Converter IC for buck- bost application, choosing between LM5576 & TPS54260 !

Posted by Michal Grabarczyk

on Dec 14 2011 15:30 PM

Prodigy30 points

Hello All,

This is my first project connected with DC/DC converters. Therefore all kind of remarks will be appreciated.

I have to design buck-boost converter (inverting configuration) meeting following requirements:

U in_min = 6V

U in_max= 36V

Uout = - 5V

I out_min = 28 mA

I out_max =1A

I out_typ = 100-200mA

Moreover an important issue is also converter footprint. It have to be as small as possible.

I tried to use WEB BENCH and SWITCHER PRO software. Once the input data is processed WEB BENCH gives only two possible solution with LM5576 & LM25576.

In SWITCHER PRO I can't define output voltage with value less than 0. It looks like inverting configurations aren’t implemented . Is that right ?

Any way I found some IC form TI side, which may meets requirements - TPS54260. In datasheet is showed that it can be used in inverting configuration. It have also small footprint.

I also found application note SLA317a and excel used to calculate parameters values for this IC.

There are few questions which I have at this stage of design :

1.Are there any difference in operating between LM5576 & TPS54260 working as buck- boost converter ( taking my requirements into account) ?

2. Does anyone have experience with TPS54260 as buck-boost converter , with similar specification?

3. What else design support I can get beside SLA317 application note? Do you know why WEB BENCH doesn't include this solution?

4. What about evaluation board for buck-boost converter? Can I use standard board for TPS54260 and just switch the output signals (OUT to GND, GND to OUT).

Posted by David Daniels

on Dec 14 2011 17:22 PM

Intellectual1125 points

Michal Grabarczyk,

Thanks for the interest in a TI power solution.

In both cases, you are using a step down regulator as an inverting supply. The enable and clock synchronization will be referenced to chip's ground which will now be the system's negative voltage. If you are coupling these pins to other components in the system you may need to level shift.

The TPS54260 has a smaller package (msop or son) and higher switching frequency which you can use smaller inductor and output capacitors.

Both devices are externally compensated which can the changed to compensate for the inverting buck boost.

Using the TPS54260 and derivative devices is a popular method to create a negative rail, hence the app note and excel sheet.

This inverting configuration can also create a positive/negative supply by using a catalog coupled inductor (see slva369).

The slva317a app note and excel sheet are best method we have to instruct on how to use the device until the webench integration.

Many users tweak a standard step down evm by adding a bulk capacitor from VIN to VOUT, changing the inductor and frequency compensation to evaluate the inverting configuration.

Thanks Again,

David