LP2998MASLASHNOPB datasheet(19/33 Pages) TI1

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VTT

LP2998

PVIN

VDDQ

VREF

AVIN

VREF = 0.75V

VSENSE

GND

VDDQ = 1.5V

VDD = 2.5V

VTT = 0.75V

CIN

COUT

CREF

VTT

LP2998

PVIN

VDDQ

VREF

AVIN

VREF = 1.25V

VSENSE

GND

47 2F

VDDQ = 2.5V

VDD = 2.5V

VTT = 1.25V

330 2F

0.01 2F

LP2998, LP2998-Q1

www.ti.com

SNVS521K – DECEMBER 2007 – REVISED AUGUST 2014

Typical Application (continued)

This circuit permits termination in a minimum amount of board space and component count. Capacitor selection

can be varied depending on the number of lines terminated and the maximum load transient. However, with

motherboards and other applications where VTT is distributed across a long plane it is advisable to use multiple

bulk capacitors and addition to high frequency decoupling. Figure 29 shown below depicts an example circuit

where 2 bulk output capacitors could be situated at both ends of the VTT plane for optimal placement. Large

aluminum electrolytic capacitors are used for their low ESR and low cost.

Figure 29. Typical SSTL-2 Application Circuit for Motherboards

In most PC applications an extensive amount of decoupling is required because of the long interconnects

encountered with the DDR-SDRAM DIMMs mounted on modules. As a result bulk aluminum electrolytic

capacitors in the range of 1000 µF are typically used.

9.2.5 HSTL Applications

The LP2998 can be easily adapted for HSTL applications by connecting VDDQ to the 1.5 V rail. This will produce

a VTT and VREF voltage of approximately 0.75 V for the termination resistors. AVIN and PVIN should be

connected to a 2.5 V rail for optimal performance.

Figure 30. HSTL Application

9.2.6 QDR Applications

Quad data rate (QDR) applications utilize multiple channels for improved memory performance. However, this

increase in bus lines has the effect of increasing the current levels required for termination. The recommended

approach in terminating multiple channels is to use a dedicated LP2998 for each channel. This simplifies layout

and reduces the internal power dissipation for each regulator. Separate VREF signals can be used for each DIMM

bank from the corresponding regulator with the chipset reference provided by a local resistor divider or one of the

LP2998 signals. Because VREF and VTT are expected to track and the part to part variations are minor, there

should be little difference between the reference signals of each LP2998.

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