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LTC4278

4278fc

One way to reduce cost and improve output ripple is to use

a simple LC filter. Figure 18 shows an example of the filter.

APPLICATIONS INFORMATION

unidirectional58Vtransientvoltagesuppressorbeinstalled

betweenthediodebridgeandtheLTC4278(D3inFigure2).

ISOLATION

The802.3standardrequiresEthernetportstobeelectrically

isolatedfromallotherconductorsthatareuseraccessible.

This includes the metal chassis, other connectors and

any auxiliary power connection. For PDs, there are two

common methods to meet the isolation requirement. If

there will be any user accessible connection to the PD,

then an isolated DC/DC converter is necessary to meet

the isolation requirements. If user connections can be

avoided, then it is possible to meet the safety requirement

by completely enclosing the PD in an insulated housing.

In all PD applications, there should be no user accessible

electrical connections to the LTC4278 or support circuitry

other than the RJ-45 port.

LAYOUT CONSIDERATIONS FOR THE LTC4278

The LTC4278’s PD front end is relatively immune to layout

problems. Excessive parasitic capacitance on the RCLASS

pin should be avoided. Include a PCB heat sink to which

the exposed pad on the bottom of the package can be

soldered. This heat sink should be electrically connected

to GND. For optimum thermal performance, make the

heat sink as large as possible. Voltages in a PD can be as

large as 57V for PoE applications, so high voltage layout

techniques should be employed. The SHDN pin should

be separated from other high voltage pins, like VPORTP,

VNEG, to avoid the possibility of leakage currents shutting

down the LTC4278. If not used, tie SHDN to VPORTN. The

load capacitor connected between VPORTP and VNEG of the

LTC4278 can store significant energy when fully charged.

The design of a PD must ensure that this energy is not

inadvertently dissipated in the LTC4278. The polarity-

protection diodes prevent an accidental short on the cable

from causing damage. However if, VPORTN is shorted

to VPORTP inside the PD while capacitor C1 is charged,

RLOAD

COUT2

1µF

VOUT

COUT

470µF

47µF

×3

FROM

SECONDARY

WINDING

L1, 0.1µH

4278 F18

Figure 18.

The design of the filter is beyond the scope of this data

sheet. However, as a starting point, use these general

guidelines. Start with a COUT 1/4 the size of the nonfilter

solution. Make C1 1/4 of COUT to make the second filter

pole independent of COUT. C1 may be best implemented

with multiple ceramic capacitors. Make L1 smaller than

the output inductance of the transformer. In general, a

0.1µH filter inductor is sufficient. Add a small ceramic

capacitor (COUT2) for high frequency noise on VOUT. For

those interested in more details refer to “Second-Stage

LC Filter Design,” Ridley, Switching Power Magazine, July

2000 p8-10.

Circuit simulation is a way to optimize output capacitance

and filters, just make sure to include the component

parasitic. LTC SwitcherCAD® is a terrific free circuit

simulation tool that is available at www.linear.com. Final

optimization of output ripple must be done on a dedicated

PC board. Parasitic inductance due to poor layout can

significantly impact ripple. Refer to the PC Board Layout

section for more details.

ELECTRO STATIC DISCHARGE AND SURGE

PROTECTION

The LTC4278 is specified to operate with an absolute

maximum voltage of –100V and is designed to tolerate

brief overvoltage events. However, the pins that interface

to the outside world (primarily VPORTN and VPORTP) can

routinely see peak voltages in excess of 10kV. To protect

the LTC4278, it is highly recommended that the SMAJ58A

LTC4264 Datasheet(PDF) 37 Page - Linear Technology

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