LTC4259A datasheet(27/32 Pages) LINER

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LTC4259A

4259Af

The LTC4259A 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 LTC4259A

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 LTC4259A to power the port, bypassing IEEE compli-

ant 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 LTC4259A. To simplify design and meet voltage

isolation requirements, the logic level supply can be

generated from the isolated – 48V supply. Figure 19

shows an example method using an LT

®1619 to control

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 is designed to control power delivery in

IEEE 802.3af compliant Power Sourcing Equipment (PSE).

Because proper operation of the LTC4259A may depend

on external signals and power sources, like the –48V

supply (VEE) or the OSCIN oscillator source, external

components such as the sense resistors (RS), and possi-

bly software running on an external microprocessor,

using the LTC4259A in a PSE does not guarantee 802.3af

compliance. Using an LTC4259A does get you most of the

way there. This section discusses the rest of the elements

that go along with the LTC4259A to make an 802.3af

complaint PSE. Each paragraph below addresses a com-

ponent which is critical for PSE compliance as well as

APPLICATIO S I FOR ATIO

DRV

100

Ω

100

µH

4.7

µH

B1100

µF

16V

4700pF

100pF

CMPZ4702B

ISOLATED

GND

ISOLATED

–48V

Si2328DS

FMMT593

GATE

VIN

S/S

SENSE

LT1619

GND

Si2328DS

510

Ω

47k

3.32k

0.100

Ω

1.24k

100k

4259A F19

910k

µF

100V

VEE

Ω

µF

10V

VDD

3.3V

300mA

µF

10V

ISOLATED

GND

Figure 19. –48V to 3.3V Boost Converter

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