LTC4278 Linear Technology, LTC4278 Datasheet - Page 14

• Manufacturer Part Number: LTC4278
• Description: IEEE 802.3at PD
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC4278
Table 1. LTC4278 Modes of Operation as a Function
of Input Voltage
V
0V to 1.4V
1.5V to 9.8V
(5.4V to 9.8V)
12.5V to ON/OFF*
ON/OFF* to 60V
>71V
*ON/OFF includes hysteresis. Rising input threshold, 37.2V Max.
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 LTC4278 (Figure 2).
14
PORTP
Falling input threshold, 30V Min.
–V
PORTN
(V)
LTC4278 MODES OF OPERATION
Inactive (Reset After 1st Classifi cation Event)
25k Signature Resistor Detection Before 1st
Classifi cation Event (Mark, 11k Signature
Corrupt After 1st Classifi cation Event)
Classifi cation Load Current Active
Inrush and Power Applied To PD Load
Overvoltage Lockout,
Classifi cation and Hot Swap Are Disabled
Figure 2. PD Front End Using Diode Bridges on Main and Spare Inputs
POWERED
DEVICE
INPUT
(PD)
RJ45
1
2
3
6
4
5
7
8
TX
TX
RX
RX
SPARE
SPARE
+
–
+
–
+
–
T1
BR2
TO PHY
The input diode bridge introduces a voltage drop that
affects the range for each mode of operation. The
LTC4278 compensates for these voltage drops so that a
PD built with the LTC4278 meets the IEEE 802.3af/IEEE
802.3at-established voltage ranges. Note the Electrical
Characteristics are referenced with respect to the LTC4278
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 LTC4278 presents its precision, temperature-com-
pensated 25k resistor between the V
pins, alerting the PSE that a PD is present and requests
power to be applied. The LTC4278 signature resistor also
compensates for the additional series resistance intro-
duced by the input diode bridge. Thus a PD built with
the LTC4278 conforms to the IEEE 802.3af/IEEE 802.3at
specifi cations.
0.1μF
100V
BR1
D3
V
LTC4278
PORTN
V
PORTP
4278 F02
PORTP
and V
PORTN
4278f