LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 20

• Manufacturer Part Number: LTC4259CGW
• Manufacturer: Linear Technology

Specifications of LTC4259CGW

• Linear Misc Type: Negative Voltage
• Operating Supply Voltage (typ): -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: Not Compliant

APPLICATIO S I FOR ATIO
LTC4259
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 LTC4259 implements a variety of current sense and
limit thresholds to control current flowing through the
port. Figure 15 is a graphical representation of these
thresholds and the action the LTC4259 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 LTC4259 forces a signal, amplified from the OSCIN pin,
out of the DETECT pins and onto the Power over Ethernet
connection. It calculates the connection impedance from
ohm’s law, Z
connect sensing circuitry controls the disconnect timer.
When the connection impedance rises (AC current falls
below I
nect 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 connection imped-
ance falls (AC current rises above I
count of the disconnect timer, the timer resets and the port
remains powered.
20
VOLTAGE
SENSE n
300mV
250mV
200mV
150mV
100mV
50mV
0mV
ACDMIN
600mA
500mA
400mA
300mA
200mA
100mA
0mA
Figure 15. LTC4259 Current Sense and Limits
CURRENT
R
S
DIS
= 0.5
PORT
, the port is turned off and the port’s discon-
) due to the removal of the PD, the discon-
CONNECT
DC DIS-
= V
DIS
U
AC
/I
timer duration can be programmed
AC
(I
CUT
CUT
ACDMIN
U
. Like DC disconnect, the AC dis-
)
LIMIT
(I
LIM
), the disconnect timer
)
ACDMIN
W
CIRCUIT
SHORT
) before maximal
PORT OFF IN 1 s
PORT OFF IN t
OR t
NORMAL
OPERATION
PORT OFF IN t
START
U
EFFECT
CURRENT
LIMIT
ICUT
DIS
4259 F15
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
disconnect signals that reset the t
together. Thus on a channel 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 0.22 F 100V X7R capacitor (C
(R
shown in Figure 16. This provides an AC path for sensing
the channel impedance. The 1k resistor, R
current 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
the other hand, C
signal at the frequency of OSCIN. For f
at least a 0.22 F 100V X7R capacitor. The sizes of C
C
an 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
values of C
LTC Applications department for additional support.
DIS1
DET
DET
OSCIN
, R
) from the port’s DETECT pin to the port’s output as
and t
DET
DIS
) shorter than the delay programmed with the
DET
and the frequency, f
duration programmed in register 16h.
DIS0
, R
DIS
bits. Note that AC disconnect and DC
DET
PSE
DET
, may be up to one cycle of OSCIN
and C
(Figure 16) controls the connection
has to be large enough to pass the
PSE
ACDMIN
OSCIN
is discouraged. Contact the
OSCIN
PSE
DET
, of the OSCIN signal if
, are chosen to create
DIS
of 0.1 F or less. On
during that period.
) and a 1k resistor
OSCIN
timer are ORed
100Hz, use
DET
, limits
PSE
4259i
,