Datasheet

LTC4263-1
15
42631fa
APPLICATIONS INFORMATION
2.2μs. During the 2.2μs that the LED pin is pulled low, cur-
rent ramps up in the inductor, limited by R
LED
. Diode D2
completes the circuit by allowing current to circulate while
the LED pin is open circuit. Since current is only drawn
from the power supply 6% of the time, power dissipation
is substantially reduced.
When V
DD5
is powered from an external supply, the PWM
signal is disabled and the LED pin will pull down continu-
ously when on. In this mode, the LED can be powered from
the 5V supply with a simple series resistor.
EXTERNAL COMPONENT SELECTION
This section discusses the other elements needed to make
a system including the LTC4263-1 function correctly. It is
recommended to adhere closely to the example application
circuits provided. For further assistance contact the Linear
Technology Applications department.
PoE System Power Delivery
The LTC4263-1 can output over 30W(typ) and is designed
to deliver 25W(min) to the PD over a 100 meter CAT5
cable for high power applications such as wireless access
points, security cameras and RFID readers. There are
several parameters external to the LTC4263-1 that limit
the power available to the PD. Figure 8 provides a simple
model used to calculate this power delivery.
The primary element affecting the delivery of power to the
PD is the supply feeding the LTC4263-1. By maximizing
this voltage, the highest and most effi cient power delivery
can be obtained. However, in order to adhere to common
safety requirements, the supply is normally limited to
60V and the IEEE 802.3af committee has chosen 57V as
a nominal maximum. In this example, a 56±1V power
supply output sets the lower limit to 55V. The LTC4263-1
overload current limit monitors port current and removes
Table 1. Port Status and LED Flash Codes
PORT STATUS LED FLASH CODE FLASH PATTERN
Non-Powered Device
0Ω < R
PORT
< 200Ω
Off LED Off
Port Open
R
PORT
> 1MΩ
Off LED Off
Port On
25kΩ
On LED On
Low Signature Resistance
300Ω < R
PORT
< 15kΩ
1 Flash
High Signature Resistance
33kΩ < R
PORT
< 500kΩ
2 Flashes
Port Overload Fault 5 Flashes
Figure 7. LED Drive Circuit with Single 48V Supply
V
DD48
D1
10mH, 21mA
COILCRAFT
DS1608C-106
D2
BAS19
V
DD5
R
LED
1k
V
DD48
LED
V
SS
LTC4263-1
4263 F07
0.1μF