Datasheet

LTC4269-1
13
42691fc
APPLICATIONS INFORMATION
Table 1. LTC4269-1 Modes of Operation as a Function
of Input Voltage
V
PORTP
–V
PORTN
(V) LTC4269-1 MODES OF OPERATION
0V to 1.4V Inactive (Reset After 1st Classifi cation Event)
1.5V to 9.8V
(5.4V to 9.8V)
25k Signature Resistor Detection Before 1st
Classifi cation Event (Mark, 11k Signature
Corrupt After 1st Classifi cation Event)
12.5V to ON/OFF* Classifi cation Load Current Active
ON/OFF* to 60V Inrush and Power Applied To PD Load
>71V Overvoltage Lockout,
Classifi cation and Hot Swap Are Disabled
*ON/OFF includes hysteresis. Rising input threshold, 37.2V Max.
Falling input threshold, 30V Min.
These modes satisfy the requirements defi ned in the
IEEE 802.3af/IEEE 802.3at specifi cation.
INPUT DIODE BRIDGE
In the IEEE 802.3af/IEEE 802.3at standard, the modes of
operation reference the input voltage at the PD’s RJ45
connector. Since the PD must handle power received in
either polarity from either the data or the spare pair, input
diode bridges BR1 and BR2 are connected between the
RJ45 connector and the LTC4269-1 (Figure 2).
The input diode bridge introduces a voltage drop that
affects the range for each mode of operation. The
LTC4269-1 compensates for these voltage drops so that a
PD built with the LTC4269-1 meets the IEEE 802.3af/IEEE
802.3at-established voltage ranges. Note the Electrical
Characteristics are referenced with respect to the
LTC4269- 1 package pins.
DETECTION
During detection, the PSE looks for a 25k signature resis-
tor which identifi es the device as a PD. The PSE will apply
two voltages in the range of 2.8V to 10V and measures
the corresponding currents. Figure 1 shows the detection
voltages V1 and V2 and the corresponding PD current.
The PSE calculates the signature resistance using the ΔV/
ΔI measurement technique.
The LTC4269-1 presents its precision, temperature-com-
pensated 25k resistor between the V
PORTP
and V
PORTN
pins, alerting the PSE that a PD is present and requests
power to be applied. The LTC4269-1 signature resistor
also compensates for the additional series resistance
introduced by the input diode bridge. Thus a PD built
with the LTC4269-1 conforms to the IEEE 802.3af/IEEE
802.3at specifi cations.
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
POWERED
DEVICE
(PD)
INPUT
42691 F02
1
7
8
5
4
SPARE
–
SPARE
+
TO PHY
BR2
0.1µF
100V
BR1
D3
LTC4269-1
V
PORTN
V
PORTP
Figure 2. PD Front End Using Diode Bridges on Main and Spare Inputs