hv110 Supertex, Inc., hv110 Datasheet - Page 4

• Manufacturer Part Number: hv110
• Description: Power-over-ethernet Interface Pd Controller Meets Ieee802.3af Tm Standard
• Manufacturer: Supertex, Inc.

PSE Power Standards
PSE powers a single link. It searches the link for a
PD and supplies power to the link only after a PD
Signature is detected. The PSE will reject any links
with an invalid PD Signature. When the PD is
removed, the PSE will also remove the power from
the link.
PSE may be able to do an optional Classification of
the PD, to detect the maximum power drawn by the
PD, to do some high level Power Management. PSE
is limited to a continuous maximum output of 15.4W.
Discovery
Key
Discovery. Discovery is the method used to
determine if a device at the end of the cable is
capable of receiving high voltage DC, before
applying high voltage. Discovery also is used for
determining when a PD device is disconnected or
removed subsequently. The reason for all of this is
that high voltages (-48V) connected to many legacy
devices can cause equipment damage. For this
reason Discovery takes place at voltages compatible
with existing legacy equipment and high voltage DC
is only applied once discovery is satisfied. The
IEEE802.3af Discovery is based upon the sensing of
a characteristic impedance. This impedance is
defined nominally as 25k (23.5k to 26.25k) with no
more than 0.1uF of capacitance in parallel with the
impedance, in a voltage range from 2.8V to 10V.
The presence of diode rectification at the PD end
forces a slope impedance method, requiring at least
two operating point measurements, to eliminate the
effect of diode level shift.
Classification (optional)
As per the IEEE802.3af standards, the PSE has to
deliver a minimum of 15.4W to a PD connected to it
while limited by the 350mA maximum operating
current. Not all PD devices, however, require this
much power to operate. For example an IP Phone
with a monochrome screen will require far less
power than an IP Phone with color display. By
identifying the power drawn through each port, PSE
can assist in the System Power Management
protocol to determine the total number of PDs it can
support, depending on the output capacity of the
system power supply.
To achieve this type of power management an
optional step was added to the IEEE802.3af
standard called ‘Classification’. Classification allows
a device to communicate the maximum power it will
ever demand to the PSE so that the Power
to
all
Power-over-Ethernet
methods
is
4
Table 2. Classification Signature measured at PD connector
Management Protocol can allocate the unused
power to other ports, enabling the full utilization of
the installed capacity. Table 1 identifies the different
classifications included in the IEEE802.3af standard.
In order to identify the class of the PD connected,
the PSE sends a second voltage signal of 15 to 20V,
slightly higher than signature detection voltage and
measures the current. Depending on the magnitude
of the current drawn, the PSE will classify the load to
one of the four Classes as shown in Table 2, and will
assume that the load will not draw any additional
power than shown for the given Class.
Note that Class 0 default will work for all devices &
Classification is only needed in the rare instance
when a multi-port switch or router wants to rate the
system supply lower than the combined Class 0 port
output; a situation which will reduce it’s potential
classification base. In fact most of the PD devices,
like Wireless Access Points, in the market today are
Class 0 devices and hence do not require any
classification methods. HV110 therefore does not
force the use of resistors and wasted silicon area to
implement a Classification current source. HV110,
however, can be made to be compatible with
classification by utilizing low cost circuitry as shown
in Figure 9.
Disconnect
The PSE must be able to remove the power from a
port once the PD is removed. The purpose of this is
to prevent damage to non-compatible devices
connected to the same link at a later time. As per the
DC
disconnect load if the current is between 5mA and
10mA and must disconnect between 0-5mA, if the
condition persists for more than 300ms. Although not
required by a PD device, HV110 includes a
“minimum circuit breaker” which when the current is
in a range less than 20mA will cause a shutdown
after the 300ms if the PSE does not react.
Class
Class
0
1
2
3
0
1
2
3
disconnect
Voltage
15-20V
15-20V
15-20V
15-20V
Probe
Table 1. PD Power Classification
Optional
Optional
Optional
Default
Usage
requirements,
Min.(mA)
0.5
17
26
9
0.44 – 12.95W
6.49 – 12.95W
0.44 – 3.84W
3.84 – 6.49W
PD Power
Max. (mA)
the
12
20
30
4
PSE
HV110
A061104
may