Datasheet

LTC4269-1
15
42691fc
APPLICATIONS INFORMATION
DETECTION V1
ON
OFF
OFF OFFON
T = R
LOAD
C1
TRACKS
V
PORTN
DETECTION V2
TIME
PD CURRENT
50
40
30
V
PORTP
(V)
20
10
40mA
50
40
30
20
10
TIME
V
PORTP
– V
NEG
(V)
–10
TIME
–20
–30
V
PORTP
– T2P (V)
–40
–50
dV
dt
INRUSH
C1
=
42691 F04
INRUSH = 100mA R
CLASS
= 30.9Ω
I
LOAD
=
V
PORTN
R
LOAD
V
PORTP
PSE
I
IN
R
LOAD
R
CLASS
C1
R
CLASS
T2P
LTC4269-1
V
NEG
V
PORTN
1st CLASS
1st MARK 2nd MARK
DETECTION V1
DETECTION V2
1st MARK 2nd MARK
2nd CLASS
1st CLASS
2nd CLASS
LOAD, I
LOAD
INRUSH
Figure 4. V
NEG
, T2P and PD Current
as a Result of 2-Event Classifi cation
SIGNATURE CORRUPT DURING MARK
As a member of the IEEE 802.3at working group, Linear
Technology noted that it is possible for a Type 2 PD to
receive a false indication of a 2-event classifi cation if a PSE
port is pre-charged to a voltage above the detection voltage
range before the fi rst detection cycle. The IEEE working
group modifi ed the standard to prevent this possibility by
requiring a Type 2 PD to corrupt the signature resistance
during the mark event, alerting the PSE not to apply power.
The LTC4269-1 conforms to this standard by corrupting
the signature resistance. This also discharges the port
before the PSE begins the next detection cycle.
PD STABILITY DURING CLASSIFICATION
Classifi cation presents a challenging stability problem due
to the wide range of possible classifi cation load current.
The onset of the classifi cation load current introduces a
voltage drop across the cable and increases the forward
voltage of the input diode bridge. This may cause the PD
to oscillate between detection and classifi cation with the
onset and removal of the classifi cation load current.
The LTC4269-1 prevents this oscillation by introducing a
voltage hysteresis window between the detection and clas-
sifi cation ranges. The hysteresis window accommodates
the voltage changes a PD encounters at the onset of the
classifi cation load current, thus providing a trouble-free
transition between detection and classifi cation modes.
The LTC4269-1 also maintains a positive I-V slope through-
out the classifi cation range up to the on-voltage. In the
event a PSE overshoots beyond the classifi cation voltage
range, the available load current aids in returning the PD
back into the classifi cation voltage range. (The PD input
may otherwise be “trapped” by a reverse-biased diode
bridge and the voltage held by the 0.1F capacitor).
INRUSH CURRENT
Once the PSE detects and optionally classifi es the PD,
the PSE then applies powers on the PD. When the
LTC4269-1 input voltage rises above the on-voltage
threshold, LTC4269-1 connects V
NEG
to V
PORTN
through
the internal power MOSFET.