Motherboard Traces

PCIe

Connector

1" Trace

1" Trace

Motherboard

Traces

CDCUN1208LP

www.ti.com

SCAS928 – MAY 2012

APPLICATION INFORMATION

PCI EXPRESS APPLICATIONS

Figure 41 shows a typical application in which the receiver is off board. The PCIe Specification (CEM2.0)

requires that all source termination is on the motherboard (not on the daughter card). For this reason, the

termination resistors are placed as shown. Additionally, source resistors are employed to eliminate ringing. In this

Figure 41. Typical Configuration – Off Board Receiver

Figure 42 shows a typical application in which the receiver is on-board. In this case, series resistors are not

are placed close to the receiver.

Figure 42. Typical Configuration – On Board Connection

DEVICE POWER SUPPLY CONNECTIONS AND SEQUENCING

VDD (pin 5) is the core power supply of the device while VDDOx (pins 11, 14, 22, and 27) provide power for the

output sections. The core supply must be present either before the application of the output power supplies or be

present simultaneously. Applying an output power supply voltage on any of the VDDOx pins prior to the

application of power to the core supply pin will potentially result in improper device operation.

VDDO2 (pin 14) and VDDO4 (pin 27) provide power for OUT1/OUT2 and OUT7/OUT8 respectively. Additionally,

these pins provide power to integrated voltage regulators that condition power for two banks of outputs. For

example, the regulator associated with OUT1–OUT4 receives power from the VDDO2 pin. Consequently, if the

application requires one or two outputs from a bank of four, then the application must use OUT3/OUT4 and apply

(pin 27). If the application uses subset of OUT5–OUT8, then OUT7/OUT8 must be used. For example, if the

application will use 6 of the 8 output channels, then VDDO1, VDDO2, and VDDO4 (along with OUT1–OUT4, and

OUT7–OUT8) must be used. If the application requires the use of 7 of the 8 output channels, the

VDDO1–VDDO4 are used, and OUT1–OUT7 or OUT1–OUT6 and OUT8 could be used.

(1)

If OUT1 or OUT2 are used and VDDO1 is powered but not VDDO2, the CDCUN1208LP will not function properly. Likewise, if OUT5 or

OUT6 are used and VDDO3 is powered but not VDDO4, then the device will not function properly either.

Copyright © 2012, Texas Instruments Incorporated

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