Inverting Switch Mode supply.

[kvresto]

Hi Everyone.

I need a negative power supply to run of a battery from 9V to 15V, to give me an output voltage of about -18V, so obviously this will be an inverting buck/boost topology. I believe most buck regulators can be used in an  inverting buck/boost topology?, or is there more to it than that. I'm looking at TI's TPS5402 3.5V to 28V Input, 1.7A Output, Non-Synchronous Step-Down Regulator with Integrated MOSFET as a possible candidate.

Has anyone had some experience in this area and can shed some info on what can and can't be used for this application (an inverting buck/boost topology).

I'm rolling my own boards so anything with a power pad/thermal pad is just a pain to use, so I have always kept away from them.

Thanks
kvresto

[ajb]

You can certainly do it, just be aware that in operation the converter sees the difference between Vin and Vout as its supply voltage, so make sure that your converter can tolerate that voltage.  So for example if you're generating a -5V rail from a 24V input, as far as the converter is concerned it's running from a 29V supply and bucking it down to 5V.  Obviously any control or status pins will also be referenced to the negative rail, so they'll require special handling to be able to interact with them from Gnd-referenced circuitry.

[bktemp]

The inverting buck configuration draws a high current on start up: The power supply must be able to supply enough current or the converter will not start up correctly or even short the power supply.
One workaround is a soft start or a a delayed start to allow the input capacitors to charge up and supply enough current.

[kvresto]

ok, thanks to all, and AcHmed99 those app notes were great.

The TPS5402 is a current mode device, and some of TI's/NS Simple Switchers are voltage mode devices, now is it true to say that early designs were voltage mode, and all latter design are current mode?

SIGSEGV is inevitable if you try to talk more than you know....................... Yep!