ASUS P4V8X-X won't POST

[momaka]

Re: ASUS P4V8X-X won't POST

It does not, there can be small coils which are not part of any buck regulator, why not? And there are also small-power regulators like those for southbridge or some additional ICs without coils too (quite often actually) so as stated, it goes both ways.

If you have a motherboard like that, please show me. I have seen hundreds of motherboards, and not one with a coil/inductor that wasn't either part of the buck circuit or pre-filter coil to some buck circuit.

And with linear regulators (i.e. chips like 1117, 1085, 1086, 317, just to name a few), there are NEVER any coils/inductors before or after that circuit. That's because linear regulators don't need them by design.

[Behemot]

Re: ASUS P4V8X-X won't POST

For example MSI K8N SLI Platinum, I see no inductor in the DRAM VRM.

[momaka]

Re: ASUS P4V8X-X won't POST

For example MSI K8N SLI Platinum, I see no inductor in the DRAM VRM.

Because it's a linear type of regulator. (and yes, I looked at this picture as a reference before commenting.)
A lot of DDR-based motherboards use linear regulation for the RAM's Vdd (2.5-2.6V). Typically, it's dropped down either from the PSU's 3.3V rail or the 3.3V stand-by rail on the motherboard (depending on the computer's sleep state).

[Behemot]

Re: ASUS P4V8X-X won't POST

Really? Gonna have to check the driver in there. It makes no sense to use 3 FETs (usual number for DC-DC buck regulator) for linear.

[momaka]

Re: ASUS P4V8X-X won't POST

Really? Gonna have to check the driver in there. It makes no sense to use 3 FETs (usual number for DC-DC buck regulator) for linear.

A buck regulator without a coil is like a car without wheels - good luck driving that!
If you remove the coil, all the caps will see is huge amounts of ripple current, and it won't take long before they get fried. I've actually tried it too, just for giggles. It was on a dead Xbox 360 board.

The 3 MOSFETs on that MSI board are all either linear regulators or pass-through switches.
One of them should convert 3.3V rail to 2.5V for RAM, like I said.
Another should be for CPU Vtt (typically about 1.2V or so)
The last one could be either 3.3V standby (generated from 5VSB) or chipset secondary Vcc. The chipset already has its own buck regulator - that is, the two MOSFETs and coil right above the MSI logo. But in standby/soft-off, the chipset might need something lower than 3.3V, so one of these MOSFETs is probably for generating that.

Also, MSI sometimes uses a series of linear regulators one after another to drop the voltage - that way, the heat output is spread across a larger area instead of having just one regulator heat up like crazy (ASUS did that a lot with their socket 939, AM2, and 775 boards, and it always ended up browning the board and overheating whatever caps were nearby).