[SI-LIST] Re: Arguments against Thevenin bias/termination for ddr2 Vtt
- From: agathon <hreidmarkailen@xxxxxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Tue, 29 May 2007 13:34:41 -0700
Bill,
au contraire, mon frere....
#2-4, 6 are also specific to the Thevenin bias/termination, as opposed to
Vtt separate regulator.
Come on, you frikkin experts. :-) This should be easy. I double dare
ya.
On 5/29/07, Bill Owsley <wdowsley@xxxxxxxxx> wrote:
>
> Only one and five are accurate. The rest are symptoms of other problems
> that should have been addressed.
>
>
> *agathon <hreidmarkailen@xxxxxxxxx>* wrote:
>
> Hello,
> Assume a 50 ohm environment so, for example, a 100 ohm pullup to Vddq (
> 1.8V)
> and pulldown to gnd, for Address/Command signal group.
> The typical method is to use a lin. regulator with outputs for Vref and
> Vtt,
> so they supposedly track each other better.
>
> The only arguments against the pullup/down I can come up with are:
>
> 1. Extra dc current (but the regulator has Vout-Vin losses, too). For
> Addr./Cmd it's around 200mA. for a single port, and doesn't increase w/
> memory size.
> 2. The PDN (Vddq) is used dc-coupled, so its behavior statistically
> influences Vtt. That is, Vtt is more subject to Vddq and other noise.
> 3. Vtt is then subject to board PDN resonances.
> 4. The Thevenin method makes Vin (rcvr) more sensitive to Vddq noise
> merely
> from the linear network analysis point of view, or to Vtt offset at worst
> case pullup/down values, than when Vtt is regulated and terminated with 50
> ohms (nom.). ... I'm verifying this now; may not be true. I assume 1%
> resistors.
> 5. Uses more pcb space and routing area.
> 6. Vtt current switching noise is injected into the PDN. Not very nice if
> layout or margins are poor.
>
>
>
> Arguments in favor:
> 1. Using Vddq actually may force Vtt to track it better. Regulators
> providing Vtt and Vref (with Vddq sense) cannot track Vddq as quickly or
> accurately.
> 2. The dc current penalty is small.
> 3. ???
>
>
>
> Thanks very much.
>
> -----------
>
>
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