LM46002 Webench value of Cboot

Nikolay Gavrikov12

Prodigy 40 points

Other Parts Discussed in Thread: LM46002

Hello All,

I have designed a step-down converter using LM46002 with following parameters:

Vin: from 14 to 22 volts

Vout: 3.3V

Iout: 2A

In schematics I got maximum voltage of Cboot is 35V but in datasheet for LM46002 absolute maximum ratings between BOOT and SW pins is only 5.5V. Mistake in Webench?

Best regards,

Nikolay Gavrikov

over 9 years ago

0 Juan Paulo Fung over 9 years ago

TI__Expert 3840 points

Hi Nikolay - The design schematic shows the values of the passive components and the DC rating for each component. In the CBOOT case, the 470nF capacitor that's readily available is rated to 35V DC; that does not mean the voltage across CBOOT-SW is 35V. I hope this answers your question and clears your concern. Thank you!

Best Regards,
-JP

0 Nikolay Gavrikov12 over 9 years ago in reply to Juan Paulo Fung

Prodigy 40 points

If you increase Vin to 30...60V, DC rating for Cboot in Webench will be increased to 100V. That means I can't place there 50V capacitor in this situation?

0 Oleksiy Pazyuk over 9 years ago in reply to Nikolay Gavrikov12

Expert 2585 points

Hi, Nikolay,

Bootstrap capacitor Cboot rating is not related with Vin. Cboot is required to create bias for internal driver of the HS switch. Roughly voltage across it is equal to the Vcc=3.2 (typ), while absolute value on the Cboot pin is VCC + Vsw. Thus Cboot could be rated for 6.3V minimum. Higher rating is possible and depend on component stock which you have.

0 Nikolay Gavrikov12 over 9 years ago in reply to Oleksiy Pazyuk

Prodigy 40 points

Ok, so it is bug in Webench. Thanks a lot!

Best regards,

Nikolay

0 Oleksiy Pazyuk over 9 years ago in reply to Nikolay Gavrikov12

Expert 2585 points

I guess it is not a bug of Webech, it is "feature". There was selected component which satisfies application from internal available database. And there was no 470nF capacitor with 6.3V rating in this database.

0 Nikolay Gavrikov12 over 9 years ago in reply to Oleksiy Pazyuk

Prodigy 40 points

I wrote early that if you will increase Vin to 30...60V, than Cboot rating will be increased to 100V. And furthermore, if you click to capacitor you can see upperbound and lowerbound limits for it: from 90V to 300V

0 Kevin Ip over 9 years ago in reply to Nikolay Gavrikov12

TI__Intellectual 2155 points

Hi Nikolay, Oleksiy,
You can actually use a 6.3V rated capacitor for Cboot. Currently, the limits in WEBENCH is using VinMax, but this may be over-rating the voltage. We will make an update to improve the component limits for Cboot.

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LM46002 Webench value of Cboot