Travis Downs (travis.downs.delete@this.gmail.com) on May 12, 2020 7:53 am wrote:
> A nice read!
>
> What is the purpose of the LDO? Does it even reduce power draw? The voltage drop is a dead loss
> in an LDO, in the sense that the reduced power I * (Vin - Vout) is just dissipated in the regulator,
> but is there a secondary effect that saves power? Or is it just to better control the voltage
> for another purpose (one could be to move power dissipation away from the core)?
>
>
>

One advantage of the FIVR is that the power from the motherboard VRMs
> to the processor is about twice as high as in a conventional system.

>
> I believe "power" should be "voltage" in that sentence.

Thanks for the correction. I fixed that.

LDO's save a bit of power. Since switching power is ~V^2, and the power dissipated by an LDO is linear WRT voltage drop, you can improve power with an LDO. So your power savings are equal to the resistive losses.

In contrast, a buck converter has a roughly constant efficiency. So the regime where an LDO helps is small swings (say <10%). Anything bigger and you want a proper buck converter.

David