What is the function of FB2 in the reference schematic for the 54L15 CSP47 package?

[zhanxy]

"What is the function of FB2 in the reference schematic for the 54L15 CSP47 package? Can it be removed? Why is it not present in QFN48? According to the latest schematic, C12 and C5 are marked as DNM. Currently, layout space is tight. Can they be directly removed?"

[Syed Maysum Abbas Zaidi]

Hi,

1. FB2 is a ferrite bead used to reduce high-frequency noise on the RF power supply (DECRF). This supply is related to the radio, which is sensitive to noise so FB2 helps in keeping digital noise away from the RF circuitry. If FB2 is removed, the device may still work, but EMC and RF performance may be reduced.

2. The CSP47 package is very small, so the internal power pins are closer together and more sensitive to noise coupling. To compensate for this, extra external filtering (FB2) is added in the reference design. Whereas the QFN48 package is larger and already provides better natural isolation between pins, so FB2 is not needed there.

3. DNM indicates Do Not Mount, meaning the component is optional and not populated by default. So these can be omitted if board space is constrained.

Best Regards,
Syed Maysum

[zhanxy]

In the CSP package reference design, are capacitors C12, C2, C13, and C5 all placed close to B1 (DECRF)? Why does A4 (DECA) not require a filter capacitor, while pin 43 (DECA) in the QFN package requires a dedicated filter capacitor C12?

[Syed Maysum Abbas Zaidi]

Hi,

Yes, in the CSP package reference design the capacitors C12, C2, C13, and C5 are placed close to B1 (DECRF) to provide local decoupling for the RF/analog supply.

One of the reasons why A4 (DECA) in the CSP package does not require a separate filter capacitor is that the package is relatively small, and the DECA pin is physically very close to DECRF and its associated decoupling capacitors. Due to these very short distances and low parasitics, the DECA node already sees sufficiently clean and stable supply filtering.

In contrast, the QFN package is larger, and the DECA pin (pin 43) is farther away from other decoupling components. To ensure proper local filtering and noise suppression at the DECA pin, a dedicated capacitor (C12) is therefore added close to the pin in the QFN reference design.

Best Regards,
Syed Maysum