TPS5450: 12V Flybuck Design

Hayden_Hast

Part Number: TPS5450
Other Parts Discussed in Thread: LM53603, LM43603, LMR23630, LM5017, LM5161

Hello,

I'm using the TPS5450 in a flybuck configuration and am seeing the Iso-12V output drop from 12V to 9.5V when 200mA of load is connected to the Isolated load and ~10mA load connected to the non-isolated 12V output. The non-isolated 12V output is good at 12V so perhaps the feedback network needs to be adjusted to take the 12V_ISO output into consideration? Any guesses to what is causing the drop in voltage and how ti can be remedied?

I've included the a screenshot of the schematic below. Please let me know if you have any additional questions.

Regards,

HaydenTPS5450_12V_Flybuck.docx

over 7 years ago

0 Frank De Stasi over 7 years ago

TI__Guru 57555 points

I am not sure that you can use a non-synchronous converter for a fly-buck.

You will need to use a synchronous device such as the LMR23630AF with FPWM option or the LM53603.

0 Hayden_Hast over 7 years ago in reply to Frank De Stasi

TI__Intellectual 1515 points

Frank,

What's the theory behind why a synchronous converter is needed over an asynchronous? Is there an app note discussing this? I've read through the design collateral here (www.ti.com/.../design_resources.html) but didn't find any detailed explanations.

Thanks,
Hayden

0 Hayden_Hast over 7 years ago in reply to Hayden_Hast

TI__Intellectual 1515 points

Also, both the LMR23630AF and LM53603 are only rated up to 10V Vout and I need 12Vout. Is FPWM mode a requirement for a fly-buck configuration, because if not I would like to use the LM43603. If it is, do you have another suggestion that would be able to provide 12V Vout?

Regards,
Hayden

0 Frank De Stasi over 7 years ago in reply to Hayden_Hast

TI__Guru 57555 points

I think that the inductor current may need to go negative under some load conditions.

However, I have not worked with the flybuck in detail. I have attache an app note that might help.

Flybuck_AN2292.pdf

0 Frank De Stasi over 7 years ago in reply to Frank De Stasi

TI__Guru 57555 points

The LM53603 has a 10V limitation. I do not think that the LMR23630 has that limitation.

You can also use the LM5017. The LM43603 does not have FPWM.

0 Hayden_Hast over 7 years ago in reply to Frank De Stasi

TI__Intellectual 1515 points

Frank, thanks for the assistance. You're correct, the LMR23630 would work, I was looking at the wrong datasheet.

We ended up going with the LM5161, which should work for this design as well.

Regards,
Hayden

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TPS5450: 12V Flybuck Design