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Applications Information

Power-Supply Filtering

The MAX3622 is a mixed analog/digital IC. The PLL

contains analog circuitry susceptible to random noise.

In addition to excellent on-chip power-supply noise

rejection, the MAX3622 provides a separate power-

supply pin, V

CCA

, for the VCO circuitry. Figure 2 illus-

trates the recommended power-supply filter network for

CCA

. The purpose of this design technique is to

ensure clean input power supply to the VCO circuitry

and to improve the overall immunity to power-supply

noise. This network requires that the power supply is

+3.3V ±5%. Decoupling capacitors should be used on

all other supply pins for best performance.

Crystal Selection

The crystal oscillator is designed to drive a fundamen-

tal mode, AT-cut crystal resonator. See Table 1 for rec-

ommended crystal specifications. See Figure 4 for

external capacitor connection.

Crystal Input Layout and Frequency

Stability

The crystal, trace, and two external capacitors should

be placed on the board as close as possible to the

MAX3622’s X_IN and X_OUT pins to reduce crosstalk

of active signals into the oscillator.

The layout shown in Figure 3 gives approximately 3pF

of trace plus footprint capacitance per side of the crys-

tal (Y1). The dielectric material is FR-4 and dielectric

thickness of the reference board is 15 mils. Using a

25MHz crystal and the capacitor values of C10 = 27pF

and C9 = 33pF, the measured output frequency accu-

racy is -10ppm at +25°C ambient temperature.

Table 1. Crystal Selection Parameters

PARAMETER SYMBOL MIN TYP MAX UNITS

Crystal Oscillation Frequency f

OSC

25 MHz

Shunt Capacitance C

2.0 7.0 pF

Load Capacitance C

18 pF

Equivalent Series Resistance

(ESR)

50 

Maximum Crystal Drive Level 300 μW

MAX3622

Low-Jitter, Precision Clock Generator

with Two Outputs

6 _______________________________________________________________________________________

CCA

10.5Ω

+3.3V ±5%

0.01μF 10μF

0.01μF

Figure 2. Analog Supply Filtering

C 9

C 10

MAX3622

25MHz

CRYSTAL

Figure 3. Crystal Layout

1 2 3 4 5 6 7 8 9

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