LM5021: Is there any risk to power VCC pin from an auxiliary, 10V regulated supply?

John11513

Part Number: LM5021
Other Parts Discussed in Thread: UCC28C42

Hi experts,

My customer is designing a DC/DC module product based on LM5021. For some reason they must use this part though it is not fully meet the application condition. To make the device fit the application, they shorted VIN and VCC pin together and powering the two pins with an 10V regulated supply, which input is powered from a bias coil. I understand this configuration will disable VIN UVLO function. They implement VIN UVLO with external circuit. Is there any other risk for this kind of configuration? There is a NOTE in datasheet says "It is not recommended that the bias winding be connected to the VCC pin of the LM5021. Doing so can cause the device to operate incorrectly or not at all.". Customer worry about the reliability of this kind of application circuit so need us help them verify.

Thank you!

John

over 2 years ago

0 Alexis_H over 2 years ago

TI__Genius 17795 points

Hello John,

Thank you for your patience. Having a VIN UVLO with an external circuit should be fine, however, VCC is not designed to handle 10V, hence the need for the 8.5V linear regulator with the VCC UVLO rising being 7V. By shorting the VIN and VCC pin together and powering it with a 10V regulated circuit, the customer will most likely damage the internal components so I would highly recommend NOT doing this. Please tell them to follow the datasheet's limits to ensure the reliability of the part.

Best Regards,

Alexis

0 John11513 over 2 years ago in reply to Alexis_H

TI__Expert 5720 points

Alexis_H said:
ot designed to handle 10V,

Hi Alexis,

Thank you for reply.

I understand it is not recommended. But it is the only way for them to use this device in one of their specific project.

Do you meant apply 8.5V on VCC and VIN is ok? what is the allowable range?

Regards,

John

+1 Alexis_H over 2 years ago in reply to John11513

TI__Genius 17795 points

Hi John,

I would recommend trying to use a different flyback controller that does not have a startup circuit like the UCC28C42 (UCC28C42 data sheet, product information and support | TI.com) instead. Is that a device they have considered for their design instead?

As the datasheet recommends against this sort of design in the first place with the LM5021, I cannot say how reliable their design will be.

Best Regards,

Alexis

0 John11513 over 2 years ago in reply to Alexis_H

TI__Expert 5720 points

Ok, thank you.

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LM5021: Is there any risk to power VCC pin from an auxiliary, 10V regulated supply?