CLKoutX

CLKoutX*

HCSL

Receiver

50:Traces

HCSL

Driver

Rs

Rs

LMK00338

SNAS636A – DECEMBER 2013 – REVISED OCTOBER 2014

www.ti.com

Typical Applications (continued)

Electrical Characteristics provides crystal interface specifications with conditions that ensure start-up of the

crystal, but it does not specify crystal power dissipation. The designer will need to ensure the crystal power

dissipation does not exceed the maximum drive level specified by the crystal manufacturer. Overdriving the

crystal can cause premature aging, frequency shift, and eventual failure. Drive level should be held at a sufficient

level necessary to start-up and maintain steady-state operation.

2

where

•

•

•

•

(4)

crystal connected to OSCout with the oscillation circuit active.

then a larger resistor value is mandatory to avoid overdriving the crystal. However, if the power dissipated in the

9.2.2 Detailed Design Procedure

9.2.2.1 Termination and Use of Clock Drivers

When terminating clock drivers keep in mind these guidelines for optimum phase noise and jitter performance:

•

Transmission line theory should be followed for good impedance matching to prevent reflections.

•

Clock drivers should be presented with the proper loads.

–

HCSL drivers are switched current outputs and require a DC path to ground via 50

Ω termination.

•

Receivers should be presented with a signal biased to their specified DC bias level (common mode voltage)

for proper operation. Some receivers have self-biasing inputs that automatically bias to the proper voltage

level; in this case, the signal should normally be AC coupled.

9.2.2.2 Termination for DC Coupled Differential Operation

For DC coupled operation of an HCSL driver, terminate with 50

Ω to ground near the driver output as shown in

Figure 20. Series resistors, Rs, may be used to limit overshoot due to the fast transient current. Because HCSL

drivers require a DC path to ground, AC coupling is not allowed between the output drivers and the 50

Ω

termination resistors.

Figure 20. HCSL Operation, DC Coupling

18

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