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LP2994M Schematic ( PDF Datasheet ) - National Semiconductor

Teilenummer LP2994M
Beschreibung DDR Termination Regulator
Hersteller National Semiconductor
Logo National Semiconductor Logo 




Gesamt 15 Seiten
LP2994M Datasheet, Funktion
May 2002
LP2994
DDR Termination Regulator
General Description
The LP2994 regulator is designed to provide a linear solution
to meet the JEDEC SSTL-2 and SSTL-3 specifications (Se-
ries Stub Termination Logic) for active termination of DDR-
SDRAM. The device utilizes an internal operational amplifier
to provide linear regulation of VTT without the need for
expensive external components. The output stage prevents
shoot through while delivering 1.5A continuous current and
maintaining excellent load regulation. The LP2994 also in-
corporates an active low shutdown pin to tri-state the output
during Suspend To Ram (STR) states.
Patents Pending
Features
n Source and sink current
n Low external component count
n Independent analog and power rails
n Linear topology
n Small package SO-8
n Low cost and easy to use
n Shutdown pin
Applications
n SSTL-2
n SSTL-3
n DDR-SDRAM Termination
n DDR-II Termination
Typical Application Circuit
FIGURE 1. SSTL-2 VTT Termination
20045904
© 2002 National Semiconductor Corporation DS200459
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LP2994M Datasheet, Funktion
Typical Performance Characteristics (Continued)
Maximum Sinking Current vs AVIN
(VDDQ = 1.8V)
Maximum Sourcing Current vs AVIN
(VDDQ = 1.8V, PVIN = 3.3V)
20045926
VTT vs IOUT (0, 25, 85, and 125˚C)
VTT vs IOUT
20045927
20045929
20045930
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LP2994M pdf, datenblatt
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 1µA combined, therefore,
a fairly large resistor such as 1kor larger can be used. A
recommended capacitor would be a 1uF X7R ceramic
capacitor.
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