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
).
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.
