LTC4258 datasheet(24/28 Pages) LINER

24 / 28 page

LTC4258

4258p

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

®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 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

(VEE) or the OSCIN oscillator source, external components

such as the sense resistors (RS), and possibly software

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

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

4258 F16

910k

µF

100V

VEE

Ω

µF

10V

VDD

3.3V

300mA

µF

10V

ISOLATED

GND

Figure 16. –48V to 3.3V Boost Converter

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