LTC4259ACGW-1#PBF Linear Technology, LTC4259ACGW-1#PBF Datasheet - Page 28

• Manufacturer Part Number: LTC4259ACGW-1#PBF
• Description: IC CTRLR POE QUAD AC DISC 36SSOP
• Manufacturer: Linear Technology

Specifications of LTC4259ACGW-1#PBF

• Controller Type: Power over Ethernet Controller (POE)
• Interface: I²C
• Voltage - Supply: 3 V ~ 4 V
• Current - Supply: 2.5mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Linear Misc Type: Negative Voltage
• Family Name: LTC4259A
• Package Type: SSOP
• Operating Supply Voltage (min): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
LTC4259A-1
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 LTC4259A-1. To simplify design and meet voltage
isolation requirements, the logic level supply can be
generated from the isolated – 48V supply. Figure 21
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
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 LTC4259A-1 is designed to control power delivery in
IEEE 802.3af compliant Power Sourcing Equipment (PSE).
Because proper operation of the LTC4259A-1 may depend
on external signals and power sources, like the –48V
supply (V
components such as the sense resistors (R
bly software running on an external microprocessor,
using the LTC4259A-1 in a PSE does not guarantee
802.3af compliance. Using an LTC4259A-1 does get you
28
ISOLATED
–48V
ISOLATED
EE
) or the OSCIN oscillator source, external
GND
V
EE
10Ω
+
U
CMPZ4702B
1µF
100V
U
910k
W
Si2328DS
510Ω
®
10µF
16V
Figure 21. –48V to 3.3V Boost Converter
1619 to control
S
), and possi-
U
V
S/S
IN
8
1
LT1619
DRV
GND
7
4
SENSE
GATE
V
C
FB
6
3
47k
4700pF
5
2
most of the way there. This section discusses the rest of
the elements that go along with the LTC4259A-1 to make
an 802.3af compliant PSE. Each paragraph below ad-
dresses a component which is critical for PSE compliance
as well as possible pitfalls that can cause a PSE to be
noncompliant. For further assistance please contact Lin-
ear Technology’s Applications department.
Sense Resistors
The LTC4259A-1 is designed to use a 0.5Ω sense resistor,
R
the sense resistor has been minimized in order to reduce
power loss and as a consequence, the voltage which the
LTC4259A-1 must measure is small. Each port may be
drawing up to 450mA with this current flowing through the
sense resistor and associated circuit board traces. To
prevent parasitic resistance on the circuit board from
obscuring the voltage drop across the sense resistor, the
LTC4259A-1 must Kelvin sense the resistor voltage. One
way to achieve Kelvin sensing is “star grounding,” shown
pictorially in Figure 1. Another option is to use a – 48V
power plane to connect the sense resistor and the
LTC4259A-1 V
voltages developed across parasitic circuit board resis-
tances from affecting the LTC4259A-1 current measure-
ment accuracy. The precision of the sense resistor directly
affects the measurement of the IEEE parameters I
S
100µH
100Ω
, to monitor the current through each port. The value of
100pF
Si2328DS
0.100Ω
1%
1W
EE
FMMT593
B1100
pin. Either of these strategies will prevent
1.24k
1%
3.32k
1%
+
FMMT593
4259A F21
100k
47µF
10V
4.7µH
+
ISOLATED
GND
47µF
10V
V
3.3V
300mA
DD
INRUSH
4259a1fa
,