LP2994_14 Datasheet, PDF (13/19 Pages) Texas Instruments – DDR Termination Regulator

Obsolete Device
FIGURE 9. Increasing V_TT by Level Shifting
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Conversely, the R2 resistor can be placed between V_SENSE
and V_DDQ to shift the V_TT output lower than the internal refer-
ence voltage of V_DDQ/2. The equations relating V_TT and the
resistors can be seen below:
V_TT = (V_DDQ/2) x ( 1 - R1/R2)
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FIGURE 10. Decreasing V_TT by Level Shifting
REFERENCE VOLTAGE
DDR-SDRAM and the motherboard chipsets usually require
a reference voltage which tracks V_TT. To implement this fea-
ture in most applications it is advisable to use two equal
resistors as a resistor divider. This prevents long V_REF traces
from running across the motherboard picking up noise which
can interfere with performance. However, in a few applica-
tions it may be desirable to use the V_TT output on the LP2994
to generate the V_REF signal. The can be accomplished by us-
ing an RC filter on the output of V_TT to create a V_REF signal.
Typically, the reference voltage required by chipsets and
memory is well under 1µA combined, therefore, a fairly large
resistor such as 1kΩ or larger can be used. A recommended
capacitor would be a 1uF X7R ceramic capacitor.
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FIGURE 11. Creating a Reference Voltage for Memory and Chipsets
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