ltc4264cde-trpbf Linear Technology Corporation, ltc4264cde-trpbf Datasheet - Page 9

• Manufacturer Part Number: ltc4264cde-trpbf
• Description: High Power Pd Interface Controller With 750ma Current Limit
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
PSE
Figure 3. Output Voltage, PWRGD, PWRGD and
PD Current as a Function of Input Voltage
I
I
LIMIT_HIGH
LIMIT_LOW
I
CLASS
R
I
I
I
I
1
CLASS
LIMIT_LOW
–20
–30
–40
–50
–20
–30
–40
–50
–20
–30
–40
–50
LOAD
–10
–10
–10
CLASS
I
20
10
=
IN
V1 – 2 DIODE DROPS
=
DEPENDENT ON R
R
V
LOAD
dV
dt
= 300mA, I
IN
UVLO
OFF
25kΩ
=
R
V
I
POWER
POWER
CLASS
IN
DETECTION V1
LIMIT_LOW
LTC4264
BAD
BAD
DETECTION V2
DETECTION I
DETECTION I
CLASSIFICATION
PWRGD
PWRGD
C1
CLASSIFICATION
V
GND
LIMIT_HIGH
OUT
UVLO
TURN-ON
CLASS
UVLO
ON
POWER
POWER
GOOD
GOOD
I
2
1
2
=
SELECTION
= 750mA
V2 – 2 DIODE DROPS
PWRGD TRACKS
V
UVLO
TURN-OFF
POWER
τ = R
IN
25kΩ
LOAD, I
(UP TO I
BAD
POWER
R
UVLO
LOAD
OFF
BAD
C1
LOAD
TIME
TIME
TIME
TIME
TIME
LOAD
LIMIT_HIGH
C1
4264 F03
V
OUT
GND
)
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
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
IN
pins. This precision, temperature-compensated
LTC4264
9
4264f