LTC4259A-1
29
4259a1fa
APPLICATIO S I FOR ATIO
WUUU
I
LIM
, I
CUT
and I
MIN
. Therefore, to maintain IEEE compli-
ance, use a resistor with 0.5% or better accuracy.
Power MOSFETs
The LTC4259A-1 controls power MOSFETs in order to
regulate current flow through the Ethernet ports. Under
certain conditions these MOSFETs have to dissipate sig-
nificant power. See the Choosing External MOSFETs sec-
tion for a detailed discussion of the requirements these
devices must meet.
Common Mode Chokes
Both nonpowered and powered Ethernet connections
achieve best performance (for data transfer, power trans-
fer and EMI) when a common mode choke is used on each
port. In the name of cost reduction, some designs share a
common mode choke between two adjacent ports. Even
for nonpowered Ethernet, sharing a choke is not recom-
mended. With two ports passing through the choke, it
cannot limit the common mode current of either port.
Instead, the choke only controls the sum of both ports’
common mode current. Because cabling from the ports
generally connects to different devices up to 200m apart,
a current loop can form. In such a loop, common mode
current flows in one port and out the other, and the choke
will not prevent this because the sum of the currents is
zero. Another way to view this interaction between the
paired ports is that the choke acts as a transformer
coupling the ports’ common modes together. In
nonpowered Ethernet, common mode current results
from nonidealities like ground loops; it is not part of
normal operation. However, Power over Ethernet sends
power and hence significant current through the ports;
common mode current is a byproduct of normal opera-
tion. As described in the Choosing External MOSFETs
section and under the Power Supplies heading below,
large transients can occur when a port’s power is turned
on or off. When a powered port is shorted (see Surge
Suppressors and Circuit Protection), a port’s common
mode current may be excessive. Sharing a common mode
choke between two ports couples start-up, disconnect
and fault transients from one port to the other. The end
result can range from momentary noncompliance with
802.3af to intermittent behavior and even to excessive
voltages that may damage circuitry (in both the PSE and
PD) connected to the ports.
Detect, AC Blocking and Transient Suppressor Diodes
During detection and classification, the LTC4259A-1 senses
the port voltage through the detect diodes D
DET
in Fig-
ure 18. Excessive voltage drop across D
DET
will corrupt
the LTC4259A-1’s detect and classification results. Select
a diode for D
DET
that will have less than 0.7V of forward
drop at 0.4mA and less than 0.9V of forward drop at 50mA.
When the port is powered, the detect diode is reverse bi-
ased. Any leakage through the detect diode prevents the
LTC4259A-1 from sensing all the current coupled through
the C
DET
capacitor. At high temperature with 70V of reverse
bias, a typical switching diode like the 1N4148 may have
more than 50µA of leakage. Such leakage can interfere with
AC disconnect because it is a large fraction of the LTC4259A-
1’s I
ACDMIN
threshold. Using a low leakage detect diode like
the CMPD3003 is recommended.
The AC blocking diodes can interfere with AC disconnect
sensing if they become leaky. If the AC blocking diode (D
AC
in Figure 18) begins leaking, it contributes to the Ethernet
port impedance, potentially bringing the impedance low
enough to draw I
ACDMIN
from the DETECT pin and keep the
port powered. More likely, leakage through the AC block-
ing diode will cause shifts in the AC disconnect threshold
that are not large enough to make the PSE noncompliant.
Generally, diode leakage is caused by voltage or tempera-
ture stress. Diodes that are rated to 100V or more and can
handle dissipating at least 0.5W should be acceptable in
this application. Other component leakages can have a
similar affect on AC disconnect and even affect DC discon-
nect if the leakage becomes severe. Among components
to be wary of are the transient surge suppressors. The
devices shown in Figure 1 are rated for less than 5µA of
leakage at 58V. However there is a potential for stress
induced leakage, so healthy margins should be used when
selecting diodes for these applications.
Capacitors
Sizing of both the C
DET
and C
PSE
capacitors is critical to
proper operation of the LTC4259A-1’s AC disconnect sens-
ing. See the AC Disconnect section for more information.
