DP83TG721S-Q1: Feritte Beads definition

Hi Katharina,

Our datasheet recommendations should be followed exactly in order to guarantee performance of the device. Please recommend customer to install the given ferrite beads, as these have been selected by design and used in validation of the silicon. This will provide proper EMC performance, we would not recommend removing them.   

Thanks,

David

Hi David,

can we disclose why 100MHz is decisive and not the 25MHz from the quartz?

Thanks!

Katharina

Hi Katharina,

Can you clarify what they mean by "decisive" frequency? 

There are different clock frequencies internally (PLLs), not simply a 25Mhz. Ferrite bead provides EMC protection for all the needed frequencies, as we conduct during compliance testing for the device.

Thanks,

David

Hi David,

David Creger said:

Our datasheet recommendations should be followed exactly in order to guarantee performance of the device. Please recommend customer to install the given ferrite beads, as these have been selected by design and used in validation of the silicon. This will provide proper EMC performance, we would not recommend removing them. 

We would like to be able to better assess whether and which ferrite beads could be considered. But also if we see an interference frequency, whether it is possibly generated by the Ethernet PHY.

David Creger said:

Can you clarify what they mean by "decisive" frequency? 

There are different clock frequencies internally (PLLs), not simply a 25Mhz. Ferrite bead provides EMC protection for all the needed frequencies, as we conduct during compliance testing for the device.

What frequencies do we have to expect from the PHY on the respective rails?
Basically, a minimum impedance curve over frequency with the necessary quality to get sufficiently expected frequencies suppressed.
There are possible simulations/calculations for this purpose. The pearls must have been chosen in a suitable way.
For example, we suggested 1k, but also 600R. 100R at a specific frequency may also be sufficient.

Do we have such estimation we can share (under NDA)?

Hi Katharina,

Ferrite beads other than the recommended ones should not be considered. Design team does simulations and the beads were chosen in a suitable way, yes. Such simulation data is confidential and not shared.  

Can you help me understand the reason for this question? Is customer looking to cost-down or these beads are not available to them for some reason? 

Thanks,

David

Hi David, 

after discussion with the customer he shared the following:

There are different backgrounds for the questions, for example.
A)Knowledge of the possible expected interference frequencies in the system
b)if necessary Potential cost savings in the event of a possible omission of the bead (the possible interference frequencies are also relevant for risk assessment)
C)Select another bead with lower RDC (direct influence on impedance vs. frequency)

Among other things, some customers have a requirement that in the event of a "soft" MLCC short circuit, the inductor/ferrite bead must have a current carrying capacity greater than the current supplied by the source. E.g. buck with 2.5A, bead =1A => request violated => other ferrite bead
They are not yet convinced that this would be a consequential error.

Hi Katharina,

Understood, thanks for sharing the background. 

We would not recommend omitting or replacing the recommended bead, as these beads were used in validation and compliance testing of this device. If customer chooses to use a different bead, they should look for one with similar impedance-vs-frequency characteristics as the recommended.

Thanks,

David