Two independent switching regulators (a buck and a boost) on one board

[Politecnico]

Hello Everyone,

In terms of noise and interference, what are the consequences of using an independent DC-DC buck converter (or a DC-DC buck-boost) and an independent DC-DC boost converter on a battery-powered design (one PCB)? Are there other consequences which need to be prevented / taken into consideration?

Thanks!

 

[treez]

Do you mean in cascade?....if so, beware their feedback loops may fight each other....unless their feedback loop bandwidths are very different.
Then again if the cap bank between them is very large then their is less chance of such instability.

 

[KlausST]

Hi,

With a good PCB layout and shielded inductors I see no problem. Maybe add filters..

Klaus

 

[Politecnico]

Thanks a lot treez and KlausST.
I think my question was not clear enough. Having battery voltage of 4.2V (fully charged) down to 3V (fully drained), I need stable 3.3V and ±5V. Based on the attached schematic, will there be any interference between the two switching regulators? Besides, in terms of the switching frequencies of the two converters, what should be the relationship between them? Thanks!

 

[treez]

It will be ok...just keep input caps close to the switching stage in each converter. Then interference between them is minimised.
Attached is layout doc for you

 

[schmitt trigger]

There are SMPS controller chips which have synchronization terminals.

But in my experience, the only time I required using them was with analog TV signals.

 

[treez]

Yes I worked in one place, where they were convinced that sync'ing the switching frequencies of converters like shown here meant better chances of passing EMC.....obviously interleaved sync'ing is best, but even plain sync'ing apparently gives slightly more chance of an EMC pass.

 

[BradtheRad]

Discussions have pointed out that inductors might interfere with one another if they are placed in close proximity on a board, especially if one carries much current.