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Output Tolerance Consideration for VTT DIMM Applications

DDR3 240 Pin Socket

TPS51200

10 mF

10 mF

10 mF

VO

UDG-08022

R

S

20 W

V

OUT

V

IN

25 W

V

TT

V

DDQ

Ouput

Buffer

(Driver)

Receiver

V

SS

UDG-08023

Q1

Q2

TPS51200

SLUS812 – FEBRUARY 2008

The TPS51200 is specifically designed to power up the memory termination rail (as shown in Figure 3). The DDR

memory termination structure determines the main characteristics of the VTT rail, which is to be able to sink and

source current while maintaining acceptable VTT tolerance. See Figure 4 for typical characteristics for a single

memory cell.

Figure 3. Typical Application Diagram for DDR3 VTT DIMM using TPS51200

Figure 4. DDR Physical Signal System Bi-Directional SSTL Signaling

In Figure 4, when Q1 is on and Q2 is off:

• Current flows from VDDQ via the termination resistor to VTT

• VTT sinks current

In Figure 4, when Q2 is on and Q1 is off:

• Current flows from VTT via the termination resistor to GND

• VTT sources current

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): TPS51200