ltc4266a Linear Technology Corporation, ltc4266a Datasheet - Page 23

• Manufacturer Part Number: ltc4266a
• Description: Quad Poe/poe+/ltpoe++ Pse Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The LTC4266A/LTC4266C will support current levels well
beyond the maximum values in the 802.3at specification.
The shaded areas in Table 6 indicate settings that may
require a larger external MOSFET, additional heat sinking,
or enabling t
MOSFET Fault Detection
LTC4266A/LTC4266C PSE ports are designed to tolerate
significant levels of abuse, but in extreme cases it is pos-
sible for the external MOSFET to be damaged. A failed
MOSFET may short source to drain, which will make the
port appear to be on when it should be off; this condition
may also cause the sense resistor to fuse open, turning
off the port but causing the LTC4266A/LTC4266C SENSE
pin to rise to an abnormally high voltage. A failed MOSFET
may also short from gate to drain, causing the LTC4266A/
LTC4266C GATE pin to rise to an abnormally high voltage.
The LTC4266A/LTC4266C OUT, SENSE and GATE pins are
designed to tolerate up to 80V faults without damage.
If the LTC4266A/LTC4266C sees any of these conditions for
more than 180μs, it disables all port functionality, reduces
the gate drive pull-down current for the port and reports
a FET Bad fault. This is typically a permanent fault, but
the host can attempt to recover by resetting the port, or
by resetting the entire chip if a port reset fails to clear the
fault. If the MOSFET is in fact bad, the fault will quickly
return, and the port will disable itself again. The remaining
ports of the LTC4266A/LTC4266C are unaffected.
An open or missing MOSFET will not trigger a FET Bad
fault, but will cause a t
attempts to turn on the port.
Voltage and Current Readback
The LTC4266A/LTC4266C measures the output voltage
and current at each port with an internal A/D converter.
Port data is only valid when the port power is on. The
converter has two modes:
• Slow mode: 14 samples per second, 14.5 bits resolution
• Fast mode: 440 samples per second, 9.5 bits resolution
In fast mode, the least significant 5 bits of the lower byte
are zeroes so that bit scaling is the same in both modes.
LIM
.
START
fault if the LTC4266A/LTC4266C
Disconnect
The LTC4266A/LTC4266C monitors the port to make sure
that the PD continues to draw the minimum specified
current. A disconnect timer counts up whenever port
current is below 7.5mA (typ), indicating that the PD has
been disconnected. If the t
be turned off and the disconnect bit in the fault event reg-
ister will be set. If the current returns before the t
runs out, the timer resets and will start counting from the
beginning if the undercurrent condition returns. As long
as the PD exceeds the minimum current level more often
than t
Although not recommended, the DC disconnect feature can
be disabled by clearing the corresponding DC Disconnect
Enable bits. Note that this defeats the protection mecha-
nisms built into the IEEE spec, since a powered port will
stay powered after the PD is removed. If the still-powered
port is subsequently connected to a non-PoE data device,
the device may be damaged.
The LTC4266A/LTC4266C does not include AC disconnect
circuitry, but includes AC Disconnect Enable bits to main-
tain compatibility with the LTC4259A. If the AC Disconnect
Enable bits are set, DC disconnect will be used.
Shutdown Pins
The LTC4266A/LTC4266C includes a hardware SHDN pin
for each port. When a SHDN pin is pulled to DGND, the
corresponding port will be shut off immediately. The port
remains shut down until re-enabled via I
reset in AUTO pin mode.
Masked Shutdown
The LTC4266A/LTC4266C provides a low latency port
shedding feature to quickly reduce the system load when
required. By allowing a pre-determined set of ports to
be turned off, the current on an overloaded main power
supply can be reduced rapidly while keeping high priority
devices powered. Each port can be configured to high or
low priority; all low-priority ports will shut down within
6.5μs after the MSD pin is pulled low. If multiple ports in
a LTC4266A/LTC4266C device are shut down via MSD,
DIS
, it will stay powered.
LTC4266A/LTC4266C
DIS
timer expires, the port will
2
C or a device
DIS
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4266acfa
timer