LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 14

• 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
LTC4258
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 AC or DC methods 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
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
interface, regardless of the state of the AUTO pin.
• In Manual mode, a port will wait for instructions from
• In Semiauto mode, the port will repeatedly attempt to
• In Auto mode, the port will detect and classify a PD
• In Shutdown mode, the port is disabled and will not detect
14
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.
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.
device connected to it, then immediately turn on the
power if detection was successful regardless of the
result of classification.
or power a PD. Also, the detect and fault event bits, status
bits and enable bits for the port are reset to zero.
U
U
W
U
2
2
C
C
Regardless of which mode it is in, the LTC4258 will
remove power automatically from any port that generates
a t
and t
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
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
150 pair-to-pair termination used in legacy 802.3 de-
vices like a computer’s network interface card (NIC) that
cannot accept power.
RESISTANCE
START
Figure 11. IEEE 802.3af Signature Resistance Ranges
START
PSE
PD
or t
0
150 (NIC)
ICUT
Timing sections). It will also automatically
overcurrent fault event (see t
10k
15k
20k
19k
23.75k
26.25k
26.5k
30k
ICUT
33k
Timing
4258 F11
4258p
2
C