Re: MGT Power supply

Hi,

Switching power supplies introduce ripple voltage on the voltage rails they are regulating, requiring multiple LC stages for filtering which further slows down the regulator's response time.

Linear regulators reject this ripple (act like an active filter) but they have finite tracking bandwidth which limits their usefulness beyond 100kHz. Linears are a good choice when the front-end switcher operates at lowish frequencies (<25kHz) where the linear regulator still offers at least 40dB supply ripple rejection - some LC filtering on the LDO's input is still necessary to remove high-frequency harmonics the LDO cannot handle.

Passive low-pass filters become increasingly impractical as frequencies go down while active filters (like linears) become increasingly effective. Because of this, precision voltage regulation is nearly impossible to achieve with switching regulators but it is nearly trivial and fairly inexpensive with linears. This is unlikely to change any time soon, if ever.

Switchers are good for noise-tolerant high-power circuits but linears will remain necessary for low-noise low-power stuff like reference voltages.

As for the actual topic of linear being necessary for MGTs, on top of inherent switcher supply noise, there will be multi-tone noise from switching inputs on input rails and heaps of other potentially nasty stuff across the whole spectrum. Given the price of V2P and V4FX parts, I would opt for not taking any chances and go with power -> LC -> LDO -> C -> MGT... and read Xilinx's MGT power decoupling appnote a few times.


Symon wrote:
Hi Heiner,
I'd be interested in the response you get for this question. As linear
regulators have a bandwidth of a 100kHz or so, I fail to see how they
provide an advantage over a filtered switcher.

--
Daniel Sauvageau
moc.xortam@egavuasd
Matrox Graphics Inc.
1155 St-Regis, Dorval, Qc, Canada
514-822-6000
.

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