Datasheet

LMK00105
SNAS579F –MARCH 2012–REVISED MAY 2013 www.ti.com
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.
The power dissipated in the crystal, P
XTAL
, can be computed by:
P
XTAL
= I
RMS
2
* R
ESR
* (1 + C
0
/ C
L
)
2
(4)
Where:
• I
RMS
is the RMS current through the crystal.
• R
ESR
is the maximum equivalent series resistance specified for the crystal.
• C
L
is the load capacitance specified for the crystal.
• C
0
is the minimum shunt capacitance specified for the crystal.
I
RMS
can be measured using a current probe (e.g. Tektronix CT-6 or equivalent) placed on the leg of the crystal
connected to OSCout with the oscillation circuit active.
As shown in Figure 11, an external resistor, R
LIM
, can be used to limit the crystal drive level if necessary. If the
power dissipated in the selected crystal is higher than the drive level specified for the crystal with R
LIM
shorted,
then a larger resistor value is mandatory to avoid overdriving the crystal. However, if the power dissipated in the
crystal is less than the drive level with R
LIM
shorted, then a zero value for R
LIM
can be used. As a starting point, a
suggested value for R
LIM
is 1.5 kΩ.
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