4608X-102-102 Datasheet, PDF (55/62 Pages) Bourns Electronic Solutions – Resistor Networks Product Selection Guide

DRAM Applications
ADDRESS
CPU
DYNAMIC
MEMORY
CONTROL
ADDRESS
RAS
CAS
WE
DYNAMIC
MEMORY
ARRAY
TIMING
CONTROLLERS
SYSTEM DATA BUS
BLOCK DIAGRAM OF DRAM SYSTEM
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.
EFFECT OF DAMPING RESISTOR
WITHOUT
DAMPING
RESISTOR
WITH
DAMPING
RESISTOR
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.
Courtesy of B. Narasimhan and J. Shaffer, Micron Techology Corporation.
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.
V
"HIGH" ¹
LOGIC
LEVEL
COMPARISON OF UNDERSHOOTS
WITHOUT
DAMPING RESISTOR
WITH
DAMPING RESISTOR
Specifications are subject to change without notice.
t1
t2
t1 - TIME TO ACCEPTABLE "LOW" LOGIC LEVEL FOR
DRIVER WITHOUT DAMPING RESISTOR
t2 - TIME TO ACCEPTABLE "LOW" LOGIC
LEVEL WITHDAMPING RESISTOR
331