LTC4259ACGW-1#PBF Linear Technology, LTC4259ACGW-1#PBF Datasheet - Page 21

• Manufacturer Part Number: LTC4259ACGW-1#PBF
• Description: IC CTRLR POE QUAD AC DISC 36SSOP
• Manufacturer: Linear Technology

Specifications of LTC4259ACGW-1#PBF

• 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
• Package Type: SSOP
• 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: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
DC DISCONNECT
DC disconnect monitors the sense resistor voltage when-
ever the power is on to make sure that the PD is drawing
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-1 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-1 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-1 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
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-1 Current Sense and Limits
timer runs out, the corresponding port will be
600mA
500mA
400mA
300mA
200mA
100mA
0mA
CURRENT
R
S
= 0.5Ω
CONNECT
DC DIS-
PORT
DIS
U
timer duration can be programmed
= V
(I
CUT
CUT
U
AC
)
/I
AC
LIMIT
(I
LIM
. Like DC disconnect, the
)
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
falls below I
connect timer counts up. If the impedance remains high
(AC current remains below I
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 LTC4259A-1 provides a higher AC disconnect thresh-
old, I
LTC4259A. This reduces the port impedance that the
LTC4259A-1’s AC disconnect circuitry senses as the ab-
sence of a PD. Consequently, in the same application
circuit the LTC4259A-1 will have less sensitivity to parasitics
like leakage and stray capacitance than an LTC4259A.
Both the LTC4259A and LTC4259A-1 (used with the
recommended application circuit) meet all the AC discon-
nect requirements of 802.3af standard. Because the
LTC4259A-1 offers improved performance, Linear Tech-
nology recommends the LTC4259A-1 for new designs and
as a replacement for the LTC4259A in existing designs.
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
DIS0
DET
OSCIN
bits. Note that AC disconnect and DC disconnect
) from the port’s DETECT pin to the port’s output as
ACDMIN
DIS
) from the delay programmed with the t
DIS
duration programmed in register 16h.
ACDMIN
, the port is turned off and the port’s discon-
DIS
(see Electrical Characteristics) than the
, may vary by up to one cycle of OSCIN
) due to the removal of the PD, the dis-
DIS
timer are ORed together. Thus on
ACDMIN
ACDMIN
ACDMIN
OSCIN
LTC4259A-1
) before the maximum
DET
), the disconnect timer
, of the OSCIN signal if
during that period.
) and a 1k resistor
DIS1
21
4259a1fa
and