AN2744 datasheet(38/56 Pages) STMICROELECTRONICS

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Board description

AN2744

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Therefore, once the output voltage is fixed by the VSENSE partitioning, the required PD can

be calculated as a function of the load current (IOUTrms). The resulting value can be

compared with the dissipation limit imposed by the

θJA value, as a function of tTX and duty

cycle, to keep the junction temperature below the 125 °C limit.

5.4

Oscillator section

The ST7538Q crystal oscillator circuitry is based on a MOS amplifier working in inverter

configuration. This circuitry requires a crystal having 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 must 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 via holes, except for the crystal

connections, should cross the ground plane.

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

humidity problems, see Figure 36.

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

signals on the oscillator.

Figure 36.

A recommended oscillator section layout for noise shielding

SGND

XOUT

XIN

ST7538Q

TOP Layer

BOTTOM Layer

Clearance

SGND

XOUT

XIN

ST7538Q

TOP Layer

BOTTOM Layer

Clearance

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