Switching between standby and main voltages to power chips

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I need to power some noise-sensitive mixed signal chips with both 3.3V and 1.8V. Noise sensitivity can be taken care of with 3.3V and 1.8V LDO.

The problem is, the chips need to be powered on all the time, and in standby mode I only have low-current 3.3V available that can only power the standby mode, but in power on mode I have plenty of 5V and 12V available.

So what would be best here, use boost converter on 3.3V standby to something larger, diode-or that with 12V, and buck it down to 3.6 and 2.1 for the LDOs?

Or should I just diode-or the 3.3V and 12V together, and use that for buck-boost to 3.6 and 2.1 for the LDOs?

Is diode-or good, or what else could be used for safe switching between power sources so that the chips can stay powered continuously?

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To diode-or the 3.3V and the 12V with a buck-boost converter is a simple and good solution. But perhaps it can be done in another way.

If you have only 3.3V in standby, you could perhaps use mosfets to switch between the voltages. If that is possible you don't have to boost-up the voltage.

Can you use that low-current 3.3V also in "power on" mode ? If so, why do you have to switch between voltages at all ?

Is is possible to power the 1.8V LDO with the 3.3V ? So you don't need the 2.1V ?

I'm not sure how good the LDO's are with noise rejection. The buck-boost converter will inject noise in your circuit.

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Is the standby 3.3V clean enough to feed directly to the 3.3V power input on the chips without going thru the 3.3V LDO?

If so, you might consider using an ideal diode (e.g. linear ltc4413) to select the chip's 3.3V supply from between the LDO's 3.3V output and the standby 3.3V. (this is quite similar to Kun.io's mosfet switch suggestion).

- S

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Kun.io wrote:

If you have only 3.3V in standby, you could perhaps use mosfets to switch between the voltages. If that is possible you don't have to boost-up the voltage.

Mosfets or these ideal diode chips, thank you both for the idea.

mnehpets wrote:

Is the standby 3.3V clean enough to feed directly to the 3.3V power input on the chips without going thru the 3.3V LDO?

Don't know yet where it comes from, switcher or linear, and no tolerance info yet. If all is good, heavy LC filter could work instead of LDO.

Kun.io wrote:

Can you use that low-current 3.3V also in "power on" mode ? If so, why do you have to switch between voltages at all ?

Is is possible to power the 1.8V LDO with the 3.3V ? So you don't need the 2.1V ?

I'm not sure how good the LDO's are with noise rejection. The buck-boost converter will inject noise in your circuit.

In power-on mode I need more power than standby line can provide (max 1A or so). I need max 1A of 1.8V, so making that from 3V3 with LDO means 1.5W wasted power. So at least that 1.8V has to come through a switched, from 3.3 in standby or 5/12V in powerup.

It could be an option to have 3.3 standby powering all 3.3V supplies directly, if all is well.