LP2996A Datasheet, PDF (16/29 Pages) Texas Instruments – DDR Termination Regulator

LP2996A
SNOSCY7 – JUNE 2014
www.ti.com
Typical Application (continued)
8.2.5 Level Shifting
If standards other than SSTL-2 are required, such as SSTL-3, it may be necessary to use a different scaling
factor than 0.5 times V_DDQ for regulating the output voltage. Several options are available to scale the output to
any voltage required. One method is to level shift the output by using feedback resistors from V_TT to the V_SENSE
pin. This has been illustrated in Figure 26 and Figure 27. Figure 26 shows how to use two resistors to level shift
V_TT above the internal reference voltage of VDDQ/2. To calculate the exact voltage at V_TT the following equation
can be used.
V_TT = VDDQ/2 ( 1 + R1/R2)
(11)
VDDQ
LP2996
VDDQ
VDD
+
CIN
AVIN
VTT
PVIN
VSENSE
GND
VTT
R1
+
COUT
R2
Figure 26. Increasing VTT by Level Shifting
Conversely, the R2 resistor can be placed between V_SENSE and V_DDQ to shift the V_TT output lower than the
internal reference voltage of VDDQ/2. The equations relating VTT and the resistors can be seen below:
V_TT = VDDQ/2 (1 - R1/R2)
(12)
VDDQ
VDD
+
CIN
LP2996
VDDQ
VSENSE
AVIN
PVIN
VTT
GND
R2
R1
VTT
+
COUT
Figure 27. Decreasing VTT by Level Shifting
8.2.5.1 Output Capacitor Selection
For applications utilizing the LP2996A to terminate SSTL-2 I/O signals the typical application circuit shown in
Figure 27 can be implemented.
This circuit permits termination in a minimum amount of board space and component count. Capacitor selection
can be varied depending on the number of lines terminated and the maximum load transient. However, with
motherboards and other applications where V_TT is distributed across a long plane it is advisable to use multiple
bulk capacitors and addition to high frequency decoupling. Figure 28 shown below depicts an example circuit
where 2 bulk output capacitors could be situated at both ends of the V_TT plane for optimal placement. Large
aluminum electrolytic capacitors are used for their low ESR and low cost.
In most PC applications an extensive amount of decoupling is required because of the long interconnects
encountered with the DDR-SDRAM DIMMs mounted on modules. As a result bulk aluminum electrolytic
capacitors in the range of 1000uF are typically used.
16
Submit Documentation Feedback
Product Folder Links: LP2996A
Copyright © 2014, Texas Instruments Incorporated