diode in LM5116?

In full synchronous I see a rectangular waveform on switch node, both in CCM  that in DCM. Why there is a rectangular waveform in DCM or no load condition ?

If I set LM5116 in Emulated diode, I see the typical wavy shape waveform on SW node, in no load condition.

Thanks everyone in advance

  • 0
    TI__Guru**** 191445 points
    I am surprised they do not give more detail in the datasheet, but I suspect that they consider this to be common knowledge, not needing any additional explanation.

    For a buck converter the switch current is a triangular waveform with a dc offset. the dc offset portion is the load current. The switch current is increasing during the high side on time and decreasing during the high side off time. As the load current is decreased towards zero, a portion of the triangular current waveform may go below zero. Since the low side switch is an FET, it can conduct current in both directions and the SW node waveform remains a rectangular pulse train. This is "forced CCM". For diode emulation, when the switch current reaches zero, the low side FET is turned off. The FET body diode, acts as a catch diode. When the inductor current (switch current) goes to zero, there can be no voltage across the inductor so the SW node will go to Vout. there will always be some ringing as eh inductor current goes discontinuous. That is the "wavy" shape you see in diode emulated mode.
  • 0 in reply to JohnTucker
    Intellectual 285 points
    Thank John, I had supposed this, but I wasn't sure.
    I guess in full synchronous operation there isn't high efficiency at light load condition, because when the inductor current is zero, the low side FET continues to "suck" current from output capacitor. It's right ? If is so i can't reach high efficiency at ligh load during full synchronous operation.