LTC4259ACGW#TRMPBF Linear Technology, LTC4259ACGW#TRMPBF Datasheet - Page 20

• Manufacturer Part Number: LTC4259ACGW#TRMPBF
• Manufacturer: Linear Technology

Specifications of LTC4259ACGW#TRMPBF

• Linear Misc Type: Negative Voltage
• Family Name: LTC4259A
• Operating Supply Voltage (min): -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: Compliant

APPLICATIO S I FOR ATIO
noncompliant PD with only a few ohms of resistance. With
foldback, the MOSFET sees a maximum of 18W for the
duration of t
The LTC4259A’s duty cycle protection enforces 15 times
longer off time than on time, preventing successive at-
tempts to power a defective PD from damaging the
MOSFET. System software can enforce even longer wait
times. When the LTC4259A is operated in semiauto or
manual mode—described in more detail under Operating
Modes—it will not power on a port until commanded to do
so by the host controller. By keeping track of t
t
port again after one of these faults even if the LTC4259A
reports a Detect Good. In this way the host controller
implements a MOSFET cooling off period which may be
programmed to protect smaller MOSFETs from repeated
thermal cycling. The LTC4259A has built-in duty cycle
protection for t
Timing sections) that is sufficient to protect the MOSFETs
shown in Figure 1.
Before designing a MOSFET into your system, carefully
compare its safe operating area (SOA) with the worst case
conditions (like powering up 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.
LTC4259A
20
ICUT
faults, the host controller can delay turning on the
START
ICUT
.
and t
U
START
U
(see t
ICUT
W
Timing and t
U
START
START
and
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
LTC4259A’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 LTC4259A 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 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
LTC4259A’s DETECT n pin positive as well and engaging
the DETECT n clamps. This will generally not damage the
LTC4259A but extreme cases can cause the LTC4259A to
reset. When it resets, the LTC4259A signals an interrupt,
alerting the host controller which can then return the
LTC4259A to normal operating mode.
A substantial transient surge suppressor can typically
protect the LTC4259A and the rest of the PSE from these
faults. Placing a polyfuse between the RJ-45 connector
and the LTC4259A 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
DET n
), bringing the
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