AN4630 datasheet(17/33 Pages) STMICROELECTRONICS

Part #	AN4630

Description	PCB design guidelines for the BlueNRG and BlueNRG-MS devices

PDF	33 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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17 / 33 page

AN4630

Design recommendations when using QFN32

package

Figure 12: Vias on the exposed pad of the QFN32 package

The ground of the two external crystals has to be isolated from the ground of the RF part of

the board. This is because the RF ground is “dirty” and this signal can disturb the correct

functionality of the two crystals. Also to reduce the coupling effects some cunning have to

be taken: In the high-frequency crystal the load capacitor of the FXTAL0, pin 18, has to be

connected to ground in series with an inductor (see Figure 13: "High-frequency crystal

inductor"); In the low-frequency crystal the ground part of two load capacitors have to be

connected together and, after, connected to the ground by an inductor. The two tracks that

connect the low-frequency crystal to the SXTAL0 and SXTAL1, pins 23 and 22, have to be

put in a layer different from the TOP.

DocID027262 Rev 1

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AN4630 Datasheet(PDF) 17 Page - STMicroelectronics