3.3-V or 1.8-V
Supply
VDDIO
100 nF
10 nF
10 F
1 F
100 nF 10 nF
3.3-V Supply
10 F
1 F
VDDIO
100 nF
10 nF
10 F
1 F
100 nF 10 nF
AVDD3V3
10 F
1 F
Ferrite Bead for
improved EMC
(Optional)
Ferrite Bead for
improved EMC
(Optional)
System Overview
14
TIDUES1A – October 2019 – Revised February 2020
Copyright © 2019–2020, Texas Instruments Incorporated
EMC Compliant 10/100-Mbps Ethernet PHY Reference Design With IEEE
802.3at Type-1 (
≤
12.95 W) PoE-PD
It is also important that the physical size of bead components be chosen to accommodate the current
necessary to supply the physical layer device supply pins. See individual component data sheets for
component current requirements. Place local bypass components (including capacitors and optionally
ferromagnetic beads) between device supply pins and power sourcing vias on the PCB. TI recommends
the PCB have at least one solid ground plane and one solid power plane to provide a low-impedance
power source to the component. This also provides a low-impedance return path for non-differential digital
RMII and clock signals. Ground vias must be placed as close to the ground pins as possible. The
DP83825I is capable of operating with a 3.3-V or 1.8-V of I/O supply voltages along with the analog supply
of 3.3 V. The DP83825I device needs VDDA3V3 after VDDIO is fully ramped. If power sequencing is not
feasible on the customer board, then an external Reset (RST_N) is needed on pin 5 when both power
VDDA3V3 and VDDIO supplies are ramped.
Figure 10. Power Supply Decoupling Recommendation
2.4.6
Magnetics
For communication on a local area network (LAN), magnetics provide electrical isolation, signal balancing,
common-mode rejection, impedance matching, and EMC improvement. The 10/100Base-T Ethernet
utilizes an Unshielded Twisted Pair (UTP) transmission cable consisting of four sets of twisted pairs
connected in a balanced configuration. The UTP wiring is prone to pick up noise that leads to conducted
and radiated noise emission. Also for human safety, the IEEE specification requires a 10/100BASE-T port
to be able to withstand 1,500 VAC at 50 Hz, 60 Hz for 1 minute between ports or from each port to the
chassis ground. Transformers simply and easily provide the balanced connection to each pair of a cable,
provide a very effective rejection of common-mode signals and inexpensively meet this isolation
requirement.
Common-mode rejection is the ability to reject a signal which, referenced to ground, has the same
amplitude and phase on both inputs. This signal, which is usually the result of noise or a small impedance
mismatch, produces a small differential error voltage at the input terminals of the Ethernet. Subsequently,
this error gets amplified right along with the desired communications signal. The magnetics play a major
role in knocking down common-mode noise down to an acceptable level. The common-mode rejection of
a transformer functions in both signal directions of a port. This common-mode rejection attenuates
common-mode signals coming both from the cable to the PHY and also from the PHY to the cable.
Reduction of the common-mode signals picked up by the cable from its environment improves the signal-
to-noise ratio of the system, thus allowing AFE and DPS inside PHY to more easily recover the data signal
and achieve the desired bit error rate. The noise from the board circuits can couple to the sensitive signal
traces going from the PHY to the magnetics, coupling equally to both signals of a differential pair and
creating common-mode noise. This noise will be attenuated by the magnetics and improves the EMC
performance.
