AN2451 datasheet(32/55 Pages) STMICROELECTRONICS

32 / 55 page

Board description

AN2451

32/55

Doc ID 12791 Rev 3

5.4

Oscillator section

The ST7540 crystal oscillator circuitry is based on a MOS amplifier working in inverter

configuration.

This circuitry requires a crystal with a maximum load capacitance of 16 pF and a maximum

ESR of 40

Ω.

It is very important to keep the crystal oscillator and the load capacitors as close as possible

to the device.

The resonant circuit should be far away from noise sources such as:

●

Power supply circuitry

●

Burst and surge protections

●

Mains coupling circuits

●

Any PCB track or via carrying a signal.

To properly shield and separate the oscillator section from the rest of the board, it is

recommended to use a ground plane on both sides of the PCB, filling all the area below the

crystal oscillator and its load capacitors. No tracks or vias should cross the ground plane

except for the crystal connections.

It is also recommended to use a large clearance on the oscillator related tracks, to minimize

humidity problems (see Figure 28).

Connecting the case to ground is also a good practice to reduce the effect of radiated

signals on the oscillator.

Figure 28.

A recommended oscillator section layout for noise shielding

5.5

Surge and burst protection

The specific structure of the coupling interface circuit of the application is a weak point

against high voltage disturbances that can come from the external environment. In fact an

efficient coupling circuit with low insertion losses consequently allows a very low impedance

path from the mains to the powerline interface of the device.

For this reason it is recommended to add some specific protection devices on the mains

coupling path, to prevent high energy disturbances coming from the mains from damaging

the internal circuitry of the ST7540.

AN2451 Datasheet(PDF) 32 Page - STMicroelectronics