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參數(shù)資料
| 型號： | LTC4259AIGW-1#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 電源管理 |
| 英文描述： | Quad IEEE 802.3af Power over Ethernet Controller with AC Disconnect; Package: SSOP; No of Pins: 36; Temperature Range: -40°C to +85°C |
| 中文描述： | 4-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO36 |
| 封裝： | 0.300 INCH, LEAD FREE, PLASTIC, SSOP-36 |
| 文件頁數(shù)： | 15/32頁 |
| 文件大小： | 416K |
| 代理商： | LTC4259AIGW-1#TRPBF |
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LTC4259A-1
22
4259a1fa
the port impedance. The 1k resistor, RDET, limits current
flowing through this path during port power on and power
off.
Sizing of capacitors is critical to ensure proper function of
AC disconnect. CPSE (Figure 18) controls the connection
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 CPSE of 0.1F. On the
other hand, CDET has to be large enough to pass the signal
at the frequency of OSCIN. For fOSCIN≈ 100Hz, use at least
a 0.47
F 100V X7R capacitor. The sizes of CPSE, CDET,
RDET and the frequency, fOSCIN, are chosen to create 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 CDET, RDET and CPSE is discouraged. Contact the
LTC Applications department for additional support.
When choosing CDET and CPSE, carefully consider voltage
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. 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.
APPLICATIO S I FOR ATIO
WU
UU
Figure 18. AC Disconnect Single Port Application Circuit (Port 4 Shown)
Figure 19. Simplified AC Disconnect Circuit with Impedances at 100Hz
LEVEL
SHIFT
DETECT4
AGND
DGND
OSCIN
1/4
LTC4259A-1
–48V
GND
OUT4
ROUT4
10k
RS4
0.5
1%
DDET4
CMPD3003
PD
RDET4
1k
RPD_D
≤26.25k
CPD_D
≥0.05F
CDET4
0.47
F
100V X7R
CPSE4
0.1
F
100V X7R
4259A F18
DAC4
S1B
Q4
GATE4
SENSE4
VEE
OSCILLATOR
INPUT
CURRENT
SENSE
RDET
1k
ZLINK < 14k
ZCABLE < 32k
PD
~16k
~7k
ZPD < 14k
CDET
0.47
F
CPSE
0.10
F
CCABLE ≤ 0.05F
<32k
CPD_D ≥ 0.05FRPD_D < 26.25k
4259A F19
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