LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 20

• 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
SERIAL DIGITAL INTERFACE
The LTC4258 communicates with a host (master) using
the standard 2-wire interface as described in the SMBus
Specification Version 2.0 (available at http://smbus.org).
The SMBus is an extension of the I
LTC4258 is also compatible with the I
Timing Diagrams (Figures 5 through 9) show the timing
relationship of the signals on the bus. The two bus lines,
SDA and SCL, must be high when the bus is not in use.
External pull-up resistors or current sources, such as the
LTC1694 SMBus accelerator, are required on these lines.
If the SDA and SCL pull-ups are absent, not connected to
the same positive supply as the LTC4258’s V
not activated when the power is applied to the LTC4258, it
is possible for the LTC4258 to see a START condition on
the I
between I
a START condition when it powers up because the SCL and
SDA lines were left floating, it will not assert an interrupt
(pull INT low) until it sees a STOP condition on the bus. In
a typical application the I
traffic and the LTC4258 will see a STOP so soon after
power up that this momentary condition will go unnoticed.
Isolating the Serial Digital Interface
IEEE 802.3af requires that network segments be electri-
cally isolated from the chassis ground of each network
20
VOLTAGE
SENSE n
300mV
250mV
200mV
150mV
100mV
50mV
0mV
2
C bus. The interrupt pin (INT) is only updated
600mA
500mA
400mA
300mA
200mA
100mA
0mA
CURRENT
R
Figure 14. LTC4258 Current Sense and Limits
S
2
= 0.5
C transactions. Therefore if the LTC4258 sees
CONNECT
DC DIS-
U
(I
CUT
CUT
U
2
)
C bus will immediately have
LIMIT
(I
LIM
)
W
CIRCUIT
SHORT
2
C bus standard. The
2
C bus, and the
PORT OFF IN t
OR t
NORMAL
OPERATION
CURRENT LIMIT
IN 1 s
PORT OFF IN t
DD
START
U
EFFECT
pin, or are
CURRENT
LIMIT
ICUT
DIS
4258 F14
interface device. However, the network segments are not
required to be isolated from each other provided that the
segments are connected to devices residing within a
single building on a single power distribution system.
For simple devices such as small powered Ethernet
switches, the requirement can be met by using an iso-
lated power supply to power the entire device. This
implementation can only be used if the device has no
electrically conducting ports other than twisted-pair
Ethernet. In this case, the SDAIN and SDAOUT pins of the
LTC4258 can be connected together to act as a standard
I
If the device is part of a larger system, contains serial
ports, or must be referenced to protective ground for
some other reason, the Power over Ethernet subsystem
including the LTC4258s must be electrically isolated
from the rest of the system. The LTC4258 includes
separate pins (SDAIN and SDAOUT) for the input and
output functions of the bidirectional data line. This eases
the use of optocouplers to isolate the data path between
the LTC4258s and the system controller. Figure 15
shows one possible implementation of an isolated inter-
face. The SDAOUT pin of the LTC4258 is designed to
drive the inputs of an optocoupler directly, but a standard
I
signals into the optocouplers from the system controller
side. Schmitt triggers must be used to prevent extra
edges on transitions of SDA and SCL.
Bus Addresses and Protocols
The LTC4258 is a read-write slave device. The master can
communicate with the LTC4258 using the Write Byte,
Read Byte and Receive Byte protocols. The LTC4258’s
primary serial bus address is (010A
nated by pins AD3-AD0. All LTC4258s also respond to the
address (0110000)b, allowing the host to write the same
command into all of the LTC4258s on a bus in a single
transaction. If the LTC4258 is asserting (pulling low) the
INT pin, it will also acknowledge the Alert Response
Address (0001100)b using the receive byte protocol.
2
2
C/SMBus SDA pin.
C device typically cannot. U1 is used to buffer I
3
A
2
A
1
A
0
)b, as desig-
4258p
2
C