LM5161-Q1: Fly-Buck-Boost the negative voltage on SW in start-up

tateo yamashiro

Part Number: LM5161-Q1

Hi,

We got a question from the customer about LM5161-Q1.
Could you help us?

[Question]
They evaluated LM5161-Q1 in Fly-buck-boost topology on their own board. Before the IC starts switching in start-up, the negative voltage occurs on SW to GND. Please refer to the attached file for the details.

- They assume this phenomenon occurs due to charge current flows through Cin to Cout. Is this their understanding correct?

- How is the level of negative voltage determined? They assume there are 2 factors.

1. The ratio of Cout to Cin.
2. Vf of the body diode on low side FET.

- Is there any solution to prevent the negative voltage beyond the rating of SW and damage to the body diode due to the rush current?

Best Regards,
tateo

over 5 years ago

0 Arief Hernadi over 5 years ago

TI__Mastermind 23815 points

Hi,

Do you have also the waveform of inductor current?

Thanks
-Arief

0 Arief Hernadi over 5 years ago in reply to Arief Hernadi

TI__Mastermind 23815 points

Hi,

Let me know if you have any more questions, feel free to open the thread if you do

Thanks
-Arief

0 tateo yamashiro over 5 years ago in reply to Arief Hernadi

Guru 10890 points

Hi,

Sorry for the delayed reply. Thank you for your support.

Now I'm asking the customer to provide the waveform of primary trance current. I'll let you know if I get the waveform.

Thank you for your cooperation.

Best Regards,

tateo

0 Katelyn Wiggenhorn over 5 years ago in reply to tateo yamashiro

TI__Genius 11040 points

Hello Yamashiro-san,

The E2E post will lock after 30 days. I am going to close this post, but please feel free to create another E2E post if you have additional questions.

Best Regards,
Katelyn

Power management

Power management forum

LM5161-Q1: Fly-Buck-Boost the negative voltage on SW in start-up