ltc4259a-1 Linear Technology Corporation, ltc4259a-1 Datasheet - Page 30

• Manufacturer Part Number: ltc4259a-1
• Description: Quad Ieee 802.3af Power Over Ethernet Controller With Ac Disconnect
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC4259A-1
Also, C
powered port. Port voltage instability is generally not a
problem if V
of these reasons be aware that many ceramic dielectrics
have dramatic DC voltage and temperature coefficients. A
0.22µF ceramic capacitor is often nowhere near 0.22µF
when operating at 50VDC or 100VDC. Use 100V or higher
rated X7R capacitors for C
duced voltage dependance while also being relatively small
and inexpensive.
Power Supplies
The LTC4259A-1 must be supplied with 3.3V (V
–48V (V
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 V
ever, if the V
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 V
or V
AC disconnection sensing. Proper bypassing and stability
of the V
Another problem that can affect the V
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
30
EE
can adversely affect detection, classification and the
PSE
DD
EE
). Poor regulation on either of these supplies
and V
may be important to the voltage stability of a
EE
, the –48V supply, is well bypassed. For both
EE
EE
supply is underdamped or otherwise
U
supplies is important.
U
DET
and C
W
PSE
EE
as these have re-
supply is insuffi-
U
EE
DD
. How-
) and
DD
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 V
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 V
few percent more than the maximum total power the PSE
will supply to PDs. Finally, the LTC4259A-1s draw current
from V
power divided by the switcher efficiency must also be
added to the V
Fast V
V
V
V
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 I
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.
EE
EE
EE
EE
supply to slew much slower than 50mV/µs.
slew rate to 50mV/µs. In most applications, existing
bypass capacitors (described above) will cause the
EE
supply capable of less than 15.4W • (number of
EE
transients can damage the LTC4259A-1. Limit the
. If the V
EE
EE
supply’s capability.
DD
supply capability should be at least a
supply is generated from V
ACDMIN
, tripping the
EE
4259a1fa
, that