CM3202-00SM Ver la hoja de datos (PDF) - California Micro Devices Corp

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CM3202-00SM

DDR VDDQ and Termination Voltage Regulator

California Micro Devices Corp 

Application Information (cont’d)

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Application Information

Powering DDR Memory

Double-Data-Rate (DDR) memory has provided a huge

step in performance for personal computers, servers

and graphic systems. As is apparent in its name, DDR

operates at double the data rate of earlier RAM, with

two memory accesses per cycle versus one. DDR

SDRAM's transmit data at both the rising falling edges

of the memory bus clock.

DDR’s use of Stub Series Terminated Logic (SSTL)

topology improves noise immunity and power-supply

rejection, while reducing power dissipation. To achieve

this performance improvement, DDR requires more

complex power management architecture than previ-

ous RAM technology.

Unlike the conventional DRAM technology, DDR

SDRAM uses differential inputs and a reference volt-

age for all interface signals. This increases the data

bus bandwidth, and lowers the system power con-

sumption. Power consumption is reduced by lower

operating voltage, a lower signal voltage swing associ-

ated with Stub Series Terminated Logic (SSTL_2) and

by the use of a termination voltage, VTT. SSTL_2 is an

industry standard, defined in JEDEC document

JESD8-9. SSTL_2 maintains high-speed data bus sig-

nal integrity by reducing transmission reflections.

JEDEC further defines the DDR SDRAM specification

in JESD79C.

DDR memory requires three tightly regulated voltages:

VDDQ, VTT, and VREF (see Figure 1). In a typical

SSTL_2 receiver, the higher current VDDQ supply volt-

age is normally 2.5V with a tolerance of ±200-mV. The

active bus termination voltage, VTT, is half of VDDQ.

VREF is a reference voltage that tracks half of VDDQ, ±

1%, and is compared with the VTT terminated signal at

the receiver. VTT must be within ±40-mV of VREF.

© 2006 California Micro Devices Corp. All rights reserved.

05/08/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: 408.263.3214 l Fax: 408.263.7846 l www.cmd.com

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