LM5145: Negative current during Diode Emulation Mode with no load

user6244360

Part Number: LM5145

Hi team,

I want to use LM5145 as a buck controller, 48V input and 12V/5A output. I connect SYNCIN to GND and find that, in Diode Emulation Mode with no load, the inductor current will be negative at VLO falling edge. I think if there is a diode, there should be no negative current. Did I test it in a wrong way? or it maybe a IC error? or is there any advantage to make inductor current be negative?

Best regards

Dong Chen

over 5 years ago

0 Xing He over 5 years ago

TI__Intellectual 2025 points

When LO turn off, the parasitic capacitance of MOSFET resonance with inductor so you find the negative current. It should be small, and the inductor current is sine wave.

Best Regards

Xing He

0 user6244360 over 5 years ago in reply to Xing He

Prodigy 40 points

Hi Xing,

Thanks for your reply.

Below is my test waveform.What I want to know is shown as red circuit. If ZCD detection threshold is exact, low-side MOSFET should be turned off as soon as IL decreases to zero. However, the inductor current is negative and low-side MOSFET still turns on. Is this phenomenon normal? What is possible reason for it?

After VLO is low level, the resonance should be caused by parasitic capacitor and inductor.

0 Xing He over 5 years ago in reply to user6244360

TI__Intellectual 2025 points

Hi,

Can you figured out all the waveform in your picture represent for what voltages or currents?

Best Regards,

Xing

0 user6244360 over 5 years ago in reply to Xing He

Prodigy 40 points

Hi Xing，

The green one is inductor current, the dark blue one is LO pin voltage，the light blue one is SW pin voltage，the purple one is syncout pin voltage.

0 Xing He over 5 years ago in reply to user6244360

TI__Intellectual 2025 points

Hi,

It is right l when ZCD detection threshold is exact, the low side switch should be turned off. The Diode emulation zero-cross threshold is between -5mV~5mV.

You can check you low side MOSFET Rdson, if the Rdson*IL is between -5mV~5mV, it will have the negative current like your waveform, this is the normal waveform.

Best Regards,

Xing

0 user6244360 over 5 years ago in reply to Xing He

Prodigy 40 points

Hi Xing,

Thanks for your support!

Rdson is 13mohm and Rdson*IL=13*392=5.096mV, a little above the range.

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LM5145: Negative current during Diode Emulation Mode with no load