NCP1081
http://onsemi.com
11
DESCRIPTION OF OPERATION
Powered Device Interface
The PD interface portion of the NCP1081 supports the
IEEE802.3af and 802.3at defined operating modes:
detection signature, current source classification, inrush and
operating current limits. In order to give more flexibility to
the user and also to keep control of the power dissipation in
the NCP1081, both current limits are configurable. The
device enters operation once its programmable Vuvlo_on
threshold is reached, and operation ceases when the supplied
voltage falls below the Vuvlo_off threshold. Sufficient
hysteresis and Uvlo filter time are provided to avoid false
power on/off cycles due to transient voltage drops on the
cable.
Detection
During the detection phase, the incremental equivalent
resistance seen by the PSE through the cable must be in the
IEEE802.3af standard specification range (23.75 kW to
26.25 kW) for a PSE voltage from 2.7 V to 10.1 V. In order
to compensate for the non-linear effect of the diode bridge
and satisfy the specification at low PSE voltage, the
NCP1081 presents a suitable impedance in parallel with the
25.5 kW R
det
external resistor connected between VPORTP
and VPORTN. For some types of diodes (especially Schottky
diodes), it may be necessary to adjust this external resistor.
When the Detection_Off level is detected (typically
11.5 V) on VPORTP, the NCP1081 turns on its internal
3.3 V regulator and biasing circuitry in anticipation of the
classification phase as the next step.
Classification
Once the PSE device has detected the PD device, the
classification process begins. The NCP1081 is fully capable
of responding and completing all classification handshaking
procedures as described next.
Classification Current Source Generation
In classification, the PD regulates a constant current
source that is set by the external resistor RCLASS value on
the CLASS pin. Figure 6 shows the schematic overview of
the classification block. The current source is defined as:
I
class
+
V
bg
R
class
, (where V
bg
is 1.2 V)
CLASS
VDDA1
1.2 V
VPORTP
VPORTN1,2
NCP1081
Rclass
Figure 6. Classification Block Diagram
The NCP1081 can handle all defined types of
classification, IEEE802.3af, 802.3at and proprietary
classification.
In the IEEE802.3af standard the classification is
performed with a Single Event Layer 1 classification.
Depending on the current level set during that single event
the power level is determined. The IEEE802.3at standard
allows two ways of classification which can also be
combined. These two approaches enable higher power
applications through a variety of PSE equipment.
For power injectors and midspans a pure physical
hardware handshake is introduced called Two Event Layer 1
classification. This approach allows equipment that has no
data link between PSE and PD to classify as high power.
Since switches can establish a data link between PSE and
PD, a software handshake is possible. This type of
handshake is called Layer 2 classification (or Data Link
Layer classification). It has the main advantage of having a
finer power resolution and the ability for the PSE and PD to
participate in dynamic power allocation.
Table 4. Single and Dual Event Classification
Standard Layer Handshake
802.3af 1 Single event physical classification
802.3at 1 Two event physical classification
802.3at 2 Data-link (IP) communication
classification
One Event Layer 1 Classification
An IEEE802.3af compliant PSE performs only One Event
Layer 1 classification event by increasing the line voltage
into the classification range only once.
Two Event Layer 1 Classification
A IEEE802.3at compliant PSE using this physical
classification performs two classification events and looks
for the appropriate response from the PD to check if the PD
is IEEE802.3at compatible.
The PSE will generate the sequence described in Figure 7.
During the first classification finger, the PSE will measure
the classification current which should be 40 mA if the PD
is at compliant. If this is the case, the PSE will exit the
classification range and will force the line voltage into the
Mark Event range. Within this range, the PSE may check the
non-valid input signature presented by the PD (using the two
point measurement defined in the IEEE802.3af standard).
Then the PSE will repeat the same sequence with the second
classification finger. A PD which has detected the sequence
“Finger + Mark + Finger + Mark” knows the PSE is
IEEE802.3at compliant, meaning the PSE will deliver more
current on the port. (Note that a PSE IEEE802.3at compliant
may apply more than two fingers, but the final result will be
the same as two fingers).
