10

SY89531L

Micrel, Inc.

M9999-010808

hbwhelp@micrel.com or (408) 955-1690

Crystal Input and Oscillator Interface

The SY89531L features a fully integrated on-board

oscillator to minimize system implementation costs. The

oscillator is a series resonant, multivibrator type design,

and thus, a series-resonant crystal is preferred, but not

required.

A parallel-resonant crystal can be used with the SY89531L

with only a minor error in the desired frequency. A parallel-

resonant mode crystal used in a series resonant circuit will

exhibit a frequency of oscillation a few hundred ppm lower

than specified; a few hundred ppm translates to kHz

inaccuracies. In a general computer application this level of

inaccuracy is immaterial.

As the oscillator is somewhat sensitive to loading on its

inputs, the user is advised to mount the crystal as close to

the SY89531L as possible to avoid any board level parasitics.

In addition, trace lengths should be matched. Figure 2 shows

how to interface with a crystal. Table 1 illustrates the crystal

specifications. Certain crystals may require a 10pF capacitor

across XTAL1 and XTAL2 for proper operation. This is

normally not required, but it is recommended that provisions

be made for it.

SY89531L

XTAL2

(Pin 26, SOIC)

XTAL1

(Pin 25, SOIC)

XTAL

16.666MHz

Quartz Crystal Selection:

(1) Raltron Series Resonant: AS-16.666-S-SMD-T-MI

(2) Raltron Parallel Resonant: AS-16.666-18-SMD-T-MI

Optional

Figure 2. Crystal Interface

External Loop Filter Considerations

The SY89531L features an external PLL loop filter that

allows the user to tailor the PLL’s behavior to their application

and operating environment. We recommend using ceramic

capacitors with NPO or X7R dielectric, as they have very-

low effective series resistance. For applications that require

ultra-low cycle-to-cycle jitter, use the components shown in

Figure 3. The PLL loop bandwidth is a function of feedback

divider ratio, and the external loop filter allows the user to

compensate. For instance, the PLL’s loop bandwidth can

be decreased by using a smaller resistor in the loop filter.

This results in less noise from the PLL input, but potentially

more noise from the VCO. Refer to

“AC Electrical

Characteristics” for target PLL loop bandwidth. The designer

should take care to keep the loop filter components on the

same side of the board and as close as possible to the

SY89531L’s LOOP_REF and LOOP_FILTER pins. To insure

minimal noise pick up on the loop filter, it is desirable to cut

away the ground plane directly underneath the loop filter

component pads and traces. However, the benefit may not

be significant in all applications and one must be careful to

not alter the characteristic impedance of nearby traces.

470pF

Loop

Filter

Loop

Reference

330

Ω

0.2

µF

Figure 3. External Loop Filter Connection

Table 1. Quartz Crystal Oscillator Specifications

Output Frequency: 14MHz-18MHz

Mode of Oscillation: Fundamental

Min

Typ

Max

Unit

Frequency Tolerance @25

°C

—

±30

±50

ppm

Frequency Stability over 0

°C to 70°C

—

±50

±100

ppm

Operating Temperature Range

–20

—

+70

°C

Storage Temperature Range

–55

—

+125

°C

Aging (per yr/first 3 yrs)

——

±5

ppm

Load Capacitance

—

18 (or series)

—

pF

Equivalent Series Resistance (ESR)

——

50

Ω

Drive Level

—

100

—

µW