CHIPSET

MEMORY

LP2995

www.ti.com

SNVS190M – FEBRUARY 2002 – REVISED MARCH 2013

DETAILED DESCRIPTION

The LP2995 is a linear bus termination regulator designed to meet the JEDEC requirements of SSTL-2 and

tight tolerance over the entire current range while preventing shoot through on the output stage.

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 RAM. The most common form of termination is Class II single parallel

termination. This involves using one Rs series resistor from the chipset to the memory and one Rt termination

resistor. This implementation can be seen below in Figure 13.

Figure 13.

from the LP2995. For determination of the current requirements of DDR-SDRAM termination please refer to the

accompanying application notes.

Pin Descriptions

AVIN AND PVIN

AVIN and PVIN are the input supply pins for the LP2995. AVIN is used to supply all the internal control circuitry

should be connected directly and tied to the 2.5V rail for optimal performance. This eliminates the need for

bypassing the two supply pins separately.

VDDQ

generated by two internal 50k

optimal implementation of VDDQ is as a remote sense for the reference input. This can be achieved by

connecting VDDQ directly to the 2.5V rail at the DIMM. This ensures that the reference voltage tracks the DDR

memory rails precisely without a large voltage drop from the power lines. For SSTL-2 applications VDDQ will be

SSTL-3 applications it may be desirable to have a different scaling factor for creating the internal reference

voltage besides 0.5. For instance a typical value that is commonly used is to have the reference voltage equal

VDDQ*0.45. This can be achieved by placing a resistor in series with the VDDQ pin to effectively change the

resistor divider.

The purpose of the sense pin is to provide improved remote load regulation. In most motherboard applications

of the LP2995, then the long trace will cause a significant IR drop, resulting in a termination voltage lower at one

middle of the bus. This will provide a better distribution across the entire termination bus.

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Product Folder Links: LP2995