LMK04031BISQ National Semiconductor, LMK04031BISQ Datasheet - Page 44

• Manufacturer Part Number: LMK04031BISQ
• Manufacturer: National Semiconductor

Specifications of LMK04031BISQ

• Operating Temperature (max): 85C
• Operating Temperature (min): -40C
• Pin Count: 48
• Mounting: Surface Mount
• Lead Free Status / RoHS Status: Not Compliant

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17.7 OPTIONAL CRYSTAL OSCILLATOR
IMPLEMENTATION (OSCin/OSCin*)
The LMK04000 family features supporting circuitry for a dis-
cretely implemented oscillator driving the OSCin port pins.
Figure 6
cillator:
This circuit topology represents a parallel resonant mode os-
cillator design. When selecting a crystal for parallel reso-
nance, the total load capacitance, C
load capacitance is the sum of the tuning capacitance
(C
(C
and is given by:
C
Skyworks model SMV1249-074. A dual diode package with
common cathode and provides the variable capacitance for
tuning. The single diode capacitance ranges from approxi-
mately 31 pF at 0.3 V to 3.4 pF at 3 V. The capacitance range
of the dual package (anode to anode) is approximately 15.5
pF at 3 V to 1.7 pF at 0.3 V. The desired value of V
to the diode should be V
typical performance curve from the data sheet for the
SMV1249-074 indicates that the capacitance at this voltage
is approximately 6 pF (12 pF/2).
TUNE
TUNE
IN
), and stray capacitance due to PCB parasitics (C
), the capacitance seen looking into the OSCin port
is provided by the varactor diode shown in
illustrates a reference design circuit for a crystal os-
CC
/2, or 1.65 V for V
FIGURE 6. Reference Design Circuit for Crystal Oscillator Option
L
, must be specified. The
30027164
CC
= 3.3 V. The
TUNE
Figure
applied
STRAY
6,
),
44
The nominal input capacitance (C
OSCin pins is 6 pF. The stray capacitance (C
PCB should be minimized by arranging the oscillator circuit
layout to achieve trace lengths as short as possible and as
narrow as possible trace width (50 Ω characteristic
impedance is not required). As an example, assume that
C
Consequently the load capacitance specification for the crys-
tal in this case should be nominally 14 pF.
The 2.2 nF capacitors shown in the circuit are coupling ca-
pacitors that block the DC tuning voltage applied by the 4.7 k
and 10 k resistors. The value of these coupling capacitors
should be large, relative to the value of C
C
For a specific value of C
quency (F
F
C
S
STRAY
TUNE
1
= Series resonant frequency
= Motional capacitance of the crystal
), so that C
is 4 pF. The total load capacitance is nominally:
L
) of the parallel resonant mode circuit is:
TUNE
30027163
becomes the dominant capacitance.
L
, the corresponding resonant fre-
IN
) of the LMK04000 family
30027165
TUNE
(C
STRAY
C1
30027166
= C
) of the
C2
>>