Inverting boost buck converter

[gmc]

I've been using a MT3608 chip for a while on some of my projects to generate 12V from a 5V USB supply which has been working great.

On a new project I need -12V and 5V so am wondering how I can use the below schematic to do this if at all possible, or do I need a specific boost chip which supports inverting? If so, any recommendations. 
Looking for -12V @ 250mA

[youngda9]

Check out the LT1372, datasheet has a couple of circuit topologies that you might be interested in on sheet 11 of the datasheet.  -5V converter, and +/-15V converter that could easily be altered to +/-12V as desired.  Something similar could probably be done with the chip that you're using for the flyback applicaiton.

[gmc]

Thanks. That looks like it will do just the job.

[mariush]

Those LT1372 chips are quite expensive... as most LT parts are.

There's Rohm BD81870 for example, 2.5$ for one piece, dual output switching regulator, one output is step up, the other is inverting : https://www.digikey.com/en/products/detail/rohm-semiconductor/BD81870EFV-ME2/8028007

Datasheet has  3.6v input,  6.2v at 200mA, -6.2v at 200mA example circuit at page 19 : https://fscdn.rohm.com/en/products/databook/datasheet/ic/power/switching_regulator_system/bd81870efv-m-e.pdf

With 5v input, you'd probably get close to +/- 9v at 200-250mA, I'm not sure, go through the datasheet and see what limitations are there.

There's also a cheaper BD8316GWL-E2 : https://www.digikey.com/en/products/detail/rohm-semiconductor/BD8316GWL-E2/4625662   - datasheet shows example circuit that does 2.5v - 5.5v to  +/- 5.6v 200mA



A cheap *34063 chip would also do the job of inverting, but no idea if the usb port can supply enough current to have both 12v and -12v at up to 250mA.  It won't do 250mA out inverting 5v to -12v, but 150mA would be possible.

For -5v, there's charge pump regulators that can invert the usb input, up to around 100mA they're cheap, around 1$ : https://www.digikey.com/short/pdrcr3q3


EDIT :  There's also much more efficient regulators compared to that MT3608 ... that one's only plus is that it's cheap.

For example MP3437 from MPS will get to 95% efficiency (datasheet shows example of 8v out up to more than 2A) :  https://www.digikey.com/en/products/detail/monolithic-power-systems-inc/MP3437GJ-Z/18088579

MP3435 is even cheaper and example even shows boosting to 12v : https://www.digikey.com/en/products/detail/monolithic-power-systems-inc/MP3435GL-Z/16639901

and loads of other chips...