LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 24

• Manufacturer Part Number: LTC4258
• Description: Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC4258
The LTC4258 can also be used to power older powered
Ethernet devices that are not 802.3af compliant and may
be detected with other methods. Although the LTC4258
does not implement these older detection methods auto-
matically, if software or external circuitry can detect the
noncompliant devices, the host controller may command
the LTC4258 to power the port, bypassing IEEE compliant
detection and classification and sending power to the
noncompliant device.
LOGIC LEVEL SUPPLY
In additon to the 48V used to source power to each port,
a logic level supply is required to power the digital portion
of the LTC4258. To simplify design and meet voltage
isolation requirements, the logic level supply can be
generated from the isolated – 48V supply. Figure 16
shows an example method using an LT
a –48V to 3.3V current mode supply. This boost con-
verter topology uses the LT1619 current mode controller
and a current mirror which reflects the 3.3V output
24
ISOLATED
–48V
ISOLATED
GND
V
EE
10
+
U
CMPZ4702B
1 F
100V
U
910k
W
Si2328DS
510
®
10 F
16V
1619 to control
Figure 16. –48V to 3.3V Boost Converter
U
V
S/S
IN
8
1
LT1619
DRV
GND
7
4
SENSE
GATE
V
C
FB
6
3
47k
4700pF
5
2
voltage to the –48V rail, improving the regulation toler-
ance over the more traditional large resistor voltage
divider. This approach achieves high accuracy with a
transformerless design.
IEEE 802.3af COMPLIANCE AND EXTERNAL
COMPONENT SELECTION
The LTC4258 is designed to control power delivery in IEEE
802.3af compliant Power Sourcing Equipment (PSE).
Because proper operation of the LTC4258 may depend on
external signals and power sources, like the –48V supply
(V
such as the sense resistors (R
running on an external microprocessor, using the LTC4258
in a PSE does not guarantee 802.3af compliance. Using an
LTC4258 does get you most of the way there. This section
discusses the rest of the elements that go along with the
LTC4258 to make an 802.3af complaint PSE. Each para-
graph below addresses a component which is critical for
PSE compliance as well as possible pitfalls that can cause
100 H
100
EE
) or the OSCIN oscillator source, external components
100pF
Si2328DS
0.100
1%
1W
FMMT593
B1100
1.24k
1%
3.32k
1%
+
FMMT593
4258 F16
100k
S
47 F
10V
4.7 H
), and possibly software
+
ISOLATED
GND
47 F
10V
V
3.3V
300mA
DD
4258p