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參數(shù)資料
| 型號： | LTC4259AIGW-1#PBF |
| 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)： | 9/32頁 |
| 文件大?。?/td> | 416K |
| 代理商： | LTC4259AIGW-1#PBF |
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LTC4259A-1
17
4259a1fa
listed in Table 2. During classification, the LTC4259A-1
controls and measures the port voltage through the
DETECT
n pin. Note that class 4 is presently specified by the
IEEE as reserved for future use. Figure 14 shows a PD load
line, starting with the shallow slope of the 25k signature re-
sistor below 10V, then drawing the classification current
(in this case, class 3) between 14.5V and 20.5V. The
LTC4259A-1’s load line for classification is also shown in
Figure 14. It has low impedance until current limit is reached
at 55mA (min).
The LTC4259A-1 will classify a port immediately after a
successful detection cycle in Semiauto or Auto modes, or
when commanded to in Manual mode. It measures the PD
classification signature current by applying 18V (typ) to
the port and measuring the resulting current. It reports the
detected class in the Class Status bits in the correspond-
ing Port Status register. Note that in Auto mode, the port
will power up regardless of which class is detected.
The classification circuitry is disabled when the port is in
Shutdown mode, powered up, or the corresponding Class
controlled manner that satisfies the PD’s power needs
while minimizing disturbances on the –48V backplane.
Gate Currents
Once the decision has been made to turn on power to a
port, the LTC4259A-1 uses a 50
A current source to pull
up on the GATE pin. Under normal power-up circum-
stances, the MOSFET gate will charge up rapidly to VT (the
MOSFET threshold voltage), the MOSFET current will rise
quickly to the current limit level and the GATE pin will be
servoed to maintain the proper IINRUSH charging current.
When output charging is complete, the MOSFET current
will fall and the GATE pin will be allowed to continue rising
to fully enhance the MOSFET and minimize its on resis-
tance. The final VGS is nominally 13V. When a port is
turned off, a 50
A current source pulls down on the GATE
pin, turning the MOSFET off in a controlled manner.
No External Capacitors
No external capacitors are required on the GATE pins for
active current limit stability, lowering part count and cost.
This also allows the fastest possible turn-off under severe
overcurrent conditions, providing maximum safety and
protection for the MOSFETs, load devices and board traces.
Connecting capacitors to the external MOSFET gates can
adversely affect the LTC4259A-1’s ability to respond to a
shorted port.
Inrush Control
The 802.3af standard lists two separate maximum current
limits, ILIM and IINRUSH. Because they have identical val-
ues, the LTC4259A-1 implements both as a single current
limit using VLIM (described below). Their functions are
differentiated through the use of tICUT and tSTART, respec-
tively (see tICUT Timing and tSTART Timing sections). To
maintain consistency with the standard, the IINRUSH term
is used when referring to an initial tSTART power-up event.
When the LTC4259A-1 turns on a port, it turns on the
MOSFET by pulling up on the gate. The LTC4259A-1 is
designed to power up the port in current limit, limiting the
inrush current to IINRUSH.
The port voltage will quickly rise to the point where the PD
reaches its input turn-on threshold and begins to draw
APPLICATIO S I FOR ATIO
WU
UU
Figure 14. PD Classification
VOLTAGE (VCLASS)
0
CURRENT
(mA)
60
50
40
30
20
10
0
510
15
20
4259A F14
25
TYPICAL
CLASS 3
PD LOAD
LINE
48mA
33mA
PSE LOAD LINE
23mA
14.5mA
6.5mA
CLASS 4
CLASS 2
CLASS 1
CLASS 0
CLASS 3
OVER
CURRENT
Enable bit is cleared.
POWER CONTROL
The primary function of the LTC4259A-1 is to control the
delivery of power to the PSE port. It does this by control-
ling the gate drive voltage of an external power MOSFET
while monitoring the current via a sense resistor and the
output voltage at the OUT pin. This circuitry serves to
couple the raw isolated –48V input supply to the port in a
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| LTC4259CGW-ES | 制造商:LINEAR_TECH 功能描述: |
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