LMX25311778EVAL National Semiconductor, LMX25311778EVAL Datasheet - Page 5

• Manufacturer Part Number: LMX25311778EVAL
• Manufacturer: National Semiconductor

Specifications of LMX25311778EVAL

• Lead Free Status / Rohs Status: Not Compliant

Connection Diagram
VregPLL1
VregPLL2
VregVCO
VrefVCO
VccVCO
VregDIG There is not really any reason to use any other values than the recommended value of 10 nF
VccBUF
Ftest/LD It is an option to use the lock detect information from this pin.
VccDIG
VccPLL
OSCin*
R2pLF
OSCin
Pin(s)
CPout
DATA
Vtune
Fout
CLK
CE
LE
These pins are inputs to voltage regulators. Because the LMX2531 contains internal regulators, the power supply
noise rejection is very good and capacitors at this pin are not critical. An RC filter can be used to reduce supply noise,
but if the capacitor is too large and is placed too close to these pins, they can sometimes cause phase noise
degradation in the 100 - 300 kHz offset range. Recommended values are from open to 1 μF. The series resistors
serve to filter power supply noise and isolate these pins from large capacitances.
If the VrefVCO capacitor is changed, it is recommended to keep this capacitor between 1/100 and 1/1000 of the value
of the VregVCO capacitor.
Because this pin is the output of a regulator, there are stability concerns if there is not sufficient series resistance. For
ceramic capacitors, the ESR (Equivalent Series Resistance) is too low, and it is recommended that a series resistance
of 1 - 3.3Ω is necessary. If there is insufficient ESR, then there may be degradation in the phase noise, especially in
the 100 - 300 kHz offset. Recommended values are from 1 μF to 10 μF.
The choice of the capacitor value at this pin involves a trade-off between integer spurs and phase noise in the 100 -
300 kHz offset range. Using a series resistor of about 220 mΩ in series with a capacitance that has an impedance of
about 150 mΩ at the phase detector frequency seems to give an optimal trade-off. For instance, if the phase detector
frequency is 2.5 MHz, then make this series capacitor 470 nF. If the phase detector frequency is 10 MHz, make this
capacitance about 100 nF.
Since the maximum voltage on these pins is less than the minimum Vcc voltage, level shifting may be required if the
output voltage of the microcontroller is too high. This can be accomplished with a resistive divider.
As with the CLK, DATA, and LE pins, level shifting may be required if the output voltage of the microcontroller is too
high. A resistive divider or a series diode are two ways to accomplish this. The diode has the advantage that no current
flows through it when the chip is powered down.
This is the high frequency output. This needs to be AC coupled, and matching may also be required. The value of the
DC blocking capacitor may be changed, depending on the output frequency.
In most cases, it is sufficient to short these together, although there always the option of adding additional poles.
C1_LF, C2_LF, and R2_LF are used in conjunction with the internal loop filter to make a fourth order loop filter.
This is the fastlock resistor, which can be useful in many cases, since the spurs are often better with low charge pump
currents, and the internal loop filter can be adjusted during fastlock.
This is the reference oscillator input pin. It needs to be AC coupled.
If the device is being driven single-ended, this pin needs to be shunted to ground with a capacitor.
Application Information
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