LTC4258
14
4258fb
APPLICATIO S I FOR ATIO
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The LTC4258 provides a complete solution for detection
and powering of PD devices in an IEEE 802.3af compliant
system. The LTC4258 consists of four independent ports,
each with the ability to detect, classify, and provide iso-
lated –48V power to a PD device connected to it. The
LTC4258 senses removal of a PD with IEEE 802.3af
compliant DC method and turns off –48V power when the
PD is removed. An internal control circuit takes care of
system configuration and timing, and uses an I
2
C interface
to communicate with the host system.
OPERATING MODES
Each LTC4258 port can operate in one of four modes:
Manual, Semiauto, Auto or Shutdown. The operating
mode for a port is set by the appropriate bits in the
Operating Mode register. The LTC4258 will power up with
all ports in Shutdown mode if the external AUTO pin is tied
low; if AUTO is high, all ports will wake up in Auto mode.
The operating mode can be changed at any time via the I
2
C
interface, regardless of the state of the AUTO pin.
•
In Manual mode, a port will wait for instructions from
the host system before taking any action. It will run
single detection or classification cycles when com-
manded, and will report results in the Port Status
registers. When the host system decides it is time to
turn on or off power to a port, it can do so by setting
the appropriate Power On/Off bits in the Power Enable
PB register regardless of the current status of detec-
tion or classification.
• In Semiauto mode, the port will repeatedly attempt to
detect and classify a PD device attached to the link. It
will report this information in its Port Status register,
and wait for the host system to set the appropriate
Power On bit in the Power Enable PB register before
applying power to the port.
• In Auto mode, the port will detect and classify a PD
device connected to it, then immediately turn on the
power if detection was successful regardless of the
result of classification.
• In Shutdown mode, the port is disabled and will not detect
or power a PD. Also, the detect and fault event bits, status
bits and enable bits for the port are reset to zero.
Regardless of which mode it is in, the LTC4258 will
remove power automatically from any port that generates
a t
START
or t
ICUT
overcurrent fault event (see t
ICUT
Timing
and t
START
Timing sections). It will also automatically
remove power from any port that generates a disconnect
event if the appropriate Disconnect Enable bit is set in the
Disconnect Enable register. The host controller may also
remove power at any time by setting the appropriate
Power Off bit in the Power Enable PB register.
Power-On RESET
At turn-on or any time the LTC4258 is reset (either by
pulling the RESET pin low or writing to the global Reset All
bit), all the ports turn off and all internal registers go to a
predefined state, shown in Table 1.
Several of the registers assume different states based on
the state of the AUTO pin at reset. The default states with
AUTO high allow the LTC4258 to detect and power up a PD
in Automatic mode, even if nothing is connected to the I
2
C
interface.
SIGNATURE DETECTION
The IEEE defines a specific pair-to-pair PD signature
resistance that identifies a device that can accept
Power over Ethernet in accordance with the 802.3af
specification. When the port voltage is below 10V, an
802.3af compliant PD will have a 25k signature resistance.
Figure 11 illustrates the relationship between the PD
signature resistance (white box from 23.75k to 26.25k)
and required resistance ranges the PSE must accept
(white box) and reject (gray boxes). According to the
802.3af specification, the PSE may or may not accept
resistances in the two ranges of 15k to 19k and 26.5k to
33k. Note that the black box in Figure 11 represents the
150Ω pair-to-pair termination used in legacy 802.3 de-
vices like a computer’s network interface card (NIC) that
cannot accept power.
Figure 11. IEEE 802.3af Signature Resistance Ranges
RESISTANCE
PD
PSE
0Ω 10k
15k
4258 F11
19k 26.5k
26.25k23.75k
150Ω (NIC)
20k 30k
33k
