Dec 2002 New Power for Ethernet Powered Devices (Part 2 of a 3-Part Series)

DESIGN FEATURES
New Power for Ethernet—Powered
Devices (Part 2 of a 3-Part Series) by Dave Dwelley
Introduction
An IEEE 802.3af Powered Ethernet
connection provides both the familiar
10/100/1000MB/s data link and
13W worth of 48V DC power to a
connected device. Such a device,
known as a PD (for Powered Device),
can be a digital Voice-Over-IP phone,
a network wireless access point, a
PDA charging station, an HVAC
thermostat, or almost any small
Ethernet-connected data device that
would otherwise be powered by a wall
transformer. A PD need not use the
data link at all; something as simple
as a cell phone battery charger or an
illuminated exit sign could draw its
power from an Ethernet connection.
This article is the second in a threepart series on Powered Ethernet. This
issue covers the operation of the PD
in detail. Part 1 appeared in the last
issue of the Linear Technology magazine and covered the power details of
the system, with a focus on the PSE
(Power Sourcing Equipment) and its
characteristics. Part 3 will discuss
the nuances of detection and classification—the mechanism that the
802.3af standard uses to ensure that
PDs receive power while legacy dataonly devices remain unpowered.
Characteristics of a PD
Power arrives at the PD on a standard
CAT-5 network cable via an RJ-45
connector. A CAT-5 wire contains four
twisted pairs of 24-gauge wire (8 conductors in total). Two of the pairs—the
“signal” pairs, at pin pairs (1, 2) and
(3, 6) on the RJ-45 connector, shown
in Figure 1—are used for the standard 10/100 Ethernet transmit and
receive links. The two other pairs—
the “spare” pairs, at pin pairs (4, 5)
and (7, 8) are unused in 10/100 networks. 1000BASE-T networks use all
four pairs. 48V appears on the cable
as a difference in the common-mode
voltages between the two signal pairs
or the two spare pairs (but never
Linear Technology Magazine • December 2002
Powered Ethernet Promises to Remove Warts
An excerpt from Part 1 of this series:
For years, data has passed over Ethernet CAT-5 networks, primarily to and
from servers and workstations. The IEEE 802.3 group, the originator of the
Ethernet standard, is currently at work on an extension to the standard,
known as 802.3af, which will allow DC power to be delivered simultaneously over the same wires. This promises a whole new class of Ethernet
devices, including IP telephones, wireless access points, and PDA charging
stations, which do not require additional AC wiring or external power
transformers (“wall warts”).
SIGNAL
PAIRS
48V
*SIGNAL PAIRS
CAN BE
EITHER
POLARITY
±* ±*
CAT-5
CABLE
RJ-45
CONNECTOR
8 7 6 5 4 3 2 1
–
+
48V
SPARE
PAIRS
Figure 1. Signal and spare
pairs on RJ-45 connector
both). The signal pairs are transformer-isolated as they enter the PD
to strip the DC out of the data signal
path; the power is taken from center
taps on these transformers and passed
to the PD input circuitry, as shown in
Figure 2. The spare pairs may or may
not be transformer isolated.
To be considered an IEEE-802.3af
PD, a device must meet several criteria. A PD must be able to accept power
over either the signal pairs or the
spare pairs, since a PSE is allowed to
power either set. This is typically accomplished by diode ORing the two
power inputs, as shown in Figure 3a.
This circuit has the additional advantage of removing the signature from
the unused set of pairs when the
power is applied to the other set, a
requirement of the IEEE spec. PDs
are not allowed to draw power from
both sets of pairs simultaneously.
Diode bridges can be used to implement auto-polarity; this is useful since
many CAT-5 cables are wired as crossover cables, so voltage polarity is likely
to arrive reversed instead of forward.
An alternate connection, shown in
Figure 3b, uses single diodes and a
third reverse biased diode to present
an invalid signature when the polarity is reversed. This circuit will work
with 802.3af systems, although it will
not be powered if a crossover cable is
used.
All PDs must present a characteristic 25kΩ signature impedance at
the power inputs when probed with
voltages between 2.8V and 10V. The
signature impedance is allowed to
3
TO PHY
6
+ OR –*
SIGNAL PAIRS
1
48V FROM
SIGNAL PAIRS
+ OR –*
TO PHY
2
4
5
SPARE PAIRS
+
48V FROM
SPARE PAIRS
7
8
–
*SIGNAL PAIRS CAN BE EITHER POLARITY
Figure 2. Deriving power from the cable
9
DESIGN FEATURES
+ OR –
Table 1. PD power classifications and signature currents
FROM
SIGNAL
PAIRS
+ OR –
+
48V TO PD
–
+ OR –
FROM
SPARE
PAIRS
Class
PD Maximum Power
Nominal Classification Signature Current
0
0.44W–12.95W
< 5mA
1
0.44W–3.84W
10.5mA
2
3.84W–6.49W
18.5mA
3
6.49W–12.95W
28mA
4
Class 4 is currently reserved
and should not be used
40mA
+ OR –
Figure 3a. Autopolarity input circuit
+
FROM
SIGNAL
PAIRS
–
+
48V TO PD
–
+
FROM
SPARE
PAIRS
–
Figure 3b. Non-autopolarity input circuit
have up to three diodes in series, to
allow for diode-based power steering
and autopolarity circuits. This signature is an indication to the PSE,
typically the Ethernet switch or hub,
that the device on the end of the wire
is, in fact, a PD, and won’t be damaged if the PSE applies 48V to it.
Older Ethernet devices, such as network interface cards and non-powered
hubs, typically present common-mode
impedances of around 150Ω, well
away from the valid PD impedance.
A second, optional signature may
be presented at the terminals when
probed with between 15V and 20V.
This “classification” signature indicates to the PSE the maximum power
the PD will draw so the PSE can
budget power if it chooses to. The
classification signature appears as a
constant current drawn by the PD at
the input terminals. Table 1 shows
the classes and their constant current signatures. Classes 1, 2 and 3
are used when the power is known.
Class 0 is assigned if the PD chooses
not to implement the classification
signature. Class 0 means the PSE
does not know how much power the
PD may draw, although it’s generally
wise to budget Class 3 power for such
a PD. Class 4 is reserved for future
use.
Once the PD has identified itself to
the PSE, the PSE will apply a voltage
between 44V and 57V to the wire. The
PD now has several obligations. It
should not draw significant load current until the terminal voltage rises
above 30V (to avoid interfering with
the classification signature), yet it
must be fully operational by the time
the line voltage reaches 42V. It can
never draw more than 350mA or
12.95W continuously, whichever is
less (brief surges to 400mA are allowed under some circumstances). It
needs to operate with as much as 20Ω
of wire in series with the input, which
can cut the input voltage by as much
as 8V during a 400mA current surge.
This mandates adequate hysteresis
between the turn-on and turn-off
voltages to prevent motorboating—
1N4002
RTN
POWER
SOURCING
EQUIPMENT
(PSE)
LTC4257
GND
SMAJ58A
RCLASS SIGDISA
+
PWRGD
RCLASS
–48V
C1
RPULLUP 4.7µF
51k
100V
SWITCHING
POWER SUPPLY
SHDN
GND
VIN
VOUT
Figure 4. Typical PD application
10
VIN
+
3.3V
TO LOGIC
–
oscillating on and off—when the load
is first applied and the input voltage
is low. The PD must have an input
capacitance below 180µF to keep the
power-on current surge to a reasonable level; if this input capacitance is
larger than 180µF, the PD must actively limit the inrush current to keep
it under 350mA. Finally, the PD must
maintain at least 10mA of current
draw and must maintain an AC impedance of 33kΩ or less to avoid being
disconnected.
LTC4257 PD Power Interface
Controller
The LTC4257, shown in Figure 4, is
designed to satisfy the specific demands that the IEEE standard places
on a PD, allowing designers to focus
on overall system design without worrying about compliance. The LTC4257
includes a trimmed 25k signature
resistance on-board, and a full
classification signature circuit, programmable to classes 0, 1, 2, 3 or 4
with a single external resistor. An onchip power MOSFET keeps the PD
circuitry disconnected from the line
until the voltage rises above 40V. An
inrush current limiting circuit keeps
the line current below 400mA at all
times, and thermal limiting protects
the circuit from extreme fault conditions. The only task passed on to the
rest of the PD circuitry is keeping the
continuous power drain under
12.95W (or lower, if Class 1 or 2),
something a switching regulator, such
as the LT1871, does automatically.
The regulator circuit must also maintain the required minimum 10mA
current draw, a requirement usually
met by the quiescent operating current of the system. continued on page 27
Linear Technology Magazine • December 2002
DESIGN FEATURES
13.0
13.0
12.8
12.8
100
90
VIN = 7V
VOUT2 LOAD = 185mA
80
12.4
VIN = 9V
VOUT1 LOAD = 215mA
12.2
12.0
VIN = 7V
VOUT2 LOAD = 185mA
12.2
VIN = 5V
VOUT2 LOAD = 150mA
12.0
VIN = 7V
VOUT1 LOAD = 185mA
VIN = 5V
VOUT1 LOAD = 150mA
11.8
12.4
50
100
150
200
VOUT2 LOAD CURRENT (mA)
250
LTC4257, continued from page 10
Two additional features add flexibility to LTC4257 designs. An
open-drain PWRGD output indicates
that the voltage drop across the internal power MOSFET has dropped below
1.5V, indicating that any input capacitance has charged, the output
has reached its final value, and it is
safe to turn on the system. This helps
systems that draw the maximum input power stay below the inrush limits
at turn on. A SIG_DISA input allows
Linear Technology Magazine • December 2002
40
VIN = 9V
VOUT2 LOAD = 215mA
10
50
100
150
200
VOUT1 LOAD CURRENT (mA)
250
Figure 9. Cross-regulation of Figure 7’s
circuit with fixed VOUT2 load current and
varying VOUT1 load current.
MAXIMUM MATCHED LOAD CURRENT (mA)
put. Such a transformer, at the power
level required (1.5A total parallel current and 3.3µH to 22µH per winding),
negates most of the space savings
provided by the high frequency
LT1961. The solution is to capacitively couple energy from the input to
the output transformer like the single
output of the low-profile SEPIC using
two separate inductors. This not only
gets the job done, but reduces the
height of the inductive components
and provides layout flexibility. 1:1
transformers with only two windings
are more readily available and much
smaller than transformers with at
least three windings.
Cross-regulation is excellent in this
converter as shown in Figures 8 and
9. With only a single feedback pin, the
positive output voltage always maintains regulation, but the negative
output voltage (VOUT2) regulation
changes as a function of the differ-
VIN = 5V
VOUT2 LOAD = 150mA
50
0
0
Figure 8. Cross-regulation of Figure 7’s
circuit with fixed VOUT1 load current and
varying VOUT2 load current.
60
20
11.6
0
70
30
VIN = 9V
VOUT2 LOAD = 215mA
11.8
11.6
EFFICIENCY (%)
12.6
|VOUT2 (V)|
|VOUT2 (V)|
12.6
250
200
150
LOAD CURRENT = IVOUT1 = IVOUT2
100
50
0
5
6
7
VIN (V)
8
9
Figure 11. Maximum individual output load
current (with equal loads on VOUT1 and VOUT2)
for the circuit of Figure 7, at various input
voltages.
ence in the load currents of the two
outputs. As one output becomes
heavily loaded and other lightly
loaded, cross-regulation can become
slightly compromised due to differences in losses in the catch diodes
and inductors. Figure 8 shows that
extremely light loads on VOUT2 can
the PD to disable the 25k signature
resistance if desired, allowing it to opt
not to receive power from the PSE if it
is getting it from another source, such
as a wall transformer.
Conclusion
The LTC4257 contains virtually all of
the circuitry needed to connect a powered device to an IEEE 802.3af Power
Over Ethernet network. Signature,
classification, power switching, inrush, and fault protection are all
0
50
100
150
200
VOUT1 LOAD CURRENT (mA)
250
Figure 10. Efficiency of the circuit
shown in Figure 7 is typically 75%.
result in a loss of regulation, so a
preload may be required. However,
Figure 9 shows that VOUT1 can go to
zero load current without a loss in
regulation on VOUT2. The overall converter efficiency remains high for a
flyback or SEPIC-type design as shown
in Figure 10. The maximum load current on each output varies as a
function of the load on the other
output. Figure 11 shows the maximum matched load current (the same
load current on both outputs). If one
load current is decreased, the other
can be increased without exceeding
current limit.
Conclusion
The LT1961 is a tiny, monolithic,
1.5A boost converter with a wide input voltage range that can be used in
many applications. Its high switch
frequency and onboard switch help
minimize circuit size and cost.
included, thus simplifying the required circuitry between the input
transformers and the PD voltage regulator. The LTC4257 accomplishes all
of this in a space-saving 8-pin SO or
DFN package with only one external
component, a resistor to program the
class current (not needed for class 0).
Part 3 of this series will cover the
details of detection and classification
from the PSE end of the power network.
27
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