LTC4259ACGW Linear Technology, LTC4259ACGW Datasheet - Page 21

• Manufacturer Part Number: LTC4259ACGW
• Description: IC CTRLR POE QUAD AC DISC 36SSOP
• Manufacturer: Linear Technology

Specifications of LTC4259ACGW

• Controller Type: Power over Ethernet Controller (POE)
• Interface: I²C
• Voltage - Supply: 3 V ~ 4 V
• Current - Supply: 2.5mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Linear Misc Type: Negative Voltage
• Family Name: LTC4259A
• Operating Supply Voltage (min): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
the minimum specified current. The disconnect timer
counts up whenever port current is below 7.5mA (typ). If
the t
turned off and the disconnect bit in the fault register will be
set. If the undercurrent condition goes away before the
t
counting from the beginning if the undercurrent condition
occurs again. The undercurrent circuit includes a glitch
filter to filter out noise.
The DC disconnect feature can be disabled by clearing the
corresponding DC Discon Enable bits in the Disconnect
register (13h). The t
by bits 1 and 0 of register 16h.
The LTC4259A implements a variety of current sense and
limit thresholds to control current flowing through the
port. Figure 17 is a graphical representation of these
thresholds and the action the LTC4259A takes when
currrent crosses the thresholds.
AC DISCONNECT
AC disconnect is an alternate method of sensing the pres-
ence or absence of a PD by monitoring the port impedance.
The LTC4259A forces a signal, amplified from the OSCIN
pin, out of the DETECT pins and onto the Power over Eth-
ernet connection. It calculates the connection impedance
from ohm’s law, Z
AC disconnect sensing circuitry controls the disconnect
timer. When the connection impedance rises (AC current
falls below I
connect timer counts up. If the impedance remains high
(AC current remains below I
VOLTAGE
DIS
SENSE n
300mV
250mV
200mV
150mV
100mV
50mV
0mV
timer runs out, the timer will reset. The timer will start
DIS
Figure 17. LTC4259A Current Sense and Limits
timer runs out, the corresponding port will be
600mA
500mA
400mA
300mA
200mA
100mA
0mA
CURRENT
R
S
= 0.5Ω
ACDMIN
CONNECT
DC DIS-
PORT
) due to the removal of the PD, the dis-
DIS
U
timer duration can be programmed
= V
(I
CUT
CUT
U
AC
)
ACDMIN
/I
AC
LIMIT
(I
LIM
. Like DC disconnect, the
)
), the disconnect timer
W
CIRCUIT
SHORT
PORT OFF IN t
OR t
NORMAL
OPERATION
CURRENT LIMIT
IN 1µs
PORT OFF IN t
START
U
EFFECT
CURRENT
LIMIT
ICUT
DIS
4259A F17
counts to t
nect bit in the Fault Register is set. If the impedance falls
(AC current rises above I
count of the disconnect timer, the timer resets and the port
remains powered.
Like DC disconnect, AC disconnect can also be disabled by
clearing the corresponding AC Discon Enable bits in the
Disconnect register (13h). AC disconnect is also affected
by the t
Unlike DC disconnect, AC disconnect has no continuous
time output to the timer. Rather, AC disconnect will reset
the timer once every cycle, 1/f
the port draws more than I
Because of this behavior, the time to turn off the port after
PD removal, t
(1/f
t
signals that reset the t
a port where both disconnect modes are enabled, either
disconnect sensing method can keep the port
powered even if the other reports that there is no PD
connected.
The AC disconnect circuitry senses the port and Power
over Ethernet connection from the DETECT pins. Connect
a 0.47µF 100V X7R capacitor (C
(R
shown in Figure 18. This provides an AC path for sensing
the port impedance. The 1k resistor, R
flowing through this path during port power on and power
off.
Sizing of capacitors is critical to ensure proper function of
AC disconnect. C
impedance on the PSE side. Its capacitance must be kept
low enough for AC disconnect to be able to sense the PD.
For operation near 100Hz, use a C
other hand, C
at the frequency of OSCIN. For f
a 0.47µF 100V X7R capacitor. The sizes of C
R
economical, physically compact and functionally
robust system. Moreover, the complete Power over Ether-
net AC disconnect system (PSE, transformers, cabling,
PD, etc.) is complex; deviating from the recommended
DIS0
DET
DET
OSCIN
bits. Note that AC disconnect and DC disconnect
and the frequency, f
) from the port’s DETECT pin to the port’s output as
DIS
) from the delay programmed with the t
DIS
duration programmed in register 16h.
DET
, the port is turned off and the port’s discon-
DIS
, may vary by up to one cycle of OSCIN
has to be large enough to pass the signal
PSE
(Figure 18) controls the connection
DIS
timer are ORed together. Thus on
OSCIN
ACDMIN
ACDMIN
OSCIN
OSCIN
, are chosen to create an
) before the maximum
DET
, of the OSCIN signal if
PSE
LTC4259A
≈ 100Hz, use at least
during that period.
) and a 1k resistor
DET
of 0.1µF. On the
, limits current
PSE
DIS1
21
, C
4259afb
DET
and
,