Datasheet

LMK00105
OSCin
OSCout
C
1
C
2
XTAL
R
LIM
0.1 PF
50:Trace
50:
CMOS
Driver
0.1 PF
R
S
LMK00105
OSCin
OSCout
LMK00105
www.ti.com SNAS579F –MARCH 2012–REVISED MAY 2013
If the crystal oscillator circuit is not used, it is possible to drive the OSCin input with an single-ended external
clock as shown in Figure 10. The input clock should be AC coupled to the OSCin pin, which has an internally
generated input bias voltage, and the OSCout pin should be left floating. While OSCin provides an alternative
input to multiplex an external clock, it is recommended to use either differential input (CLKin) since it offers
higher operating frequency, better common mode, improved power supply noise rejection, and greater
performance over supply voltage and temperature variations.
Figure 10. Driving OSCin with a Single-Ended
Crystal Interface
The LMK00105 has an integrated crystal oscillator circuit that supports a fundamental mode, AT-cut crystal. The
crystal interface is shown in Figure 11.
Figure 11. Crystal Interface
The load capacitance (C
L
) is specific to the crystal, but usually on the order of 18 to 20 pF. While C
L
is specified
for the crystal, the OSCin input capacitance (C
IN
= 1 pF typical) of the device and PCB stray capacitance (C
STRAY
~ 1 to 3 pF) can affect the discrete load capacitor values, C
1
and C
2
. For the parallel resonant circuit, the discrete
capacitor values can be calculated as follows:
C
L
= (C
1
* C
2
) / (C
1
+ C
2
) + C
IN
+ C
STRAY
(1)
Typically, C
1
= C
2
for optimum symmetry, so Equation 1 can be rewritten in terms of C
1
only:
C
L
= C
1
2
/ (2 * C
1
) + C
IN
+ C
STRAY
(2)
Finally, solve for C
1
:
C
1
= (C
L
- C
IN
- C
STRAY
) * 2 (3)
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