LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 21

• 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
When choosing C
derating of the capacitors. Capacitors built around an X7R
dielectric will have about 60% of the specified capacitance
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. In order to produce the desired
capacitance at the operating bias, 100V X7R capacitors
should be used with the LTC4259.
As illustrated in Figure 17, 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
OSCILLATOR
CURRENT
SENSE
DET
INPUT
U
and C
–48V
GND
U
PSE
~11k
Figure 17. Simplified AC Disconnect Circuit with Impedances at 100Hz
LTC4259
, carefully consider voltage
1/4
Figure 16. AC Disconnect Single Channel Application Circuit
V
EE
W
R
1k
0.5 1%
DET
OSCIN
C
0.22 F
C
DET
PSE
R
LEVEL
SHIFT
S4
SENSE4
0.10 F
<14k
GATE4
DGND
U
Q4
<30k
DETECT4
AGND
OUT4
R
10k
D
S1B
OUT4
AC4
C
CABLE
D
1N4148
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
impedance of the cable capacitance at the frequency used
for AC disconnect can be a cause for concern. Assumimg
that f
capacitance gives a port impedance of 10k at 100Hz. The
PD AC signature resistance is about 25k. Connecting a PD
with the maximum allowed resistance of 26.25k brings the
connection impedance to about 8k. The presence of a PD
only makes a 20% reduction in the port impedance. With
some methods used for AC disconnect, cable capacitance
can fool the PSE into keeping the port powered. Fortu-
nately, the LTC4259 has a solution to this problem. When
DET4
0.05 F
OSCIN
100V X7R
R
1k
DET4
C
0.22 F
100V X7R
C
0.1 F
DET4
PSE4
<30k
is 100Hz, the 0.1 F of C
Rpd_d
C
26.25k
pd_d
PD
0.05 F
PD
Cpd_d
0.05k
4259 F16
R
4259 F17
PSE
pd_d
< 26.25k
plus 0.05 F of cable
LTC4259
21
4259i