multiple voltage regulators ?

[AndNowTheBastard]

Hi All,

I'm just wondering if it would be a good idea instead of having one big linear regulator to have several smaller ones spread across the PCB maybe near IC's that require more stable current.. I dont know... 
I've seen designs that do this, is there any benefit or is it a waste ?
I've a mixed design and I thought it would be a good idea to have a separate regulator, is it crazy ?

Thanks in advance.

[Bored@Work]

It is called point-of-load regulation, and there are several reasons for doing it. This PDF lists a few http://www.semtech.com/images/promo/Developments-in-Point-of-Load-Regulation.pdf (including the obligatory advertising for a particular part).

[Simon]

if by mixed you mean analogue and digital then I think separate regulators will be a good idea but I'd not go overboard unless it is really neccessary

[ejeffrey]

Yeah, separate regulators (with separate input filters) for analog and digital sections is pretty standard, and mostly required if you are doing low noise analog.  Another reason is if you need a low power standby supply like computers use to support wake-on-lan and so forth.  Generally there should be a pretty clear reason why you want to do it.  Up to a reasonable point a single larger regulator will be cheaper and easier than multiple small regulators.  If you start getting into really high currents, maybe there is a narrow region where it is easier to put multiple standard regulators on board rather than custom designing a high power regulator with discrete pass transistors.

[AndNowTheBastard]

It is called point-of-load regulation, and there are several reasons for doing it. This PDF lists a few http://www.semtech.com/images/promo/Developments-in-Point-of-Load-Regulation.pdf (including the obligatory advertising for a particular part).

Hi BoredAtWork !

Thanks you for your feedback, that was exactly what I was looking for !

if by mixed you mean analogue and digital then I think separate regulators will be a good idea but I'd not go overboard unless it is really neccessary

I really don't know how necessary they are, the figures of my project:

My input opamp must sense a very low current going to a shut resistor, the voltage drop is from 0 to 80mv (full range) and the ADC is 12 bits, there are a lot of leading zeroes everywhere !, most LDO's I've seen have a ground noise of about 20mv that's bad.
I thought about using a couple of very precise voltage references to power up the opamp locally, they are really small and one of them could power up a few opamps on it's own !

What do you think is it reasonable to use local regulation in this case ?

Regards.

[Simon]

if you need the precision and into the bargain are getting rid of noise from a lower quality reg yea I'd go for it. Your working with quite small input voltages

[ejeffrey]

20 mV seems like way too much ripple for an LDO.

Having a separate regulator for your op-amp is a good idea here.  The op-amp should ideally have its gain set so that your 80 mV full scale reading is around 80% full-scale on your ADC, so you shouldn't have a problem with leading zeros.  In that case, one LSB will be around 25 microvolts.  This isn't a terribly small number, but it is enough that you need to put a little care into noise if you want maximum performance.  Op-amps have fairly good power-supply rejection, at least at low frequency, but it is always best to start off with a quiet supply.

I wouldn't use voltage references as power supplys as a general rule.  They can vary quite a bit in behavior, but they work best as references: with a small, constant load.  Use the reference for your ADC reference, or for a virtual earth if necessary, but any linear or LDO regulator should perform fine as an analog supply as long as you follow the recommendations for capacitance.