LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 16

• 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
measures the port voltage through the DETECT n pin. Note
that class 4 is presently specified by the IEEE as reserved
for future use. Figure 13 shows a PD load line, starting with
the shallow slope of the 25k signature resistor below 10V,
then drawing the classification current (in this case, class 3)
between 14.5V and 20.5V. The LTC4258’s load line for clas-
sification is also shown in Figure 13. It has low impedance
until current limit at 65mA (typ).
The LTC4258 will classify a port immediately after a
successful detection cycle in Semiauto or Auto modes, or
when commanded to in Manual mode. It measures the PD
classification signature current by applying 18V (typ) to
the port and measuring the resulting current. It reports the
detected class in the Class Status bits in the correspond-
ing Port Status register. Note that in Auto mode, the port
will power up regardless of which class is detected.
The classification circuitry is disabled when the port is in
Shutdown mode, powered up, or the corresponding Class
Enable bit is cleared.
POWER CONTROL
The primary function of the LTC4258 is to control the
delivery of power to the PSE port. It does this by control-
ling the gate drive voltage of an external power MOSFET
while monitoring the current via a sense resistor and the
output voltage at the OUT pin. This circuitry serves to
couple the raw isolated –48V input supply to the port in a
controlled manner that satisfies the PD’s power needs
while minimizing disturbances on the –48V backplane.
16
60
50
40
30
20
10
0
0
Figure 13. PD Classification
5
U
PD LOAD
TYPICAL
CLASS 3
VOLTAGE (V
LINE
10
U
CURRENT
CLASS 4
CLASS 3
OVER
CLASS 2
CLASS 1
CLASS 0
CLASS
15
)
PSE LOAD LINE
W
48mA
33mA
20
23mA
14.5mA
6.5mA
4258 F13
25
U
Gate Currents
Once the decision has been made to turn on power to a
port, the LTC4258 uses a 50 A current source to pull up
on the GATE pin. Under normal power-up circumstances,
the MOSFET gate will charge up rapidly to V
threshold voltage), the MOSFET current will rise quickly to
the current limit level and the GATE pin will be servoed to
maintain the proper I
put charging is complete, the MOSFET current will fall and
the GATE pin will be allowed to continue rising to fully
enhance the MOSFET and minimize its on resistance. The
final V
50 A current source pulls down on the GATE pin, turning
the MOSFET off in a controlled manner.
No External Capacitors
No external capacitors are required on the GATE pins for
active current limit stability, lowering part count and cost.
This also allows the fastest possible turn-off under severe
overcurrent conditions, providing maximum safety and
protection for the MOSFETs, load devices and board traces.
Connecting capacitors to the external MOSFET gates can
adversely affect the LTC4258’s ability to respond to a
shorted port.
Inrush Control
The 802.3af standard lists two separate maximum current
limits, I
ues, the LTC4258 implements both as a single current
limit using V
differentiated through the use of t
tively (see t
maintain consistency with the standard, the I
is used when referring to an initial t
When the LTC4258 turns on a port, it turns on the MOSFET
by pulling up on the gate. The LTC4258 is designed to
power up the port in current limit, limiting the inrush
current to I
The port voltage will quickly rise to the point where the PD
reaches its input turn-on threshold and begins to draw
current to charge its bypass capacitance, slowing the rate
of port voltage increase.
GS
LIM
is nominally 13V. When a port is turned off, a
INRUSH
and I
ICUT
LIM
Timing and t
INRUSH
(described below). Their functions are
.
INRUSH
. Because they have identical val-
charging current. When out-
START
ICUT
START
Timing sections). To
and t
power-up event.
T
START
(the MOSFET
INRUSH
, respec-
term
4258p