LMX2571 Datasheet, PDF (37/61 Pages) Texas Instruments – Low-Power, High-Performance PLLatinum RF Synthesizer

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OSCin TYPE
Connection
Diagram
Register Setting
Table 36. OSCin Configuration
SINGLE-ENDED CLOCK
DIFFERENTIAL CLOCK
OSCin
OSCin*
IPBUF_SE_DIFF_SEL = 0
V_T
0.1µF
V_T
OSCin
OSCin* ^503
503
IPBUF_SE_DIFF_SEL = 1
IPBUFDIFF_TERM = 1
Single-ended and differential input clock definitions are as follows:
LMX2571
SNAS654 – MARCH 2015
CRYSTAL
C₁
OSCin ^Rd
OSCin*
C₂
XTAL_EN = 1
XTAL_PWRCTRL = Crystal dependent
V_OSCin
V_OSCin
V_OSCin
CMOS
Sine wave
Differential
Figure 54. Input Clock Definition
The integrated crystal-oscillator circuit supports a fundamental mode, AT-cute crystal. 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 crystal, the OSCin input
capacitance, C_IN (1 pF typical), of the device and PCB stray capacitance, C_STRAY (approximately 1 to 3 pF), can
affect the discrete load capacitor values, C₁ and C₂.
For the parallel resonant circuit, the discrete capacitor values can be calculated as follows:
C_L = (C₁ * C₂) / (C₁ + C₂) + C_IN + C_STRAY
(8)
Typically, C₁ = C₂ for optimum symmetry, so Equation 8 can be rewritten in terms of C₁ only:
C_L = C₁₂ / (2 * C₁) + C_IN + C_STRAY
(9)
Finally, solve for C₁:
C₁ = 2 * (C_L – C_IN – C_STRAY)
(10)
Electrical Characteristics provide 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. Over-driving 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_RMS2 * R_ESR * (1 + C_o / C_L)²
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_o is the minimum shunt capacitance specified for the crystal
• I_RMS can be measured using a current probe (for example, Tektronix CT-6 or equivalent) placed on the leg of
the crystal connected to OSCin pin with the oscillation circuit active.
(11)
The internal configurable resistor, R_d, 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_d shorted, then a
larger resistor value is mandatory to avoid over-driving the crystal. However, if the power dissipated in the crystal
is less than the drive level with R_d shorted, then a zero value for R_d can be used. As a starting point, a suggested
value for R_d is 200 Ω.
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