Datasheet

LTC4264
9
4264f
APPLICATIONS INFORMATION
DETECTION V1
CLASSIFICATION
UVLO
TURN-ON
UVLO
OFF
POWER
BAD
UVLO
OFF
UVLO
ON
UVLO
TURN-OFF
τ = R
LOAD
C1
PWRGD TRACKS
V
IN
DETECTION V2
–10
TIME
–20
–30
V
IN
(V)
–40
–50
–10
TIME
–20
–30
V
OUT
(V)
–40
–50
–10
TIME
–20
–30
PWRGD (V)PWRGD – V
OUT
(V)
–40
–50
20
10
I
CLASS
PD CURRENT
I
LIMIT_HIGH
I
LIMIT_LOW
dV
dt
I
LIMIT_LOW
C1
=
POWER
BAD
POWER
BAD
POWER
BAD
TIME
TIME
POWER
GOOD
POWER
GOOD
DETECTION I
1
CLASSIFICATION
DETECTION I
2
LOAD, I
LOAD
(UP TO I
LIMIT_HIGH
)
4264 F03
I
CLASS
DEPENDENT ON R
CLASS
SELECTION
I
LIMIT_LOW
= 300mA, I
LIMIT_HIGH
= 750mA
I
1
=
V1 – 2 DIODE DROPS
25kΩ
I
LOAD
=
V
IN
R
LOAD
I
2
=
V2 – 2 DIODE DROPS
25kΩ
GND
PSE
I
IN
R
LOAD
R
CLASS
V
OUT
C1
GND
R
CLASS
PWRGD
PWRGD
LTC4264
V
OUT
V
IN
Figure 3. Output Voltage, PWRGD, PWRGD and
PD Current as a Function of Input Voltage
SERIES DIODES
The IEEE 802.3af-defi ned operating modes for a PD refer-
ence the input voltage at the RJ45 connector on the PD.
The PD must be able to handle power received in either
polarity. For this reason, it is common to install diode
bridges BR1 and BR2 between the RJ45 connector and
the LTC4264 (Figure 4). The diode bridges introduce an
offset that affects the threshold points for each range of
operation. The LTC4264 meets the IEEE 802.3af-defi ned
operating modes by compensating for the diode drops
in the threshold points. For the signature, classifi cation,
and the UVLO thresholds, the LTC4264 extends two diode
drops below the IEEE 802.3af specifi cations. Note that
the voltage ranges specifi ed in the LTC4264 Electrical
Specifi cations are referenced with respect to the IC pins.
The LTC4264 threshold points support the use of either
traditional or Schottky diode bridges.
DETECTION
During detection, the PSE will apply a voltage in the range
of –2.8V to –10V on the cable and look for a 25k signature
resistor. This identifi es the device at the end of the cable
as a PD. With the PSE voltage in the detection range, the
LTC4264 presents an internal 25k resistor between the GND
and V
IN
pins. This precision, temperature-compensated
resistor provides the proper characteristics to alert the PSE
that a PD is present and requests power to be applied.
The IEEE 802.3af specifi cation requires the PSE to use
a ΔV/ΔI measurement technique to keep the DC offset
voltage of the diode bridge from affecting the signature
resistance measurement. However, the diode resistance
appears in series with the signature resistor and must
be included in the overall signature resistance of the PD.
The LTC4264 compensates for the two series diodes in
the signature path by offsetting the internal resistance so
that a PD built with the LTC4264 meets the IEEE 802.3af
specifi cation.
In some designs that include an auxiliary power option,
such as an external wall adapter, it is necessary to control
whether or not the PD is detected by a PSE. With the
LTC4264, the 25k signature resistor can be enabled or
disabled with the SHDN pin (Figure 5). Taking the SHDN