VTT Voltage Rail
4
V
TT
Voltage Rail
For a given topology, the worst case V
TT
current should be derived. Assuming the use of a typical R
T
parallel termination resistor and the worst case parameters given in
Table
2
, sink and source currents can
be calculated.
The driver sources (V
TT
plane would sink) the following based on this termination scheme:
(V
DD_max
–
V
TT_min
)/(R
T
+ R
DRVR
)
=
(1.575
–
0.702 V)
/
(47
+
20)
=
13 mA
The driver sinks (V
TT
plane would source) the following based on this termination scheme:
(V
TT_max
–
V
OL
/ (R
T
+ R
S
+ R
DRVR
)
=
(0.798
–
0 V)
/
(47
+
20)
=
12 mA
A bus with balanced number of high and low signals places no real demand on the V
TT
supply. However,
a bus with all DDR address/command/control signals low (~ 28 signals) causes a transient current demand
of approximately 350 mA on the V
TT
rail. The V
TT
regulator must provide a relatively tight voltage
regulation of the rail per the JEDEC specification. Besides a tight tolerance, the regulator must also allow
V
TT
along with V
REF
(if driven from a common IC), to track variations in V
DDQ
over voltage, temperature,
and noise margins.
5
Layout Guidelines for the Signal Groups
To help ensure the DDR interface is properly optimized, Freescale recommends the following sequence
for routing the DDR memory channel:
1.
Route data
2.
Route address/command/control
3.
Route clocks
The data group is listed before the command, address, and control group because it operates at twice the
clock speed, and its signal integrity is of higher concern. In addition, the data group constitutes the largest
portion of the memory bus and comprises most of the trace matching requirements (those of the data
lanes). The address/command, control, and data groups all have a relationship to the routed clock.
Therefore, the effective clock lengths used in the system must satisfy multiple relationships. The designer
should perform simulation and construct system timing budgets to ensure that these relationships are
properly satisfied.
Table
2. Worst Case Parameters for V
TT
Current Calculation
Parameter
Values
Comment
V
DDQ
(max)
1.575 V
From JEDEC spec
V
TT(max)
0.798 V
From JEDEC spec
V
TT(min)
0.702 V
From JEDEC spec
R
DRVR
20
Ω
Nominally, full strength is ~ 20
Ω
s
R
T
47
Ω
Can vary. T
ypically 25–47
Ω
s.
V
OL
0 V
Assumes driver reaches 0
V in the low state.