LP2994 Datasheet, PDF(12/15 Page) National Semiconductor (TI)

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LP2994 Datasheet, PDF (12/15 Pages) National Semiconductor (TI) – DDR Termination Regulator

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Typical Application Circuits

(Continued)

how to use two resistors to level shift VTT above the internal

reference voltage of VDDQ/2. To calculate the exact voltage

at VTT the following equation can be used.

VTT = (VDDQ/2) x ( 1 + R1/R2)

20045909

FIGURE 9. Increasing VTT by Level Shifting

Conversely, the R2 resistor can be placed between VSENSE

and VDDQ to shift the VTT output lower than the internal

reference voltage of VDDQ/2. The equations relating VTT and

the resistors can be seen below:

VTT = (VDDQ/2) x ( 1 - R1/R2)

20045910

FIGURE 10. Decreasing VTT by Level Shifting

Reference Voltage

DDR-SDRAM and the motherboard chipsets usually require

a reference voltage which tracks VTT. To implement this

feature in most applications it is advisable to use two equal

resistors as a resistor divider. This prevents long VREF 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 VTT output on the

LP2994 to generate the VREF signal. The can be accom-

plished by using an RC filter on the output of VTT to create a

VREF signal. Typically, the reference voltage required by

chipsets and memory is well under 1ÂµA combined, therefore,

a fairly large resistor such as 1kâ¦ or larger can be used. A

recommended capacitor would be a 1uF X7R ceramic

capacitor.

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