LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 19

• Manufacturer Part Number: LTC4258
• Description: Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
face. Using transient suppressors, polyfuses and ex-
tended wait times after disconnecting a PD are effective
strategies to reduce the extremes applied to the external
MOSFETs.
Surge Suppressors and Circuit Protection
IEEE 802.3af Power over Ethernet is a challenging Hot Swap
application because it must survive the (probably unin-
tentional) abuse of everyone in the building. While hot
swapping boards in a networking or telecom card cage is
done by a trained technician or network administrator,
anyone in the building can plug a device into the network.
Moreover, in a card cage the physical domain being pow-
ered is confined to the card cage. With Power over Ether-
net, the PSE supplies power to devices up to 100 meters
away. Ethernet cables could potentially be cut, shorted
together, and so on by all kinds of events from a contrac-
tor cutting into walls to someone carelessly sticking a
screwdriver where it doesn’t belong. Consequently, the
Power over Ethernet power source (PSE) must be designed
to handle these events.
The most dramatic of these is shorting a powered port.
What the PSE sees depends on how much CAT-5 cable is
between it and the short. If the short occurs on the far end
of a long cable, the cable inductance will prevent the cur-
rent in the cable from increasing too quickly and the
LTC4258’s built-in short-circuit protection will take con-
trol of the situation and turn off the port. Some energy is
stored in the cable, but the transient suppressor on the
port clamps the port voltage when the cable inductance
causes the voltage to fly back after the MOSFET is turned
off. Because the cable only had 600mA or so going through
it, an SMAJ58A or equivalent device can easily control the
port voltage during flyback. With no cable connected at all,
a powered port shorted at the PSE’s RJ-45 connector can
reach high current levels before the port is shut down. There
is no cable inductance to store energy so once the port is
shut down the situation is under control.
A short—hence low inductance—piece of CAT-5 will not
limit the rapid increase of current when the port is shorted.
Even though the LTC4258 short-circuit shutdown is fast,
the cable may have many amps flowing through it before
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the MOSFET can be turned off. Due to the high current,
this short piece of cable flies back with significant energy
behind it and must be controlled by the transient suppres-
sor. Choosing a surge suppressor that will not develop
more than a few volts of forward voltage while passing
more than 10A is important. A positive port voltage may
forward bias the detect diode (D
LTC4258’s DETECT n pin positive as well and engaging
the DETECT n clamps. This will generally not damage the
LTC4258 but extreme cases can cause the LTC4258 to
reset. When it resets, the LTC4258 signals an interrupt,
alerting the host controller which can then return the
LTC4258 to normal operating mode.
A substantial transient surge suppressor can typically
protect the LTC4258 and the rest of the PSE from these
faults. Placing a polyfuse between the RJ-45 connector
and the LTC4258 and its associated circuitry can provide
additional protection. To meet safety requirements, place
the polyfuse in the ground leg of the PSE’s output.
DC DISCONNECT
DC disconnect monitors the sense resistor voltage when-
ever the power is on to make sure that the PD is drawing
the minimum specified current. The disconnect timer
counts up whenever port current is below 7.5mA (typ). If
the t
turned off and the disconnect bit in the fault register will be
set. If the undercurrent condition goes away before the
t
counting from the beginning if the undercurrent condition
occurs again. The undercurrent circuit includes a glitch
filter to filter out noise.
The DC disconnect feature can be disabled by clearing the
corresponding DC Discon Enable bits in the Disconnect
register (13h). The t
by bits 1 and 0 of register 16h.
The LTC4258 implements a variety of current sense and
limit thresholds to control current flowing through the
port. Figure 14 is a graphical representation of these
thresholds and the action the LTC4258 takes when
currrent crosses the thresholds.
DIS
timer runs out, the timer will reset. The timer will start
DIS
timer runs out, the corresponding port will be
DIS
timer duration can be programmed
DET n
LTC4258
), bringing the
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