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LM2673: Negative output- Softstart Cap and IAdj connections

ee tech

Prodigy 220 points

Part Number: LM2673

Need help with Positive to Negative design.

I've read AN-1157. The schematic, Fig. 6, shows one end of the Iadj resistor and Css (softstart) capacitor connected to -Out. It seems if connected this way that the varying output of -Out would alter the programmed values. Since these are programming components, shouldn't these remain connected to supply ground?

thanks

over 5 years ago

0 Frank De Stasi over 5 years ago

TI__Guru 72835 points

Hello

The connection is correct. All of the internal circuits of the

buck regulator are referred to its "GND" pin. In the case of the

inverting-buck-boost configuration, the "GND" pin is at the negative

Vout level. That is why if you need to control the device, such as using the enable input,

that you must level shift it to the system ground.

This is true of all buck regulators when used as an IBB.

The attached app note may also be helpful.

Thanks

1222.snva856.pdf

0 ee tech over 5 years ago in reply to Frank De Stasi

Prodigy 220 points

Hi

I'm confused....

AN-1157, Section 2, "Design considerations" states:

"Therefore no extra level shift and inversion of feedback signal is needed to properly regulate the negative output"

Please clarify.

Also,

Are the Positive to Negative Converter boards still available for purchase?

Thanks

0 Frank De Stasi over 5 years ago in reply to ee tech

TI__Guru 72835 points

Hello

That is correct; you don't need any level shift for the FB.

The level shift is required only if you need to control the device

from an external controller.

Thanks

0 ee tech over 5 years ago in reply to Frank De Stasi

Prodigy 220 points

OK. Thanks.

Question:

is the Positive to Negative demo board shown in AN-1157 still available for purchase?

0 Marshall_Beck over 5 years ago in reply to ee tech

TI__Mastermind 35210 points

Hello EE tech,

No, the EVM of the device can be used to perform the analysis as it was done here.

Appropriate care in how connections are made to input and output is all that has to be done.