LTC4278CDKD#PBF Linear Technology, LTC4278CDKD#PBF Datasheet - Page 37

• Manufacturer Part Number: LTC4278CDKD#PBF
• Description: IC PD IEEE 802.3AT 25.5W 32-DFN
• Manufacturer: Linear Technology

Specifications of LTC4278CDKD#PBF

• Controller Type: Powered Device Interface Controller (PD)
• Interface: IEEE 802.3af
• Voltage - Supply: 4.5 V ~ 20 V
• Current - Supply: 6.4mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 32-DFN
• Input Voltage: 60V
• Supply Current: 1.35mA
• Digital Ic Case Style: DFN
• No. Of Pins: 32
• Duty Cycle (%): 88%
• Frequency: 100kHz
• Operating Temperature Range: °C To +70°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

One way to reduce cost and improve output ripple is to use a
simple LC fi lter. Figure 18 shows an example of the fi lter.
The design of the fi lter is beyond the scope of this data
sheet. However, as a starting point, use these general
guidelines. Start with a C
solution. Make C1 1/4 of C
pole independent of C
with multiple ceramic capacitors. Make L1 smaller than
the output inductance of the transformer. In general, a
0.1μH fi lter inductor is suffi cient. Add a small ceramic
capacitor (C
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 fi lters, just make sure to include the component
parasitic. LTC SwitcherCAD
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
signifi cantly impact ripple. Refer to the PC Board Layout
section for more details.
ELECTRO STATIC DISCHARGE AND SURGE
PROTECTION
The LTC4278 is specifi ed 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 V
see peak voltages in excess of 10kV. To protect the LTC4278,
it is highly recommended that the SMAJ58A unidirectional
58V transient voltage suppressor be installed between the
diode bridge and the LTC4278 (D3 in Figure 2).
APPLICATIONS INFORMATION
SECONDARY
WINDING
FROM
OUT2
) for high frequency noise on V
+
L1, 0.1μH
C1
47μF
×3
OUT
Figure 18.
OUT
PORTN
. C1 may be best implemented
+
OUT
®
1/4 the size of the nonfi lter
C
470μF
OUT
to make the second fi lter
and V
is a terrifi c free circuit
V
PORTP
OUT
C
1μF
OUT2
4278 F18
) can routinely
R
LOAD
OUT
. For
ISOLATION
The 802.3 standard requires Ethernet ports to be electrically
isolated from all other conductors that are user accessible.
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 R
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 V
V
down the LTC4278. If not used, tie SHDN to V
load capacitor connected between V
LTC4278 can store signifi cant 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, V
to V
current will fl ow through the parasitic body diode of the
internal MOSFET and may cause permanent damage to
the LTC4278.
In order to minimize switching noise and improve output
load regulation, connect the GND pin of the LTC4278 directly
SwitcherCAD is a registered trademark of Linear Technology Corporation.
NEG
PORTP
, to avoid the possibility of leakage currents shutting
inside the PD while capacitor C1 is charged,
PORTP
PORTN
LTC4278
and V
PORTN
is shorted
NEG
37
PORTP
CLASS
of the
. The
4278fa
,