ltc4259a-1 Linear Technology Corporation, ltc4259a-1 Datasheet - Page 22

• Manufacturer Part Number: ltc4259a-1
• Description: Quad Ieee 802.3af Power Over Ethernet Controller With Ac Disconnect
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC4259A-1
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
values of C
LTC Applications department for additional support.
When choosing C
derating of the capacitors. Capacitors built around an X7R
dielectric will have about 60% of the specified capacitance
22
DET
and the frequency, f
DET
OSCILLATOR
DET
CURRENT
, R
SENSE
INPUT
DET
has to be large enough to pass the signal
PSE
DET
–48V
GND
U
and C
(Figure 18) controls the connection
and C
LTC4259A-1
U
OSCIN
PSE
PSE
1/4
Figure 18. AC Disconnect Single Port Application Circuit (Port 4 Shown)
Figure 19. Simplified AC Disconnect Circuit with Impedances at 100Hz
~16k
V
~7k
EE
is discouraged. Contact the
, carefully consider voltage
OSCIN
0.5Ω 1%
, are chosen to create an
OSCIN
R
LEVEL
SHIFT
S4
SENSE4
W
PSE
R
1k
≈ 100Hz, use at least
DET
C
0.47µF
C
0.10µF
DET
PSE
DET
of 0.1µF. On the
GATE4
DGND
Q4
, limits current
Z
Z
CABLE
DETECT4
LINK
AGND
OUT4
U
PSE
< 14k
< 32k
R
10k
D
S1B
OUT4
, C
AC4
DET
D
CMPD3003
DET4
,
C
CABLE
at their rated voltage. Operated at half their rated voltage,
X7R capacitors exhibit more than 80% of their specified
capacitance. With other ceramic dielectrics commonly
used in 50V and 100V chip capacitors, capacitance falls
much more dramatically with voltage. At their rated volt-
age, Y5V or Z5U capacitors exhibit less than 30% of their
zero-bias capacitance. Ceramic capacitors can also have
significantly less capacitance at elevated temperatures. In
order to produce the desired capacitance at the operating
bias, 100V or 250V X7R capacitors should be used with
the LTC4259A-1.
As illustrated in Figure 19, the Power over Ethernet con-
nection between the PSE and PD includes a large amount
of capacitance. Cable capacitance is particularly troubling
because CAT-3 and CAT-5 pair-to-pair capacitance is not
tightly specified by the IEEE 802.3 standard or well con-
trolled by cable manufacturers. Considering that patch
panels, additional connectors, old wiring, etc. are likely to
be placed between the PSE and PD, pair-to-pair capaci-
tance is a pretty nebulous quantity. Consequently, the
cable’s contribution to the port impedance (at the fre-
quency used for AC disconnect) can be a concern.
≤ 0.05µF
100V X7R
R
1k
DET4
C
0.47µF
100V X7R
C
DET4
0.1µF
PSE4
<32k
Z
PD
< 14k
R
≤26.25k
C
PD_D
PD_D
≥ 0.05µF
PD
PD
C
≥0.05µF
PD_D
4259A F18
R
4259A F19
PD_D
< 26.25k
4259a1fa