LM22675 inverting buck circuit

[pardner]

On page 16 of the LM22675 datasheet (https://www.ti.com/lit/ds/symlink/lm22675.pdf) it shows the device being used in an inverting buck mode. My question in the circuit is what is Vout (top right of circuit)? THe datasheet does not have an explanation of the circuit and I didn't see an explanation of using the part in inverting mode.



I am doing a design with 24Vin and I need to drop it down to +5V and -5V. Would be great if the LM22675 was able to provide both, that way I wouldn't need to put two switchers on my board!


Thank you for any help,
~Pard

 

[andre_luis]

According to the manufacturer guideline, RFBB and RFBT defines a resistive voltage divider, so that applying values from circuit above would yield something close to 1,8v.

 

[pardner]

Right, so if I tune the feedback resistors to 5V, would I be able to get + and - 5V outputs?

 

[andre_luis]

Yes, that´s exactly the purpose, but I´m assuming you are referring just to the +5v output.

 

[D.A.(Tony)Stewart]

This chip can only generate 1 output voltage. It uses a fixed +1.285V reference for negative FB pin into a comparator to control the error.
It requires a separate external +5V regulator to pullup the generated -5V to the FB input to 1.285V.

Dual output chips exist but perhaps cost more than two single regulators unless all you need is a charge pump at 30mA or so.