LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 19

• Manufacturer Part Number: LTC4259CGW
• Manufacturer: Linear Technology

Specifications of LTC4259CGW

• Linear Misc Type: Negative Voltage
• Operating Supply Voltage (typ): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
Before designing a MOSFET into your system, carefully
compare its safe operating area (SOA) with the worst case
conditions (like powering on a defective PD) the device will
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
LTC4259’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
flies back after the MOSFET is turned off. Because the cable
only had 600mA or so going through it, an SMAJ58A or
other 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 cur-
rent 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.
U
U
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U
In the intermediate case things get exciting. 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 LTC4259 short-circuit shutdown is fast, the cable may
have many amps flowing through it before 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 contolled by the transient suppressor. 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
pin positive as well and engaging the DETECT n clamps.
This will generally not damage the LTC4259 but it can cause
the LTC4259 to reset. When it resets, the LTC4259 signals
an interrupt, alerting the host controller which can then
return the LTC4259 to normal operating mode.
A substantial transient surge suppressor can typically
protect the LTC4259 and the rest of the PSE from these
faults. Placing a polyfuse between the RJ-45 connector
and the LTC4259 and its associated circuitry can provide
additional protection. Connecting the transient surge sup-
pressor to the RJ-45 side of the fuse allows the transient
surge suppressor to control port voltage even when the
polyfuse is open. To meet safety requirements, place a
polyfuse in the ground leg of the PSE’s output.
DC DISCONNECT
DC disconnect watches 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 5mA to 10mA,
7.5mA typ. If the t
port will be turned off and the disconnect bit in the fault
register will be set. If the undercurrent condition goes
away before the t
The timer will start counting from the beginning if the
undercurrent condition occurs again. The undercurrent
circuit includes a glitch filter to filter out noise.
DET n
DIS
DIS
), bringing the LTC4259’s DETECT n
timer runs out, the timer will reset.
timer runs out, the corresponding
LTC4259
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4259i