WEBENCH® Tools/LM3481: For the EVM schematics, what is the functions of diode D4 and D5?

Martin Sun63

Prodigy 70 points

Part Number: LM3481
Other Parts Discussed in Thread: TL431,

Tool/software: WEBENCH® Design Tools

Engineer just modified R16 to 1Ku, but got 6.2V output, not 5V what he wanted. Could you analysis?

over 6 years ago

0 Youhao Xi over 6 years ago

TI__Guru 71275 points

C17 provide a soft start function, allowing the error voltage to grow up slowly through D5. D4 provide a quick discharge path of C17 after shutdown, therefore, the soft start can function even if the circuit is turned off and quickly turned back on again.

Your 2nd question regarding the R16 sounds weird. Sounds like the TL431 is somehow damaged. Can you replace the LV4341, keep R16=3,83k, but change R18=3.83k, too?

0 Martin Sun63 over 6 years ago in reply to Youhao Xi

Prodigy 70 points

Thank you for quick reply.
Customer found that it is OK when R16=3.83K, only changed it to 1K, the output was not regulated. I thought that the value of R14 is a little large, can not provide enough current flowing through TL431. Am I right?

0 Garrett Roecker over 6 years ago in reply to Martin Sun63

TI__Mastermind 21720 points

Hi Martin,

That is correct. R14 will need to be sized based on the output voltage selection. This application note about using the LM3481 as a flyback will help with the calculations.
www.ti.com/.../snva761.pdf

Thanks,

Garrett

0 Martin Sun63 over 6 years ago in reply to Garrett Roecker

Prodigy 70 points

Very good, thank you very much!

Power management

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WEBENCH® Tools/LM3481: For the EVM schematics, what is the functions of diode D4 and D5?