Datasheet

LTC4263
17
4263fe
APPLICATIONS INFORMATION
When active, the LED fl ash codes are repeated every 1.2
seconds. The duration of each LED fl ash is 75ms. Multiple
LED fl ashes occur at a 300ms interval.
The LTC4263 includes a feature for effi ciently driving the
LED from a 48V power supply without the wasted power
caused by having to drop over 45V in a current limit
resistor. When operating the V
DD5
supply internally, the
LTC4263 drives the LED pin with a 6% duty cycle PWM
signal. This allows use of the simple LED drive circuit in
Figure 9 to minimize power dissipation. The modulation
frequency of the LED drive is 28kHz, making the on period
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.
IEEE 802.3af COMPLIANCE AND EXTERNAL
COMPONENT SELECTION
This section discusses the other elements that go along
with the LTC4263 to make an IEEE 802.3af compliant PSE.
The LTC4263 is designed to control power delivery in IEEE
802.3af compliant Power Sourcing Equipment. Because
proper operation of the LTC4263 also depends on external
components and power sources like the 48V supply, using
the LTC4263 in a PSE does not in itself guarantee IEEE
802.3af compliance. To ensure a compliant PSE design,
it is recommended to adhere closely to the example ap-
plication circuits provided. For further assistance contact
the Linear Technology Applications department.
Figure 9. LED Drive Circuit with Single 48V Supply
Table 3. 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
Power Management
Allocation Exceeded
9 Flashes
V
DD48
D1
10mH, 21mA
COILCRAFT
DS1608C-106
D2
BAS19
V
DD5
R
LED
1k
V
DD48
LED
V
SS
LTC4263
4263 F09
0.1μF