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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)： | 24/32頁 |
| 文件大?。?/td> | 416K |
| 代理商： | LTC4259AIGW-1#TRPBF |
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LTC4259A-1
30
4259a1fa
Also, CPSE may be important to the voltage stability of a
powered port. Port voltage instability is generally not a
problem if VEE, the –48V supply, is well bypassed. For both
of these reasons be aware that many ceramic dielectrics
have dramatic DC voltage and temperature coefficients. A
0.22
F ceramic capacitor is often nowhere near 0.22F
when operating at 50VDC or 100VDC. Use 100V or higher
rated X7R capacitors for CDET and CPSE as these have re-
duced voltage dependance while also being relatively small
and inexpensive.
Power Supplies
The LTC4259A-1 must be supplied with 3.3V (VDD) and
–48V (VEE). Poor regulation on either of these supplies
can lead to noncompliance. The IEEE requires a PSE
output voltage between 44V and 57V. When the LTC4259A-
1 begins powering an Ethernet port, it controls the current
through the port to minimize disturbances on VEE. How-
ever, if the VEE supply is underdamped or otherwise
unstable, its voltage could go outside of the IEEE specified
limits, causing all ports in the PSE to be noncompliant.
This scenario can be even worse when a PD is unplugged
because the current can drop immediately to zero. In both
cases the port voltage must always stay between –44V
and –57V. In addition, the 802.3af specification places
specific ripple, noise and load regulation requirements on
the PSE. Among other things, disturbances on either VDD
or VEE can adversely affect detection, classification and the
AC disconnection sensing. Proper bypassing and stability
of the VDD and VEE supplies is important.
Another problem that can affect the VEE supply is insuffi-
cient power, leading to the supply voltage drooping out of
the specified range. The 802.3af specification states that
if a PSE powers a PD it must be able to provide the
maximum power level requested by the PD based on the
APPLICATIO S I FOR ATIO
WU
UU
PD’s classification. The specification does allow a PSE to
choose not to power a port because the PD requires more
power than the PSE has left to deliver. If a PSE is built with
a VEE supply capable of less than 15.4W (number of
PSE’s Ethernet ports), it must implement a power alloca-
tion algorithm that prevents ports from being powered
when there is insufficient power. Because the specifica-
tion also requires the PSE to supply 400mA at up to a 5%
duty cycle, the VEE supply capability should be at least a
few percent more than the maximum total power the PSE
will supply to PDs. Finally, the LTC4259A-1s draw current
from VEE. If the VDD supply is generated from VEE, that
power divided by the switcher efficiency must also be
added to the VEE supply’s capability.
Fast VEE transients can damage the LTC4259A-1. Limit the
VEE slew rate to 50mV/s. In most applications, existing
VEE bypass capacitors (described above) will cause the
VEE supply to slew much slower than 50mV/s.
OSCIN Input
AC disconnect also relies on an oscillating signal applied
to the OSCIN pin. Requirements for this signal are pro-
vided in the OSCIN Input and Oscillator Requirements
section. Out-of-band noise on the OSCIN pin will disrupt
the LTC4259A-1’s ability to sense the absence of a PD. Any
noise present at the OSCIN pin is amplified by the
LTC4259A-1 and driven out of the DETECT pins (of pow-
ered ports with AC disconnect enabled). Due to the amount
of capacitance connected to the DETECT pins, driving this
noise can easily require more than IACDMIN, tripping the
DETECT pin current sense and keeping the port powered.
During circuit board layout, keep wiring from the oscillator
to the OSCIN pin away from noise sources like digital clock
and data lines. A single-stage RC lowpass filter (shown in
Figure 22) will attenuate out-of-band noise.
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