LP2994 datasheet(12/15 Pages) NSC

12 / 15 page

Typical Application Circuits

(Continued)

how to use two resistors to level shift V

TT above the internal

reference voltage of V

DDQ/2. To calculate the exact voltage

at V

TT the following equation can be used.

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

Conversely, the R2 resistor can be placed between V

SENSE

and V

DDQ to shift the VTT output lower than the internal

reference voltage of V

DDQ/2. The equations relating VTT and

the resistors can be seen below:

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

Reference Voltage

DDR-SDRAM and the motherboard chipsets usually require

a reference voltage which tracks V

TT. To implement this

feature 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 accom-

plished by using an RC filter on the output of V

TT to create a

REF signal. Typically, the reference voltage required by

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

a fairly large resistor such as 1k

Ω or larger can be used. A

recommended capacitor would be a 1uF X7R ceramic

capacitor.

20045909

FIGURE 9. Increasing V

TT by Level Shifting

20045910

FIGURE 10. Decreasing V

TT by Level Shifting

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