DN350 - Fully Autonomous IEEE 802.3af Power over Ethernet Midspan PSE Requires No Microcontroller

Fully Autonomous IEEE 802.3af Power over Ethernet Midspan
PSE Requires No Microcontroller – Design Note 350
Dilian Reyes
Introduction
The IEEE802.3af Power over Ethernet (PoE) standard
defines how power will be delivered over CAT5 lines.
Despite the differences between legacy devices and
those that adhere to the new standard, there is no need
to completely replace existing systems. The nominal
48V required by powered devices (PDs) can be delivered
by midspan power sourcing equipment (PSE), which
is connected to the front end in series with legacy
routers and switches. The LTC ®4259A is a quad PSE
controller designed for both endpoint and midspan
PSEs that integrates PD signature detection, power
level classification, AC and DC disconnect detection
and current limit without the need for a microcontroller.
VDD
A PSE’s Duties
The responsibilities of the PSE are to correctly detect if
a compliant PD has been connected to a port, optionally classify the PD and properly apply power to the PD
while protecting the port from fault conditions. Once a
PD is powered on, the PSE monitors a PD’s presence
and switches off power when the device is removed. A
PSE must also provide overcurrent protection to prevent
damage to the PSE and PD.
Traditional PSE solutions use a microcontroller to
perform the detection measurements and calculations
and control additional circuitry that switches power to
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RESET
10k
160Ω
OSCIN
3
4
6
5
10
9
8
7
17
18
19
20
INT SHDN1
SCL
SDAIN
SDAOUT
AD0
AD1
AD2
AD3
SHDN2
SHDN3
SHDN4
35
2
VDD AUTO
16
BYP
+
10k
10μF
100V
1
RESET
LTC4259A
DGND AGND VEE SENSE1 GATE1 OUT1 SENSE2 GATE2 OUT2
15
21 28
32
33
34
29
30
31
VIN
–48V TO
–57V
0.1μF
0.1μF
50V
0.1μF
SENSE3 GATE3 OUT3
25
26
27
10k
36
OSCIN
DETECT1
DETECT2
DETECT3
DETECT4
11
12
13
14
SENSE4 GATE4 OUT4
22
23
24
10k
10k
0.50Ω
VEE
1μF
100V
IRFM120A
tBACKOFF
2N7002
1k
CMPD3003
0.47μF
100V
X7R
2N7002
1k
CMPD3003
0.47μF
100V
X7R
2N7002
1k
CMPD3003
0.47μF
100V
X7R
2N7002
1k
CMPD3003
0.47μF
100V
X7R
8
7
SMAJ58A
S1B
0.1μF
100V
X7R
0.50Ω
IRFM120A
SMAJ58A
S1B
0.1μF
100V
X7R
0.50Ω
IRFM120A
SMAJ58A
S1B
0.1μF
100V
X7R
0.50Ω
IRFM120A
8
7
RJ45
5
4
8
7
RJ45
5
4
8
7
RJ45
SMAJ58A
S1B
18M
VDD
1.1M
18M
VDD
1.1M
18M
VDD
1.1M
18M
VDD
0.1μF
100V
X7R
1.1M
Si2343DS
Si2343DS
Si2343DS
Si2343DS
300Ω
300Ω
300Ω
300Ω
DN350 F01
Figure 1. Autonomous 4-Port Power over Ethernet Midspan PSE
11/04/350_conv
RJ45
5
4
5
4
a PD. The LTC4259A in Figure 1, by contrast, requires
no microcontroller and runs autonomously carrying
out signature detection. It automatically interprets the
loading conditions and powers on a valid PD.
The midspan PSE also must not interfere with an endpoint’s operation. An endpoint PSE applies power on
either the signal pairs or the spare pairs of the CAT5
cable, while the midspan PSE must apply power to
only the spare pairs. To avoid conflict if the two were
to be connected at the same time, the circuit in Figure 2
implements an LTC1726 watchdog timer to periodically
disable the LTC4259A’s detection scheme for two seconds. Midspan devices are required to have a back off
capability after a failed attempt of detection to allow
for a potentially present endpoint PSE to detect and
power on a port.
After the back off interval is complete, LTC4259A detection is re-enabled for at least one full detection cycle.
If a midspan or endpoint PSE is able to detect a valid
signature 25kΩ (RSIG) and power up the PD, a compliant PD would no longer display the RSIG to prevent any
further good signature detects and power ups from a
second PSE. Hardware implementation of the back off
timer eliminates the need for a microcontroller software
timing routine.
VDD
The LTC4259A auto mode uses the AC disconnect
method by default. The LT®1498 in Figure 3 is a dual
rail-to-rail op amp used to output a sine wave to drive
OSCIN of the LTC4259A. The LTC4259A applies the
AC signal to the lines and detects its presence when
a PD has been removed and the port power is to be
switched off.
VDD
590k
1%
750k
2200pF
NPO
2
3
10k
0.1μF
6
0.1μF
–
1/2
LTC1498CS8
10k
1
2
1
3
4.99k
VC1 VC2 VCA
22k
6
RT
RST
LTC1726-2.5
7
5 150k
WT
WDI
GND
C1, C2
330nF
C3, C4
12nF
4
1k
Supplying 3.3V from –48V
A 3.3V supply powers the digital portion of the
LTC4259A. The LTC3803 circuit in Figure 4 converts
–48V to 3.3V eliminating the need for a second power
supply. This boost regulator circuit achieves a tight
2% regulation and outputs 400mA, enough for up to
twelve LTC4259As and port indicator LEDs in a 48-port
application.
+
0.22μF
100V
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0.22μF
100V
10μF
63V
10k
1%
10μF
16V
5
FDC2512
VCC
6
ITH/RUN NGATE
LTC3803ES6
3
4 1k
SENSE
VFB
GND
0.1Ω
1W
2
1%
B1100
10μH
VDD
3.3V
400mA
10μF
6.3V
3.32k
1%
100μF
6.3V
1
5Ω
DN350 F02
DC disconnect measures a minimum current drawn
from a port to determine if a PD is present and requires
power. While this is easier to implement, AC disconnect is considered a more accurate detection of a PD’s
presence. AC disconnect measures the PD impedance
and would keep a port powered even for PDs that idle
at low power.
DN350 F03
Figure 3. Sinewave Circuit for AC Disconnect
VEE
Disconnect Detection
When a PD is unplugged from a powered port, the IEEE
standards specify that a PSE must implement at least
one of two power disconnect detections modes for port
power removal: DC disconnect and/or AC disconnect.
+
OSCIN
300k
Figure 2. Midspan PSE Backoff Timer
7
2.4k
VSIN = 2.1VP-P
1.2VOS
105Hz
MMBT2907
VEE
5
2200pF
+
100μH
tBACKOFF
–
1/2
LTC1498CS8
681k
1%
0.1μF
8
2200pF
NPO
56k
2.2nF
FMMT723
FMMT723
806Ω
1%
47.5k
1%
DN350 F04
Figure 4. –48V to 3.3V Boost Converter
LTC4259A Options
The LTC4259A also allows flexibility when designing
an endpoint or midspan PSE. Internal registers can be
accessed via I2C for additional control and settings,
including the option of DC disconnect. The LTC4259A
aids in IEEE-compliant power management by providing
PD classification—a better method than guessing via
monitoring current—of devices that present a class,
such as an LTC4257 PD Interface Controller.
For applications help,
call (408) 432-1900
dn350f_conv LT/TP 1104 409K • PRINTED IN THE USA
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