-

+

VTT

PVIN

VDDQ

SD

GND

AVIN

VSENSE

50k

50k

+

-

VREF

LP2998, LP2998-Q1

www.ti.com

SNVS521K – DECEMBER 2007 – REVISED AUGUST 2014

8 Detailed Description

8.1 Overview

The LP2998 linear regulator is designed to meet JEDEC SSTL-2 and JEDEC SSTL-18 specifications for

termination of DDR-SDRAM and DDR2 memory. The device also supports DDR3 and DDR3L VTT bus

termination with VDDQ min of 1.35 V. The device contains a high-speed operational amplifier to provide excellent

response to load transients. The output stage prevents shoot through while delivering 1.5 A continuous current

and transient peaks up to 3 A in the application as required for DDR-SDRAM termination.

8.2 Functional Block Diagram

8.3 Feature Description

The LP2998 is a linear bus termination regulator designed to meet the JEDEC requirements of SSTL-2 and

SSTL-18. The output, VTT is capable of sinking and sourcing current while regulating the output voltage equal to

VDDQ / 2. The output stage has been designed to maintain excellent load regulation while preventing shoot

through. The LP2998 also incorporates two distinct power rails that separates the analog circuitry from the power

output stage. This allows a split rail approach to be utilized to decrease internal power dissipation. It also permits

the LP2998 to provide a termination solution for DDR3-SDRAM and DDR3L-SDRAM memory.

8.4 Device Functional Modes

The LP2998 can also be used to provide a termination voltage for other logic schemes such as SSTL-3 or HSTL.

Series Stub Termination Logic (SSTL) was created to improve signal integrity of the data transmission across the

memory bus. This termination scheme is essential to prevent data error from signal reflections while transmitting

at high frequencies encountered with DDR-SDRAM. The most common form of termination is Class II single

parallel termination. This involves one RS series resistor from the chipset to the memory and one RT termination

resistor. Typical values for RS and RT are 25 Ω, although these can be changed to scale the current

requirements from the LP2998. This implementation can be seen below in Figure 16.

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Product Folder Links: LP2998 LP2998-Q1