Looking for a special kind of LDO

Other Hardware General Electronics
1

Hi,

I need your help to find the ideal component.

I want to power a wireless circuit on battery, a lipo one.
The lipo voltage should go from 4,2V when fully charged, to 3V when dead.
All the components of the circuit will behave nicely at 3V and more, but some will be fried over 3,6V. So I need to keep voltage between 3 and 3.6V.

-> I first thought using a 3.6 zener diode to clamp voltage, but it seems quite inefficient for a wireless circuit.

-> I could use a LDO regulator at 3V. But why wasting energy to get voltage down to 3V whereas when between 3 and 3.6V raw voltage would be ok for direct powering…

-> I have been reading something about some LDO regulators that would let raw voltage flow through them when under the regulated voltage.

This would be perfect, while over 3.6V, the regulator does its job and gets it down to 3.6V. When voltage drop under 3.6V, the circuit is powered by raw voltage passing through the regulator.

But I just can’t find this type of LDO regulators ? How is it named ?
All the regulators I see just shut the power down when under the minimum voltage.

Thanks,

Julien

2

Sounds like you could do with a step up and step down switch mode regulator...

Plenty on ebay.

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-> I could use a LDO regulator at 3V. But why wasting energy to get voltage down to 3V whereas when between 3 and 3.6V raw voltage would be ok for direct powering…

If you use a 3.0V LDO then the voltage at the load is a minimum the whole time, so
the current consumption will be a minimum (current depends on the load, not the
regulator), so you'll get the best battery life (short of using a switch-mode regulator).

Allowing 3.6V direct to your circuit will mean rather more current will be flowing,
hence wasting battery capacity.

With such a small drop-out voltage a switch-mode regulator may fare worse, since
efficiencies are rarely as high as 90%, and a 3.0V LDO running from 3.3V is the
same efficiency.

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