參數(shù)資料
型號(hào)： LTC4274AIUHF-2#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 電源管理
英文描述： POWER SUPPLY MANAGEMENT CKT, PQCC38
封裝： 7 X 5 MM, LEAD FREE, PLASTIC, MO-220WHKD, QFN-38
文件頁(yè)數(shù)： 10/28頁(yè)
文件大?。?/td> 313K
代理商： LTC4274AIUHF-2#TRPBF
LTC4274A/LTC4274C
18
4274acf
ApplicAtions inForMAtion
between 19k and 26.5k, and it must reject resistances
above 33k or below 15k (shaded regions in Figure 11).
The PSE may choose to accept or reject resistances in the
undefinedareasbetweenthemust-acceptandmust-reject
ranges.Inparticular,thePSEmustrejectstandardcomputer
network ports, many of which have 150Ω common-mode
termination resistors that will be damaged if power is ap-
plied to them (the black region at the left of Figure 11).
corresponding Port Status register. Values outside this
range,includingopenandshort-circuits,arealsoreported.
If the port measures less than 1V at the first forced-current
test, the detection cycle will abort and Short Circuit will
be reported. Table 4 shows the possible detection results.
Table 4. Detection Status
MEASURED PD SIGNATURE
DETECTION RESULT
Incomplete or Not Yet Tested
Detect Status Unknown
<2.4k
Short Circuit
Capacitance > 2.7F
CPD Too High
2.4k < RPD < 17k
RSIG Too Low
17k < RPD < 29k
Detect Good
>29k
RSIG Too High
>50k
Open Circuit
Voltage > 10V
Port Voltage Outside Detect Range
More On Operating Modes
Theport’soperatingmodedetermineswhentheLTC4274A/
LTC4274C runs a detection cycle. In manual mode, the
port will idle until the host orders a detect cycle. It will
then run detection, report the results, and return to idle
to wait for another command.
In semi-auto mode, the LTC4274A/LTC4274C autono-
mously polls a port for PDs, but it will not apply power
until commanded to do so by the host. The Port Status
register is updated at the end of each detection cycle. If a
valid signature resistance is detected and classification is
enabled, the port will classify the PD and report that result
as well. The port will then wait for at least 100ms (or 2
seconds if midspan mode is enabled), and will repeat the
detection cycle to ensure that the data in the Port Status
register is up-to-date.
If the port is in semi-auto mode and high power opera-
tion is enabled, the port will not turn on in response to
a power-on command unless the current detect result is
Detect Good. Any other detect result will generate a tSTART
fault if a power-on command is received. If the port is not
in high power mode, it will ignore the detection result and
apply power when commanded, maintaining backwards
compatibility with the LTC4259A.
Figure 12. PD Detection
Figure 11. IEEE 802.3af Signature Resistance Ranges
4-Point Detection
TheLTC4274A/LTC4274Cusesa4-pointdetectionmethod
todiscoverPDs.False-positivedetectionsareminimizedby
checkingforsignatureresistancewithbothforced-current
and forced-voltage measurements. Initially, two test cur-
rents are forced onto the port (via the OUT pin) and the
resulting voltages are measured. The detection circuitry
subtractsthetwoV-Ipointstodeterminetheresistiveslope
while removing offset caused by series diodes or leakage
at the port (see Figure 12). If the forced-current detection
yields a valid signature resistance, two test voltages are
then forced onto the port and the resulting currents are
measured and subtracted. Both methods must report
valid resistances for the port to report a valid detection.
PD signature resistances between 17k and 29k (typically)
are detected as valid and reported as Detect Good in the
RESISTANCE
PD
PSE
0
10k
15k
4274AC F11
19k
26.5k
26.25k
23.75k
150 (NIC)
20k
30k
33k
FIRST
DETECTION
POINT
SECOND
DETECTION
POINT
VALID PD
25k SLOPE
275
165
CURRENT
(A)
0V-2V
OFFSET
VOLTAGE
4274AC F12
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LTC4274C 制造商:LINER 制造商全稱:Linear Technology 功能描述:12-Port PoE/PoE+/LTPoE++ PSE Controller
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