AN4630

AN4630 STMICROELECTRONICS | Alldatasheet

Document overview

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• PDF pages: 33

Technical content

Datasheet sections

• 1 Reference schematics
• 2 Components dimensioning
• 2.1 Capacitors
• 2.2 Inductors
• 2.3 External quarts
• 3 Two or multi-layer application boards
• 3.1 QFN32 package
• 3.2 WLCSP34 package
• 4 Design recommendations when using QFN32 package
• 5 Design recommendations when using WLCSP34 package
• 6 References
• 7 Revision history

PCB design guidelines for the BlueNRG and BlueNRG-MS devices Introduction The BlueNRG and BlueNRG-MS are very low power Bluetooth low energy (BLE) single-mode network processor devices which can act as master or slave. Both devices em bed a 2.4 GHz RF transceiver, a Cortex M0 microcontroller, and a by-passable DC-DC step-down converter to achieve optimum power consumption. The BlueNRG/MS devices are housed in two different packages:

• QFN 32 pins
• WLCSP 34 balls ST provides all necessary source files (reference designs) for those customers who want to speed- up their developing. This application note is intended to accompany the reference designs and provides detailed information regarding the design decisions employed within STMicroelectronics designs. In addition, it details the design guidelines for developing a generic radio frequency application using a BlueNRG and BlueNRG - MS devices. As the layout guidelines are exactly the same for both devices, from now on we use BlueNRG to refer to the two different devices. The RF performance and the critical maximum peak voltage, spurious and harmonic emission, receiver matching strongly depend on the PCB layout as well as the selection of the matching network components. For optimal performance, STMicroelectronics recommends the use of the PCB layout design hints described in the following sections. Also, last but not least, STMicroelectronics strongly suggest using the BOM defined in the reference design, BOM that guarantees, with a good PCB design, the correct RF performance. For further information, visit the STMicroelectronics web site at www.st.com.

AN4630 Reference schematics

1 Reference schematics

Different application boards have been developed to show the BlueNRG device functionalities. The schematics of the different application boards are reported in the following pictures and refer to the various possible combinations:

• QFN32 package, SMD discrete balun, DC-DC converter ON (Figure 1: "QFN32 package, SMD discrete balun, DC-DC converter ON");
• QFN32 package, SMD discrete balun, DC-DC converter OFF (Figure 2: "QFN32 package, SMD discrete balun, DC-DC converter OFF");
• QFN32 package, integrated balun , DC-DC converter ON (Figure 3: "QFN32 package, integrated balun, DC-DC converter ON");
• CSP34 package, SMD discrete balun, DC-DC converter ON (Figure 4: "CSP34 package, SMD discrete balun, DC-DC converter ON");
• CSP34 package, SMD discrete balun, DC-DC converter OFF (Figure 5: "CSP34 package, SMD discrete balun, DC-DC converter OFF");
• CSP34 package , integrated balun , DC-DC converter ON (Figure 6: "CSP34 package, integrated balun, DC-DC converter ON"); All the layout guidelines described in the next sections are to be applied to all these application boards. Figure 1: QFN32 package, SMD discrete balun, DC-DC converter ON C17 C20 XTAL1 C21 C10 C18 C13 C14 C12 BlueNRG GNDPAD C11 C15 Application MCU C19 XTAL2 C16 1.7V to 3.6V Power Supply SPI_MOSI SPI_CLK SPI_I RQ TEST1 VB AT3 TEST2 TEST3 TEST4 TEST5 TEST6 TEST7 VDD1V8 TEST8 TEST9 TEST11 TEST12 VB AT1 SXTAL0 SXTAL1 RF0 RF1 VB AT2 FXTAL0 FXTAL1 SPI_MISO SPI_CS TEST10 VDD1V2 SMPSFI LT2 NO_SMPS SMPSFI LT1 RES ETN SPI_MOSI SPI_CLK SPI_I RQ SPI_CS SPI_MISO RESETN GAMSEC2014129-911

Reference schematics AN4630 Figure 6: CSP34 package, integrated balun, DC-DC converter ON Table 1: BlueNRG application board external componenents description Components Description C1, C6, C12 Decoupling capacitors for battery voltage C2, C3 DC-DC converter filtering capacitors C4, C5 Decoupling capacitor for on-chip 1.2 V voltage regulator C19, C20 Decoupling capacitor for on-chip 1.2 V voltage regulator C9, C10, C11, C14, C15, C16, C21 RF discrete balun filter/matching capacitors C7, C8 XTAL1 capacitors C17, C18 XTAL2 capacitors L1 DC-DC converter inductor L2, L3, L4 RF discrete balun filter/matching inductors L5 XTAL1 filtering inductor L6 XTAL2 filtering inductor R1 IRQ pull-down resistor XTAL1 Low-frequency crystal XTAL2 High-frequency crystal U1 High-frequency crystal BlueNRG QFN/CSP device U2 Micro controller GAMSEC2014129-1112 C19 BALF-NRG-01D3 Application MCU XTAL1 C20 XTAL2 C17 C12 C18 BlueNRG_WLCSP 1.7V to 3.6V Power Supply B11 B22 A2 3A1 4 SPI_MOSI SPI_CLK SPI_IRQ SPI_CS SPI_MISO SPI_MOSIE2 SPI_CLKE1 SPI_IRQD2 TEST1D1 VBAT3C1 TEST2C2 TEST3B1 TEST4B2 TEST5A1 TEST6B3 TEST7A2 VDD1V8A3 TEST8A4 TEST9A5 TEST11B4 TEST12B5 VBAT1 D5 SXTAL0 E5 SXTAL1 E6 RF0 D6 RF1 C6 GND C3 FXTAL0 B6 FXTAL1 A6 SPI_MISO F1 SPI_CS F2 TEST10 E3 VDD1V2 F3 SMPSFILT2 F5 SMPS_GND F4 SMPSFILT1 F6 RESETN E4 GNDD3 GND D4 RESETN

AN4630 Reference schematics Components Description U3 Integrated balun

Components dimensioning AN4630

2 Components dimensioning

The choice of the external components is essential for the correct application functionality. In the next paragraph, the description of the main components, their functionality and how to choose them is described.

2.1 Capacitors

A capacitor is a passive electrical component used to store energy in an electrical field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric. Capacitors differ from each other in construction techniques and materials used to manufacture. A lot of different types of capacitors exist (double-layer, polyester, polypropylene and so on), but this document focuses on the surface mount versions of ceramics only. The other types of capacitors are not indicated for characteristic or cost for the application targeted in this document. A capacitor, as a practical device, exhibits not only capacitance but also resistance and inductance. A simplified schematic for the equivalent circuit is shown in Figure 7: "Capacitor equivalent circuit" . Figure 7: Capacitor equivalent circuit Typically for the capacitors the ESR (equivalent series resistance) and the ESL (equivalent series inductance) are defined. The term ESR combines all losses, both series and parallel, in a capacitor at a given frequency so that the equivalent circuit is reduced to a simple R -C series connection. Same considerations for the ESL that is the equivalent series inductor comprised of three components: pad layout, capacitor height and power plane spreading inductance. The main differences between ceramic dielectric types are the temperature coefficient of capacitance and the dielectric loss. COG and NP0 (negative-positive-zero, i.e ± 0) dielectrics have the lowest losses and are used for filtering, matching and so on. For RF parts it is generally recommended that multilayer (or monolithic) ceramic capacitors with a COG dielectric material, which is a highly stable class I dielectric offering a linear temperature coefficient, low loss and stable electrical properties over time, voltage and frequency.

AN4630 Components dimensioning For RF decoupling purposes select a capacitor value such that for the frequency to be decoupled is close to or just above the series resonant frequency (SRF) of the capacitor. At SRF the parasitic impedance resonates with the device capacitance to form a series tuned circuit and the impedance presented by the capacitor is the effective series resistance (ESR). For DC blocking or coupling applications at RF, typically a capacitor with low insertion loss and a good quality factor is required. Since a capacitor’s quality factor is inversely proportional to its ESR, select a capacitor with a low ESR and ensure that the SRF of the capacitor is greater than the frequency of operation. If the working frequency is above the SRF of the capacitor, it appears inductive. All the capacitors of the BlueNRG application board used for the matching network and for the crystals have to be COG.

2.2 Inductors

An inductor is a passive electrical component used to store energy in its magnetic field. Any conductor has inductance. An inductor is typically made of wire or other conductor wound into a coil, to increase the magnetic field. Inductors differ from each other for construction techniques and materials used to manufacture. A lot of different types of inductors exist (air core inductor, ferromagnetic core inductor, and variable inductor), but this document will focus on the inductors useful for RF only. Usually in RF the air core inductors are used. The term air core describes an inductor that does not use a magnetic core made of ferromagnetic material, but coil wound on plastic, ceramic, or another nonmagnetic form. They are lower inductance than ferromagnetic core coils but are used at high frequencies because they are free from energy losses called core losses. Usually, the real circuit of an inductor is composed of a series resistance and a parallel capacitor. The parallel capacitor is considered to be the inter-winding capacitance that exists the turns of the inductor. If the inductor is placed over a ground plane then, this capacitance also includes the capacitance that exists between the inductor and the ground plane. The series resistor can be considered as the resistance of the inductor winding. In term of circuit performance, as already mentioned for the capacitors, the self-resonant frequency and the quality factor are the main inductor parameters, especially for the circuit where the losses need to be minimized. At the self-resonant frequency, the inductor impedance is at maximum. For frequency above the self-resonance the inductor behavior change and it appears capacitive. In general wire wound inductors have a higher quality factor than a multilayer equivalent. They also reflect and radiate more energy that can give rise to higher emission levels, especially in term of self-coupling. Inductive coupling can give rise to undesired circuit operation: to minimize coupling mount the inductors in sensitive circuit areas at 90 degrees to one another. In the BlueNRG application board two different inductor types are used: 1. DC-DC converter coil: the nominal value is 10 uH for the BlueNRG and 4.7 uH for the BlueNRG-MS. The DCR has to be less than 1 ohm; the rated current has to be higher than 100 mA. 2. RF matching and filtering coil: in this case the best solutions are the high Q coils, but a good compromise between application cost versus RF performances is to choose an inductor with a medium Q.

Components dimensioning AN4630

2.3 External quarts

The BlueNRG includes a high frequency and a low-frequency integrated oscillators that required two external crystals. The BlueNRG includes a fully integrated, low power 16/32 MHz Xtal oscillator wit h an embedded amplitude regulation loop. In order to achieve low power operation and good frequency stability of the Xtal oscillator, certain considerations with respect to the quartz load capacitance C0 need to be taken into account. Figure 8: "Diagram of the BlueNRG amplitude regulated oscillator" shows a simplified block diagram of the amplitude regulated oscillator used on the BlueNRG. Figure 8: Diagram of the BlueNRG amplitude regulated oscillator Low power consumption and fast startup time are achieved by choosing a quartz crystal with a small load capacitance C0. A reasonable choice for capacitor C0 is 15 pF. To achieve good frequency stability, the following equation needs to be satisfied:

AN4630 Components dimensioning Equation 1 Where C1’=C1+CPCB1+CPAD, C2’= C2+CPCB2+CPAD, where C1 and C2 are external (SMD) components, CPCB1 and CPCB2 are PCB routing parasites and CPAD is the equivalent small-signal pad-capacitance. The value of CPAD is around 0.5 pF for each pad. The routing parasites should be minimized by placing quartz and C1/C2 capacitors close to the chip, not only for an easier matching of the load capacitance C0, but also to ensure robustness against noise injection. Connect each capacitor of the Xtal oscilla tor to ground by a separate via. Regarding the low-frequency crystal oscillator the same consideration has to be done. It is important to underline that the BlueNRG and BlueNRG-MS integrate an internal low- frequency RC oscillator that can be used without external quartz. The customer can choose to use the internal or an external one. The BlueNRG and BlueNRG-MS also integrate an internal high-frequency RC oscillator, but it is disabling after an initial system bootstrap, and it is necessary to use an external quartz for radio operations.

Two or multi-layer application boards AN4630

3 Two or multi-layer application boards

Two or multi-layers application boards Different approach has to be taken when an application board is designed using the QFN32 or the WLCSP34 package.

3.1 QFN32 package

In this case the best solution is to use a two-layer application board. Figure 9: QFN32 package application board stack-up layer The two layers have to be so distributed: 1. TOP layer: used for routing. 2. BOTTOM layer: used for grounding under the RF zones and for routing on the rest.

3.2 WLCSP34 package

In this case, it is not possible to design a two layers board and the stack-up layer is a multilayer one due to the complexity of the package. In Figure 10: "WLCSP34 package application board stack-up layer " the suggested stack-up layer. Figure 10: WLCSP34 package application board stack-up layer

AN4630 Two or multi-layer application boards In this case a four-layer solution is used. Also, a more complicated and expensive technology has to be used to connect the tracks to the internal balls. The four layers have to be distributed as follows: 1. TOP layer: used for routing. 2. INNER2 layer: used for routing. 3. INNER3 layer: used only for ground. 4. BOTTOM layer: used for routing. The filled laser vias and the buried vias have to be used in this case. The thickness between the TOP layer and the INNER2 layer has to be 80 um. The thickness between the TOP layer and the INNER3 layer has to be 300 um. The thickness between the INNER3 layer and the BOTTOM layer can be chosen according to the customer necessity.

Design recommendations when using QFN32 package AN4630

4 Design recommendations when using QFN32

Design recommentations when using QFN32 package The application board TOP layer layout using the QFN32 package is shown in Figure 11: "QFN32 package application board TOP layer". Figure 11: QFN32 package application board TOP layer It is crucial to connect very well the ground of the exposed pad of the QFN32 to the ground on the application board. So many vias are necessary to be sure that the parasitic inductor introduced from each via is negligible.

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.

Design recommendations when using QFN32 package AN4630 Figure 13: High-frequency crystal inductor

AN4630 Design recommendations when using QFN32 package Figure 14: Low-frequency crystal inductor and tracks The DC-DC converter area is very sensitive, and it is necessary to pay attention to the layout of this part. This is because the DC-DC converter generates GND noise that can get coupled on surrounding ground reducing the sensitivity, and high-frequency components can be coupled onto RF part. So to ensure a correct layout it is necessary of: Providing efficient filtering by placing capacitors as close as possible from the BlueNRG; Reducing parasitic ensuring wide and short connections to BlueNRG. In Figure 15: "DC-DC converter layout zone" the suggested layout is shown DC-DC converter inductor DC-DC converter filtering capacitors

Design recommendations when using QFN32 package AN4630 Figure 15: DC-DC converter layout zone Particular care has to be taken in the placement of the supply voltage filtering capacitors. It is, in fact, important to ensure efficient filtering placing these capacitors as close as possible from their dedicated pins on the BlueNRG. The TX/RX part of the BlueNRG is a very sensitive part. The discrete balun has to be placed as close as possible to the TX/RX pins. The traces that connect the RF pins to the balun network (differential trace) should be of equal length. If the two differential signals are unbalanced, common-mode issues can be generated. The differential traces have to be routed firmly together. Differential receivers are designed to be sensitive to the difference between a pair of inputs, but also to be insensitive to a common-mode shift of those input. Therefore, if any external noise is coupled equally into the differential traces, the receiver will be insensitive to this ( standard mode coupled ) noise. More closely differential traces are routed together, more equal will any coupled noise be on each trace, therefore better will be the rejection of the noise in the circuit.The parallel inductors in the balun (and in general) should be mutually perpendicular to avoid mutual couplings. If no perpendicular position is possible, turn away their interposing capacitors or resistors.The interconnections between the elements are not considered transmission lines because their lengths are much shorter than the wavelength and, thus, their impedance is not critical. As results, their recommended width is smallest possible. In this way, the parasitic capacitances to ground can be minimized.

AN4630 Design recommendations when using QFN32 package Figure 16: Discrete balun layout zone An application board using an integrated balun was also designed . The integrated balun was developed internally to STMicroelectronics and can be used only with the BlueNRG/MS device. It is mandatory to follow the layout rules described in the balun datasheet (BALF-NRG-01D3).

Design recommendations when using WLCSP34 package AN4630

5 Design recommendations when using WLCSP34

Design recommendations when using WLCSP34 package The application board for the WLCSP34 package was designed in a four layers due the more complexity to treat the CSP package. The print of the four layers is shown in Figure 17: "WLCSP34 four layers board". Figure 17: WLCSP34 four layers board A good ground connection is essential for the RF performances. This point is important for all RF devices in all packages, but it is critical in a CSP package due to the reduced dimension of the device. A good ground connection, that means low resistance b etween the ground balls of the device and the ground layer, produces a low cross talk among critical blocks that means RF performance not impacted. The ground connection of the BlueNRG in WLCSP package on the TOP layer is shown in Figure 18: "WLCSP34 ground connection (TOP layer view)". Laser vias are used to connect TOP layer ground and INNER2 layer ground. The ground laser vias have to be put under GND balls, under the GND_SMPS ball and under the unused balls. In the TOP layer the GND_SMPS has to be kept separated from the GND.

AN4630 Design recommendations when using WLCSP34 package Figure 18: WLCSP34 ground connection (TOP layer view) In the INNER2 layer, Figure 19: "WLCSP34 ground connection (INNER2 layer view)" , buried filled vias have to be used to connect INNER2 layer ground and INNER 3 layer ground. The buried filled vias have to be put with 400 um of offset (both in x and y directions) respect to laser vias. In the INNER2 layer the GND_SMPS has to be kept separated from the GND. It is necessary not put ground layer under the RF discrete balun. This is because due to the very little distance between TOP and INNER2 layers; 80 um, the parasitic capacitances would be too big and would not be possible to find a working solution of the matching network.

Design recommendations when using WLCSP34 package AN4630 Figure 19: WLCSP34 ground connection (INNER2 layer view) In the INNER3 layer, Figure 20: "WLCSP34 ground connection (INNER3 layer view)" , GND and GND_SMPS have to be connected together. This layer has to be kept as continue as possible to obtain a good ground that means less noise.

AN4630 Design recommendations when using WLCSP34 package Figure 20: WLCSP34 ground connection (INNER3 layer view) To guarantee the RF performances with the DC-DC converter ON it is important to have an effective filtering between VBAT1 and SMPS_GND pins. This filtering is obtained using some filtering capacitors are shown in Figure 21: "Focus on power supply pins". The C6 and C8 capacitors between the VBAT1 and the SMPS_GND have to be put as close as possible to the WLCSP. The width of the connection tracks has to be increased as much as possible. The TOP layer or the INNER2 layer has to be used to route these tracks.

Design recommendations when using WLCSP34 package AN4630 Figure 21: Focus on power supply pins To guarantee the RF performances with the DC-DC converter ON it is important also to have an effective filtering between SMPSFILT2 and GND pins. This filtering is obtained using some filtering capacitors are shown in Figure 22: "Focus on BlueNRG biasing pins". The C9 and C10 capacitors between the SMPSFILT2 and the GND have to be put as close as possible to the WLCSP. The width of the connection tracks has to be increased as much as possible. The TOP layer or the INNER2 layer has to be used to route these tracks. The L1 inductor has to be also put as close as possible to the BlueNRG device. The capacitors C4 and C7 to filter the VDD_1V2 and VDD_1V8 balls have to be also put as near as possible to the device.

AN4630 Design recommendations when using WLCSP34 package The high-frequency crystal has to be put as far as possible from the matching network. The tracks that connect the crystal pins to the BlueNRG FXTAL0 and FXTAL1 pins have to be routed as far as possible from the matching network, and also have to be kept as far as possible one from each other. The INNER2 layer has to be used to connect the crystal pins to the BlueNRG FXTAL0 and FXTAL1 pins. The INNER2 layer has to be keeping empty under the crystal. All these recommendations are shown in Figure 26: "High-frequency crystal connection". Figure 26: High-frequency crystal connection The low-frequency crystal has to be put as far as possible from the matching network. The tracks that connect the crystal pins to the BlueNRG SXTAL0 and SXTAL1 pins have to be routed as far as possible from the matching network, and also have to be kept as far as possible one from each other. The INNER2 layer has to be used to connect the crystal pins to the BlueNRG FXTAL0 and FXTAL1 pins. The INNER2 layer has to be keeping empty under the crystal. All these recommendations are shown in Figure 27: "Low-frequency crystal connection".

6 References

[1] BlueNRG datasheet Rev 5 [2] BlueNRG-MS datasheet Rev 1 [3] BALF-NRG-01D3 datasheet Rev 3

7 Revision history

Revision history

15-Jan-2015 1 Initial release.