LP2994MX Schematic ( PDF Datasheet ) - National Semiconductor

Teilenummer	LP2994MX
Beschreibung	DDR Termination Regulator
Hersteller	National Semiconductor
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Gesamt 15 Seiten 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 www.national.com 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 www.national.com 6 6 Page 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 1kΩ or larger can be used. A recommended capacitor would be a 1uF X7R ceramic capacitor. www.national.com 12 12 Page
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Besondere Datenblatt

Teilenummer	Beschreibung	Hersteller
LP2994M	DDR Termination Regulator	National Semiconductor
LP2994M	DDR Termination Regulator	National Semiconductor
LP2994MX	DDR Termination Regulator	National Semiconductor
LP2994MX	DDR Termination Regulator	National Semiconductor

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