LP2998MRX Datasheet, PDF (14/26 Pages) Texas Instruments – LP2998 DDR-I and DDR-II Termination Regulator

LP2998
SNVS521I – DECEMBER 2007 – REVISED APRIL 2013
www.ti.com
DDR-II APPLICATIONS
With the separate VDDQ pin and an internal resistor divider it is possible to use the LP2998 in applications
utilizing DDR-II memory. Figure 22 and Figure 23 show several implementations of recommended circuits with
output curves displayed in the Typical Performance Characteristics. Figure 22 shows the recommended circuit
configuration for DDR-II applications. The output stage is connected to the 1.8V rail and the AVIN pin can be
connected to either a 2.5, 3.3V or 5.5V rail.
SD
AV_IN = 2.5V
V_DDQ = 1.8V
+
47 PF
LP2998
SD
AV_IN
V_REF
V_DDQ V_SENSE
PV_IN
V_TT
GND
V_REF = 0.9V
+
0.01 PF
V_TT = 0.9V
+
220 PF
Figure 22. Recommended DDR-II Termination
If it is not desirable to use the 1.8V rail it is possible to connect the output stage to a 3.3V rail. Care should be
taken to not exceed the maximum operating junction temperature as the thermal dissipation increases with lower
V_TT output voltages. For this reason it is not recommended to power PVIN with a rail higher than the nominal
3.3V. The advantage of this configuration is that it has the ability to source and sink a higher maximum
continuous current.
SD
V_DDQ = 1.8V
AV_IN = 3.3V or 5.5V
PV_IN = 3.3V
+
C_IN
LP2998
SD
V_DDQ
V_REF
AV_IN
V_SENSE
PV_IN
V_TT
GND
+
C_REF
V_REF = 0.9V
+
C_OUT
V_TT = 0.9V
Figure 23. DDR-II Termination With Higher Voltage Rails
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 24 and Figure 25. Figure 24 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)
14
Submit Documentation Feedback
Product Folder Links: LP2998
Copyright © 2007–2013, Texas Instruments Incorporated