Datasheet
V
TT
LP2998
PV
IN
V
DDQ
V
REF
AV
IN
V
REF
=
0.9V
V
SENSE
GND
+
+
+
V
DDQ
=
1.8V
AV
IN
= 3.3V or 5.5V
V
TT
=
0.9V
SD
SD
PV
IN
= 3.3V
C
IN
C
OUT
C
REF
V
TT
LP2998
PV
IN
V
DDQ
V
REF
AV
IN
V
SENSE
GND
+
+
+
SD
SD
0.01 PF
AV
IN
=
2.5V
V
DDQ
=
1.8V
V
TT
=
0.9V
V
REF
=
0.9V
220 PF
47 PF
LP2998/LP2998-Q1
SNVS521J –DECEMBER 2007–REVISED DECEMBER 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.
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.
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)
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Product Folder Links: LP2998/LP2998-Q1