4116R-R2R-503 datasheet(55/62 Pages) BOURNS

55 / 62 page

Specifications are subject to change without notice.

331

Use Bourns Networks To:

• Match impedance between memory driver and the DRAM

array.

• Minimize reflections and ringing in DRAM inputs.

• Prevent undershoot of RAS, CAS, and WE signals which

may result in latch-up of DRAM inputs

• Improve system performance by allowing faster setting

times for DRAM inputs.

Need For Damping

The address lines (RAS, CAS) and control lines (WE) of

dynamic RAM arrays are driven in parallel, causing significant

loading on the driver of the DRAM arrays. Each DRAM control

input (WE) has capacitive loading between 5pF to 7pF, while

each address line input has about a 10pF load.

Thus each DRAM input can be modeled as a transmission line

with distributed inductance and capacitance. If not properly ter-

minated, signal reflections and ringing on the line will result,

adversely affecting the performance of the memory system. The

effects on signal transitions will be:

1. Increased settling time delay on low-to-high transitions.

2. Voltage undershoot on high-to-low transitions.

Increased settling time due to ringing reduces system perfor-

mance because the design has to allow for the settling delay

before sampling the signal. Undershoot, by bringing the input

voltage below 0 volts, can damage the driver IC as well as alter

the DRAM’s internal address register contents, causing potential

loss of data.

SYSTEM DATA BUS

TIMING

CONTROLLERS

CPU

ADDRESS

DYNAMIC

MEMORY

CONTROL

ADDRESS

RAS

CAS

DYNAMIC

MEMORY

ARRAY

BLOCK DIAGRAM OF DRAM SYSTEM

EFFECT OF DAMPING RESISTOR

WITHOUT

DAMPING

RESISTOR

WITH

DAMPING

RESISTOR

Courtesy of B. Narasimhan and J. Shaffer, Micron Techology Corporation.

"HIGH"

LOGIC

LEVEL

COMPARISON OF UNDERSHOOTS

WITHOUT

DAMPING RESISTOR

WITH

DAMPING RESISTOR

t1 - TIME TO ACCEPTABLE "LOW" LOGIC LEVEL FOR

DRIVER WITHOUT DAMPING RESISTOR

t2 - TIME TO ACCEPTABLE "LOW" LOGIC

LEVEL WITHDAMPING RESISTOR

DRAM Applications

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