LM27403 suitability for inverting buck-boost

An update:

In the meantime, I remembered that I have some TINA simulations of circuits using LM27403 in a conventional situation (i.e. NOT configured as inverting buck-boost).  I thought I might rewire one of these for IBB and see what happened.  It didn't work.  After about 100us, the gate drives start clocking, but the duty cycle increases until the upper MOSFET is on all the time, resulting in 200A flowing from VIN to ground through the inductor.

However, I think that I am seeing either a problem with TINA, or with the LM27403 spice model, or with my own ability to drive TINA, because  I notice that if I revert to the working non-IBB circuit (which simulates fine) and then simply move the (only) ground symbol from the 'normal' ground net onto the nomal output net (the IBB ground), the above behaviour ensues.  I can't see why moving the only ground symbol from one net to another should make any difference? Perhaps I don't understand the workings of TINA.

Incidentally, if I revert to the working circuit again (with the ground symbol on the 'normal' ground net) and then attempt IBB operation by simply moving the negative power input from ground to the 'normal' output, then it still doesn't work: now there is no gate action at all.  I am investigating further.

HELP!

I will move this to the simulations and modeling forum. But generally speaking, the models cannot be used for inverting buck boost operation. Internally, the GND blocks are hard coded to be ground.

Hi Ian,

The LM27403 spice model does not support IBB simulations (the reason is that the GND pin is internally tied to node 0, hence it can not be floating or work with that node tied to a non zero voltage). We are adding IBB capability to some of the newer models.

If you are following a modeling based design approach and need a model that supports IBB, I would recommend going to the link below for all models filtering out by Model Description.

If you want us to look into the TINA-TI related issue, please send us the TSC file.

Hi John, Nikhil

Thank you for your valuable help.

First, regarding John's point about the Spice models making internal connection to the ground net: I had begun to suspect this after my previous experiments, however, I still don't understand why it doesn't work if I leave TINA's ground on the controller ground and then instigate IBB by simply connecting the negative end of the power input to the output from the inductor. In the mean time, having noticed another application note which specifically covers the use of TPS40170 in IBB mode (PMP6867), I set up a webench design using that controller (in conventional mode) and exported it as a TINA design (cool feature by the way), which worked fine... until I converted it to IBB, which didn't work - in exactly the same was as my previous attempt with LM27403, even with TINA's ground connected to the controller's ground.

Regarding Nikhil's suggestion to look in the spice rack for a device whose model supports inverting mode: I can't do that since I need a controller solution with external MOSFETs, and all the devices in that table, which filtered for 'inverting' are internal switch or module. In any case, it would be preferable not to have to select a device by the availability of a working model.

So, I suppose in summary I am wondering what is the recommended design methodology for using an external-MOSFET-buck-controller in IBB mode?

PS: I found John's article on the IBB topology (SLYT286) really useful when I was first researching POL design. Thanks!

Hi Ian,

I would also recommend looking at some reference designs (TIDesigns). These are proven designs where BOM is available. For your case, filter by "Buck Boost" and then "Inverting". Then look for the one that is closest to what you want.

www.ti.com/.../refdesignsearchresults.tsp