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.

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